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Specializations of the chorioallantoic placenta in the Brazilian scincid lizard,Mabuya heathi: A new placental morphotype for reptiles
Abstract
New World skinks of the genus Mabuya exhibit a unique form of viviparity that involves ovulation of tiny (1 mm) eggs and provision of virtually all of the nutrients for embryonic development by placental means. Studies of the Brazilian species M. heathi reveal that the chorioallantoic placenta is unlike those reported in any other squamate genus and exhibits striking specializations for maternal-fetal nutrient transfer. The uterine lining is intimately apposed to the chorioallantois, with no trace of an intervening shell membrane or of epithelial erosion; thus, the placenta is epitheliochorial. The uterus exhibits multicellular glands that secrete organic material into the uterine lumen. Opposite the openings of these glands, the chorion develops areolae, invaginated pits that are lined by absorptive, columnar epithelium. A single, mesometrial placentome develops, formed by radially oriented uterine folds that project into a deep invagination of the chorion. Uterine epithelium of the placentome appears to be syncytial and secretory and overlies a rich vascular supply. The apposed chorionic epithelium is absorptive in morphology and contains giant binucleated cells that bear microvilli. Several specializations of the placental membranes of M. heathi are found among eutherian mammals, signifying evolutionary convergence that extends to histological and cytological levels. The chorioallantoic placenta of M. heathi and its relatives warrants recognition as a new morphotype for reptiles, defined here as the "Type IV" placenta. This is the first new type of chorioallantoic placenta to be defined formally for reptiles in over half a century.
... Together with the present identification of its cognate receptor, these results show that syncytin capture is not restricted to mammals and is likely to be a major driving force for placenta emergence. cells throughout gestation (19,20,(24)(25)(26)(27). Yet these differences can be considered minor in view of the tremendous qualitative transition that must have taken place between egg-laying and placental lizards that have a mode of reproduction so closely related to that of placental mammals. ...
... On the maternal side, the uterine epithelium is replaced by large syncytial structures, most probably resulting from the cellular fusion of uterine cells. At the cellular interface, maternal and fetal cells form microvilli, further increasing the exchange surface of the placentome (19,20,(24)(25)(26). Periodically the microvilli are interrupted by the presence of smaller invasive fetal cells located between the fetal and maternal epithelia, allowing a more intimate contact between fetal tissues and the maternal circulatory system (38). ...
... The organization of fetal tissues in the paraplacentome does not differ significantly from that observed in the placentome, although tissues are not folded. The materno-fetal interface is still marked by interdigitated microvilli, although they are smaller than in the placentome (19,20,(24)(25)(26). ...
Significance
Retroviral envelope gene capture and exaptation for a placental function has been demonstrated in mammals. Remarkably, placental structures have also emerged on rare occasions in nonmammalian vertebrates, resulting in related modes of reproduction. The Mabuya lizard, which emerged 25 Mya, possesses a placenta closely related to that of mammals. Here, we identified a specific retroviral envelope gene capture that shows all the characteristic features of a bona fide mammalian syncytin, being conserved in Mabuya evolution, expressed in the placenta, and fusogenic. Together with the present identification of its cognate receptor, these results show that syncytin capture is not restricted to mammals and is likely to be a major driving force for placenta emergence.
... In contrast, viviparity allows mothers to supplement embryonic nutrition via placentotrophy; the transfer of maternal nutrients across a placenta (Stewart and Castillo 1984). The extent of placentation in lizards is highly variable, ranging from predominantly lecithotrophic (dependent on yolk nutrition) in some species (Stewart and Castillo 1984;Stewart 1989;Stewart et al. 1990) to predominantly placentotrophic in others (Thompson et al. 2000;Blackburn and Vitt 2002;Thompson and Speake 2003). Indeed, lizards exhibit some of the most extreme examples of placentotrophy outside mammals, with placental hypertrophy and mammal-like embryonic development and pregnancy (Thompson and Speake 2006). ...
... Indeed, lizards exhibit some of the most extreme examples of placentotrophy outside mammals, with placental hypertrophy and mammal-like embryonic development and pregnancy (Thompson and Speake 2006). For example, the Brazilian skink, Mabuya heathi, has a true epitheliochorial placenta and placentotrophic nutrients account for >99% of increases in fetus dry mass during gestation (Blackburn et al. 1984;Blackburn and Vitt 2002). The ability of females to supplement embryonic provisioning throughout gestation has a number of important implications for both offspring and parents. ...
... Placentation also allows for the transport of inorganic ions (Blackburn 1993;Blackburn and Lorenz 2003) and histotrophic transfer (Stewart and Brasch 2003). For example, the skink Mabuya heathi, has a highly modifi ed chorioallantois which is responsible for transport of organic nutrients (Blackburn et al. 1984;Blackburn and Vitt 2002). Southern grass skinks, Pseudemoia entrecasteauxii, transfer dietary β-carotenes ingested during gestation to developing embryos, which increase the innate immune response of neonates postpartum (Itonaga et al. 2011). ...
... Features that enhance maternalfetal gas exchange (such as increased vascularity and decreased diffusion distances) can likewise be considered. Structural analysis also can lead to new, testable functional hypotheses, such as the inference of sodium-coupled water transport across the yolk sac placenta of viviparous snakes (Blackburn et al., 2002). Relevant techniques include histology, scanning, and transmission electron microscopy, histochemistry, and confocal microscopy, in conjunction with physiological and biochemical methods that have become subsumed within contemporary morphology. ...
... Ultrastructural studies on amphibian adaptations for viviparity also proliferated during this period (Greven, 1977(Greven, , 1980a(Greven, , b, 1998Wake, 1980;Greven and R€ uterbories, 1984). As for research on reptiles, despite an early ultrastructural study (Hoffman, 1970), three decades elapsed before EM was again applied to squamate placentation (Blackburn et al., 2002;Blackburn and Vitt, 2002). However, several ultrastructural studies have since accumulated (for reviews see Blackburn and Stewart, 2011;Stewart and Blackburn, 2014). ...
... Ultrastructural studies on amphibian adaptations for viviparity also proliferated during this period (Greven, 1977(Greven, , 1980a(Greven, , b, 1998Wake, 1980;Greven and R€ uterbories, 1984). As for research on reptiles, despite an early ultrastructural study (Hoffman, 1970), three decades elapsed before EM was again applied to squamate placentation (Blackburn et al., 2002;Blackburn and Vitt, 2002). However, several ultrastructural studies have since accumulated (for reviews see Blackburn and Stewart, 2011;Stewart and Blackburn, 2014). ...
In many viviparous vertebrates, pregnant females sustain their developing embryos and provide them with nutrients by means of placentas and a diversity of other types of specializations. With this article, we introduce a virtual (online) issue of the Journal of Morphology that presents 12 recent papers on fetal maintenance in viviparous vertebrates. We also outline the history of research in this area and document the central role of morphology in helping to explain the function and evolution of specializations for fetal nutrition. This virtual issue of the Journal of Morphology is an outgrowth of a symposium held under auspices of the International Congress of Vertebrate Morphology. The included papers reflect a diversity of taxa, research methods, and biological issues. To celebrate the publication of this virtual issue of the Journal of Morphology, the publisher is making freely available to readers a number of other relevant papers published in the journal over the past 128 years. J. Morphol., 2015. © 2015 Wiley Periodicals, Inc.
© 2015 Wiley Periodicals, Inc.
... Unlike eutherian mammals, most squamate embryos exhibit both a chorioallantoic placenta on the embryonic pole (Fig. 1A and B) and some form of yolk-sac placenta on the abembryonic pole (Fig. 1C; Weekes 1935, Hoffman 1970, Guillette et al. 1981, Blackburn 1992, Stewart 1992, 1993). Both placentae are formed via close appositions of uterine and embryonic tissues, and are defined as epitheliochorial in most viviparous reptiles (Blackburn & Vitt 2002, Adams et al. 2005) because the embryonic tissues do not breach or invade uterine epithelia. In extant mammals, epitheliochorial placentation has likely evolved secondarily from moreinvasive hemochorial or endotheliochorial placentation (Mess & Carter 2007). ...
... rely on yolk for nutrition) and exhibit simple chorioallantoic (Fig. 1A) and yolk-sac placentae (Fig. 1C) with little morphological specialization (Weekes 1935, Bellairs et al. 1955, Hoffman 1970, Yaron 1977, Guillette et al. 1981, Stewart 1990, 1993). In contrast, viviparous skinks of the genera Niveoscincus, Pseudemoia , Chalcides, Eumecia, Mabuya, and Trachylepis exhibit complex chorioallantoic placentae and increasing reliance on placentotrophy (Weekes 1935, Blackburn et al. 1984, Ghiara et al. 1987, Blackburn 1992, Flemming & Branch 2001, Blackburn & Vitt 2002, Stewart & Thompson 2004, Adams et al. 2005, Blackburn & Flemming 2012). The chorioallantoic placenta is differentiated into a paraplacentome (Fig. 1A) and a placentome (Fig. 1B) in Pseudemoia, Chalcides, and Mabuya (Blackburn 1993b, Stewart & Thompson 1996, Blackburn & Vitt 2002), but not in Niveoscincus, Eumecia, or Trachylepis (Flemming & Branch 2001, Stewart & Thompson 2004, Blackburn & Flemming 2012). ...
... In contrast, viviparous skinks of the genera Niveoscincus, Pseudemoia , Chalcides, Eumecia, Mabuya, and Trachylepis exhibit complex chorioallantoic placentae and increasing reliance on placentotrophy (Weekes 1935, Blackburn et al. 1984, Ghiara et al. 1987, Blackburn 1992, Flemming & Branch 2001, Blackburn & Vitt 2002, Stewart & Thompson 2004, Adams et al. 2005, Blackburn & Flemming 2012). The chorioallantoic placenta is differentiated into a paraplacentome (Fig. 1A) and a placentome (Fig. 1B) in Pseudemoia, Chalcides, and Mabuya (Blackburn 1993b, Stewart & Thompson 1996, Blackburn & Vitt 2002), but not in Niveoscincus, Eumecia, or Trachylepis (Flemming & Branch 2001, Stewart & Thompson 2004, Blackburn & Flemming 2012). The placentome is a region in the dorsal center of the chorioallantoic placenta, which is defined by thickened, occasionally interdigitating uterine and chorioallantoic tissues directly beneath the uterine mesometrium (Weekes 1935, Blackburn et al. 1984, Blackburn 1993b). ...
Squamate reptiles (lizards and snakes) are an excellent model system for testing hypotheses regarding the evolution of viviparity (live birth) in amniote vertebrates. Viviparity has evolved over 100 times in squamates, resulting in major changes in reproductive physiology. At a minimum, all viviparous squamates exhibit placentae formed by appositions of maternal and embryonic tissues, which are homologous in origin with the tissues that form the placenta in therian mammals. These placentae facilitate adhesion of the conceptus to the uterus as well as exchange of oxygen, carbon dioxide, water, sodium, and calcium. However, most viviparous squamates continue to rely on yolk for nearly all of their organic nutrition. In contrast, some species, which rely on the placenta for at least a portion of organic nutrition, exhibit complex placental specializations associated with the transport of amino acids and fatty acids. Some viviparous squamates also exhibit reduced immunocompetence during pregnancy, which could be the result of immunosuppression to protect developing embryos. Recent molecular studies using both candidate-gene and next-generation sequencing approaches suggest that at least some of the genes and gene families underlying these phenomena play similar roles in the uterus and placenta of viviparous mammals and squamates. Therefore, studies of the evolution of viviparity in squamates should inform hypotheses of the evolution of viviparity in all amniotes, including mammals.
... Vrcibradic and Rocha 1998; Rocha et al. 2002) the mean SVL is given only for females at the minimum size at parturition or larger, whereas in others (e.g. Vitt and Blackburn 1983; Ramirez-Pinilla et al. 2002) it was given for all females containing ova/embryos in the oviducts (i.e. stages 3–6 of Rocha and Vrcibradic 1999), and in only one (Vitt and Blackburn 1991) it was given separately for both groups. ...
... The relationship between brood size and female SVL was positive and significant for M. macrorhyncha (r 2 = 0.30; F = 33.95; df = 1,80; p < 0.001, n = 82) and for M. dorsivittata (r 2 = 0.44; F = 14.70; df = 1,19; p = 0.001; n = 21), as has been reported for other Neotropical congeners (Vanzolini and Rebouças-Spieker 1976; Vitt and Blackburn 1983, 1991; Stevaux 1993; Vrcibradic and Rocha 1998; Ramírez-Pinilla et al. 2002; Rocha et al. 2002). This seems to be a general trend for the genus, even in small-brooded species. ...
... All Neotropical Mabuya studied so far are sexually dimorphic in size, with females growing larger than males (Vitt and Blackburn 1983, 1991; Stevaux 1993; Vrcibradic and Rocha 1998; Rocha and Vrcibradic 1999; Ramírez-Pinilla et al. 2002; Teixeira et al. 2003). This indicates that strong selective pressures for increased fecundity have favoured the evolution of large female size. ...
The viviparous New World Mabuya (Reptilia, Scincidae) are characterized by a peculiar suite of reproductive traits associated with an extreme degree of matrotrophy and a specialized placenta. We gathered information on female reproductive traits of South American Mabuya based on a compilation of literature data plus original information on two species (Mabuya dorsivittata and Mabuya macrorhyncha). Considering brood size, we identified one “large-brooded” group (brood sizes up to eight to ten) and one “small-brooded” group (brood sizes up to four to six). Brood size tended to be positively correlated with female body size among species, although the correlation is not as strong as it is within species. There were some general trends for the reproductive ecology of New World Mabuya, such as early sexual maturation, brood sizes of usually four to six young, and parturition occurring around the dry–wet season transition.
... las especies de Mabuya en américa tienen un patrón reproductor característico que incluye además de la viviparidad, un periodo de gestación prolongado (9-11 meses), ovocitos preovulatorios pequeños (aprox. 2 mm de diámetro máximo), formación de una placenta corioalantoica especializada asociada con una provisión de todos los nutrientes necesarios para el desarrollo, es decir, una placentotrofía sustancial (vitt y Blackburn, 1983;Blackburn et al., 1984;vitt y Blackburn, 1991;vrcibradic y duarte Rocha, 1998;Ramírez-pinilla et al., 2002). este grado de placentotrofia en las especies americanas de Mabuya excede todos los casos conocidos para escamados y converge en un patrón usualmente reconocido sólo en mamíferos eutherios (vitt y Blackburn, 1991;Blackburn y vitt, 1992Blackburn y vitt, , 2002. teniendo en cuenta la importancia de la nutrición placental al desarrollo fetal tanto en mamíferos eutherios como en las Mabuya americanas (Blackburn y vitt, 2002), varias de las particularidades morfológicas similares entre ambos grupos pueden reflejar atributos funcionales comunes. ...
... este grado de placentotrofia en las especies americanas de Mabuya excede todos los casos conocidos para escamados y converge en un patrón usualmente reconocido sólo en mamíferos eutherios (vitt y Blackburn, 1991;Blackburn y vitt, 1992Blackburn y vitt, , 2002. teniendo en cuenta la importancia de la nutrición placental al desarrollo fetal tanto en mamíferos eutherios como en las Mabuya americanas (Blackburn y vitt, 2002), varias de las particularidades morfológicas similares entre ambos grupos pueden reflejar atributos funcionales comunes. así, las especializaciones compartidas son vistas como una convergencia evolutiva sobre un patrón de placentotrofía extrema en los dos grupos. ...
... Figura 7b Blackburn y vitt, 2002). aunque los cuatro tipos pueden ser arreglados conceptualmente para reflejar una morfoclina en especialización placentaria, esto no significa que representen una serie evolutiva ya que cada una se ha especializado en diferentes direcciones (Blackburn y vitt, 2002). ...
... Extant lepidosaurs would be the closest living viviparous relatives of ichthyosaurs. In viviparous squamates (as in their oviparous counterparts), fetuses are tightly curled through lateral flexion, forming a compact rounded mass tightly enclosed within the fetal membranes [37,38]. In contrast, ichthyosaur fetuses are only curled up early in development (i.e., stage 1 Stenopterygius [15]); subsequently they are stretched out (elongated) in a cranial/ caudal direction [2,4,9,14]. ...
... As stated, the major difference between ichthyosaur fetuses and those of extant lepidosaurs lies in their conformation during development, whereby lepidosaurs are generally curled in utero and ichthyosaurs are not [37,38]. The ichthyosaur uncurled morphology has been attributed to either maternal uterine restrictions or more likely the stiffening of the vertebral column over ossification [4]. ...
According to a longstanding paradigm, aquatic amniotes, including the Mesozoic marine reptile group Ichthyopterygia, give birth tail-first because head-first birth leads to increased asphyxiation risk of the fetus in the aquatic environment. Here, we draw upon published and original evidence to test two hypotheses: (1) Ichthyosaurs inherited viviparity from a terrestrial ancestor. (2) Asphyxiation risk is the main reason aquatic amniotes give birth tail-first. From the fossil evidence, we conclude that head-first birth is more prevalent in Ichthyopterygia than previously recognized and that a preference for tail-first birth likely arose in derived forms. This weakens the support for the terrestrial ancestry of viviparity in Ichthyopterygia. Our survey of extant viviparous amniotes indicates that fetal orientation at birth reflects a broad diversity of factors unrelated to aquatic vs. terrestrial habitat, further undermining the asphyxiation hypothesis. We propose that birth preference is based on parturitional mechanics or carrying efficiency rather than habitat.
Supplementary Information
The online version contains supplementary material available at 10.1186/s12862-023-02110-4.
... [6,[30][31][32][33]). Specializations for placentation in squamates have also been the subject of focused research (e.g. [18,[34][35][36]). Additionally, there is a growing body of work on teleost placentation (e.g. ...
... [161]. These lizards have unusually invasive placentae for a squamate (see §1b), with syncytial structures in the uterine epithelium [36,167]. One Brasiliscincus syncytin (syncytin-Mab1) is highly expressed in placental tissues (as well as other tissues) and is fusogenic [161]. ...
The vertebrate placenta, a close association of fetal and parental tissue for physiological exchange, has evolved independently in sharks, teleost fishes, coelacanths, amphibians, squamate reptiles and mammals. This transient organ forms during pregnancy and is an important contributor to embryonic development in both viviparous and oviparous, brooding species. Placentae may be involved in transport of respiratory gases, wastes, immune molecules, hormones and nutrients. Depending on the taxon, the embryonic portion of the placenta is comprised of either extraembryonic membranes (yolk sac or chorioallantois) or temporary embryonic tissues derived via hypertrophy of pericardium, gill epithelium, gut, tails or fins. These membranes and tissues have been recruited convergently into placentae in several lineages. Here, we highlight the diversity and common features of embryonic tissues involved in vertebrate placentation and suggest future studies that will provide new knowledge about the evolution of pregnancy.
This article is part of the theme issue ‘Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom’.
... The skinks of the Mabuya genus have a Neotropical distribution, including South America, Central America and the Antilles (Miralles et al., 2009). The species of this genus conform one of the few lineages of amniotes (excluding Mammalia) that have evolved a degree of placentotrophy in which the placenta provides most of the nutrients for embryonic development (Blackburn and Vitt, 2002;Ramírez-Pinilla, 2006;Ramírez-Pinilla et al., 2011). Females ovulate microlecithal eggs (Blackburn and Vitt, 2002;Gómez and Ramírez-Pinilla, 2004;Hernández-Franyutti et al., 2005;Vieira et al., 2010) and have an extended gestation period of 8-10 months (Ramírez-Pinilla et al., 2002;Vrcibradic and Rocha, 2011). ...
... The species of this genus conform one of the few lineages of amniotes (excluding Mammalia) that have evolved a degree of placentotrophy in which the placenta provides most of the nutrients for embryonic development (Blackburn and Vitt, 2002;Ramírez-Pinilla, 2006;Ramírez-Pinilla et al., 2011). Females ovulate microlecithal eggs (Blackburn and Vitt, 2002;Gómez and Ramírez-Pinilla, 2004;Hernández-Franyutti et al., 2005;Vieira et al., 2010) and have an extended gestation period of 8-10 months (Ramírez-Pinilla et al., 2002;Vrcibradic and Rocha, 2011). For a Colombian population it has been observed that the CLs degenerate very early during gestation (evident luteolysis when the embryos are in gastrulation stages, Gómez and Ramírez-Pinilla, 2004). ...
In squamates, progesterone (P) plays a key role in the inhibition of uterine mobility during egg retention in oviparous species, and during gestation in viviparous species. The corpus luteum (CL) is the main organ responsible for the production of P; however, in some species, the CL degenerates early and the P needed for gestation maintenance should be produced in other tissues. Mabuya sp (Scincidae) is a viviparous lizard with a prolonged gestation, it produces microlecithal eggs and, consequently, has an obligate placentotrophy related with a highly complex placenta. Its CL degenerates at early stages of gestation and therefore, other sources of P should exist. The aim of this study was to determine and localize by immunohistochemistry the production of P by detection of the enzyme 3β-Hydroxysteroid dehydrogenase (3β-HSD) and P receptors (PR) during gestation in the ovary and placenta of Mabuya sp. Positive and negative control sections were used. The ovary of this species localizes 3β-HSD and PR in the same tissues. The CL of the ovaries of females at early stages of gestation were positive for both molecules, whereas they did not localize from mid gestation to the end of pregnancy. Previtellogenic and vitellogenic follicles labelled for both molecules in the follicular epithelium and thecae. The placenta of Mabuya sp. demonstrated the potential for P production from mid gestation to the end of gestation in the uterine and chorionic tissues. PR were located in the uterine tissues throughout gestation, with a decrease towards its completion. Western blot analysis confirmed the presence of 3β-HSD mainly in the ovary of early pregnant females and in the placental tissues at mid gestation stages. Therefore, the chorioallantoic placenta of Mabuya sp. has an endocrine function producing the P needed for gestation and replacing the CL from mid gestation to the end of pregnancy.
... Blackburn (1993Blackburn ( , 1998 established four placental morphotypes based on classification proposed by Weekes (1935) that range from a simple chorioallantoic placenta (type I) to a highly specialized one (type IV). Placentation was studied in various scincid lizards (Jerez and Ramírez-Pinilla, 2001;Blackburn and Vitt, 2002;Stewart and Thompson, 2009;Blackburn and Flemming, 2012). In this paper, we present the first study on placentation in a Middle Eastern species, the scincid lizard Trachylepis vittata. ...
... The most highly placentotrophic species, Niveoscincus ocellatus, has a complex placental morphology and two species Niveoscincus microlepidotus, Niveoscincus microlepidotus were lecithotrophic (Stewart and Thompson, 2009). South American species of the former genus Mabuya were found to be highly placentotrophic with type IV chorioallantoic placenta (Jerez and Ramírez-Pinilla, 2001;Blackburn and Vitt, 2002;Ramírez-Pinilla et al., 2006). ...
Placentation was investigated in the viviparous lizard Trachylepis vittata in two populations from distinct locations in Lebanon: an isolated coastal population from Tripoli Palm Island — and a continental high altitude population from Mount Sannine. No differences in the formation of extraembryonic membranes were found between the two locations. Histological examination showed a type I chorioallantoic placenta and an omphaloplacenta. Viviparity does not limit the altitudinal distribution of T. vittata and is not accompanied by a highly specialized placental structure.
... This region is characterized by uterine folds lined by enlarged epithelial cells, apposed to an enlarged chorionic epithelium; details vary interspecifically. Placentomes are thought to function in maternal-fetal nutrient transfer in placentotrophic skinks (Blackburn, 1993b(Blackburn, , 1998aStewart and Thompson, 2000;Blackburn and Vitt, 2002). In our study, the mesometrial folds in uteri containing advanced abortive embryos appear to be the maternal component of a placentome. ...
... Taken together, these points make a strong case that the uterine granules are secretory in character. Interestingly, a carbohydrate moiety has also been documented in uterine secretions in the placentotrophic Brazilian skink, Mabuya heathi (Blackburn and Vitt, 2002). ...
Although pregnant viviparous squamates are sometimes claimed to be able to resorb inviable eggs and embryos from the uterus, definitive evidence for such resorption is not available. After placing pregnant female Pseudemoia pagenstecheri into conditions under which embryonic development is terminated, we periodically harvested the gravid oviducts and examined them histo-logically. Females contained abnormal and degenerating eggs and embryos that had died in various stages of development. Dead embryos had undergone extensive cytol-ysis, dissolution, and aseptic necrosis and vitelline masses showed signs of deterioration and passage down the ovi-duct. The uterine mucosa lay in direct contact with the vitelline material, with no intact shell membrane intervening between them. Yolk was sometimes displaced into the exocoelom and allantoic cavity due to rupture of the extraembryonic membranes. Histological examination revealed no evidence of the uptake of yolk by the uterine epithelium or its accumulation in the subepithelial connective tissue. In many specimens, the uterine epithelium showed minuscule, apical granules. The position, appearance , and staining properties of the granules suggests them to be secretory, a manifestation of placentotrophy. Our observations indicate that P. pagenstecheri females retain dead eggs and embryos for several weeks or longer, yet do not resorb them during that period. This lizard is the second placentotrophic skink species in which resorp-tion has been suspected, but in which abortive eggs appear to be retained or extruded instead of being resorbed by the oviducts. Researchers should not assume that squamates can digest and resorb oviductal eggs without definitive morphological evidence.
... Notably, placentotrophy has been documented in various scincid lizards (Blackburn and Vitt 1992;Stewart and Thompson 1993;Thompson et al. 1999;Flemming and Branch, 2001;Flemming and Blackburn 2003;Ramírez-Pinilla 2006), where it is accomplished through complex specializations for nutrient transfer (Blackburn 1993a;Flemming and Branch 2001;Jerez and Ramírez-Pinilla 2001;Blackburn and Vitt 2002;Thompson 2004, 2009a). Even in these placentotrophic species, however, placentae form through apposition of fetal membranes to the oviduct lining. ...
... placentotrophy is rare among squamates, having been found in only four to six unrelated lineages of scincid lizards on five continents. Placentation has been studied in detail in these lizards, including species from Europe (Blackburn 1993a), South America Ramírez-Pinilla 2001, 2003;Blackburn and Vitt 2002;Ramirez-Pinilla et al. 2006;Vieira et al. 2007), Africa (Flemming and Branch 2001) and Australia (Stewart and Thompson 2003, 2009aAdams et al. 2005). Their placentae show impressive specializations for nutrient transfer, including diverse adaptations for uterine secretion and fetal absorption. ...
Recent research on the African scincid lizard, Trachylepis ivensi, has significantly expanded the range of known reproductive specializations in reptiles. This species is viviparous and exhibits characteristics previously thought to be confined to therian mammals. In most viviparous squamates, females ovulate large yolk-rich eggs that provide most of the nutrients for development. Typically, their placental components (fetal membranes and uterus) are relatively unspecialized, and similar to their oviparous counterparts. In T. ivensi, females ovulate tiny eggs and provide nutrients for embryonic development almost entirely by placental means. Early in gestation, embryonic tissues invade deeply into maternal tissues and establish an intimate "endotheliochorial" relationship with the maternal blood supply by means of a yolk sac placenta. The presence of such an invasive form of implantation in a squamate reptile is unprecedented and has significant functional and evolutionary implications. Discovery of the specializations of T. ivensi illustrates why the study of a few convenient "animal models" is no substitute for broad-based studies of biological diversity as directed by phylogenetic considerations. Our study also underscores the value of museum collections to studies of biological diversity.
... they are lecithotrophic viviparous lizards that present certain similarities to therian mammals in terms of placental structure and function (Blackburn & Vitt, 1992). Studies of the reproductive biology of a few Brazilian species of Mabuya have been conducted in the last four decades, and information is available for Mabuya agilis (rocha & Vrcibradic, 1999;Rocha et al., 2002), Mabuya caissara (vanzolini & rebouças-Spieker, 1976), Mabuya dorsivittata (vrcibradic, 2001), Mabuya frenata (vrcibradic & rocha, 1998), Mabuya heathi (Vitt & Blackburn, 1983;Blackburn & vitt, 1992Blackburn & vitt, , 2002, Mabuya macrorhyncha (rocha et al., 2002), and Mabuya nigropunctata (vitt & Blackburn, 1991;Blackburn & Vitt, 1992). Mabuya arajara is placed in a large species group with a normal snout, paired frontoparientals, and no vertebral stripes on the body (rodrigues, 2000). ...
... During observations of the birth of one of these at 17:21 the female did not eat the embryonic membranes to help the newborn to be freed (Fig. 2). A brood size of four newborns documented by M. arajara seems to be very common among the South American viviparous species of Mabuya which can vary from 1-9 (Vanzolini & Rebouças-Spieker, 1976;Vitt & Blackburn, 1983;Vitt & Blackburn, 1991;Blackburn & Vitt, 1992;Vrcibradic & Rocha, 1998;Rocha & Vrcibradic, 1999;Vrcibradic, 2001;Rocha et al., 2002;Blackburn & Vitt, 2002). Mabuya nigropunctata, a closely related species to M. arajara, was documented to have a brood size of 2-9 embryos, with a gestation period of 10 to 12 months. ...
Mabuya arajara Rebouças-Spieker, 1981 has been considered an endemic species from the southern of state of Ceará, restricted to the Deciduous Dry Forests in the slopes of Plateau of Araripe (Chapada do Araripe). Here, we present an updated distributional map for the species and demonstrate that its range is not restricted as formerly believed. In addition, we had an opportunity to observe a gravid female and we describe aspects regarding parturition and number of offspring for the species.
... These placentas are formed through apposition of the chorioallantois and a specialized derivative of the yolk sac (Stewart and Blackburn, 1988) to the uterine lining. Placental tissues have been analyzed in several species via electron microscopy (Hoffman, 1970;Blackburn and Vitt, 2002;Blackburn and Lorenz, 2003a, b;Stewart and Brasch, 2003;Adams et al., 2005;Ram ırez-Pinilla et al., 2006;Anderson et al., 2011) and in many others, by light microscopy (Blackburn, 1993c;Stewart, 1993;Blackburn and Stewart, 2011;Stewart and Blackburn, 2014). In highly placentotrophic squamates, the placental membranes are responsible for maternal-fetal transfer of significant quantities of nutrients. ...
... Placental membranes in matrotrophic lizards commonly are highly specialized. Morphological studies have revealed elaborate specializations for nutrient transfer in the aforementioned Trachylepis Flemming, 2010, 2012), Eumecia (Flemming and Branch, 2001), Chalcides (Ghiara et al., 1987;Blackburn, 1993d, Blackburn andCallard, 1997), and Mabuya Ram ırez-Pinilla, 2001, 2003;Blackburn and Vitt, 2002;Ram ırez-Pinilla et al., 2006;Vieira et al., 2007;Leal and Ram ırez-Pinilla, 2008). Modest specializations are evident in Niveoscincus and Pseudemoia Thompson, 2003, 2009b;Adams et al., 2005). ...
Phylogenetic analyses indicate that viviparity (live-bearing reproduction) has originated independently in more than 150 vertebrate lineages, including a minimum of 115 clades of extant squamate reptiles. Other evolutionary origins of viviparity include 13 origins among bony fishes, nine among chondrichthyans, eight in amphibians, one in Paleozoic placoderms, six among extinct reptiles, and one in mammals. The origins of viviparity range geologically from the mid-Paleozoic through the Mesozoic to the Pleistocene. Substantial matrotrophy (maternal provision of nutrients to embryos during pregnancy) has arisen at least 33 times in these viviparous clades, with most (26) of these origins having occurred among fishes and amphibians. Convergent evolution in patterns of matrotrophy is widespread, as reflected by multiple independent origins of placentotrophy, histotrophy, oophagy, and embryophagy. Specializations for nutrient transfer to embryos are discontinuously distributed, reflecting the roles of phylogenetic inertia, exaptation (preadaptation), and constraint. Ancestral features that function in gas exchange and nutrition repeatedly and convergently have been co-opted for nutrient transfer, often through minor modification of their components and changes in the timing of their expression (heterochrony). Studies on functional and evolutionary morphology continue to play a central role in our attempts to understand viviparity and mechanisms of fetal nutrition. J. Morphol., 2014. © 2014 Wiley Periodicals, Inc.
... Sixteen species of skinks of the genus Mabuya have been described in the Americas, which occur from Mexico throughout South America (except Chile), and the West Indies (Avila-Pires, 1995;Mausfeld et al., 2002;Peters and Donoso-Barros, 1970). Mabuya species are viviparous and ovulate microlecithal eggs (1-2 mm of diameter, Blackburn and Vitt, 2002). These eggs contain only primordial yolk vacuoles and no fatty yolk platelets (Gó mez and Ramírez-Pinilla, 2004). ...
... The structure of Mabuya placentae is relatively well known, but the function of the different placental specializations has merely been inferred from the morphological complexity, suggesting that the species of this lineage have the greatest degree of placentotrophy ever seen in any reptile, and resembles that of eutherian mammals (Blackburn and Vitt, 1992). The nutritional pattern (lecitotrophy/matrotrophy) refers to the source of the nutrients invested in the embryo during its development. ...
Skinks of Mabuya genus exhibit the most specialized allantoplacenta among squamates (type IV), and the greatest degree of placentotrophy known in Reptilia. They ovulate microlecithal eggs (1-2 mm) that lack fatty yolk platelets; thus, it is suggested that virtually all of the nutrients for embryonic development are obtained by placental means. To test this inference, the net uptake of nutrients during gestation in an Andean population of Mabuya was quantified and compared with other oviparous and viviparous lizards, matrotrophic skinks, and eutherian mammals. Ionic, protein and lipid contents of recently ovulated eggs and neonates were measured. A significant net uptake of water, ions (calcium, potassium, sodium, magnesium), lipids, nitrogen (an index of protein), and dry matter was observed during development. Thus, in Mabuya the drastic reduction of egg size is related to the great reduction in the contribution of lecitotrophic nutrients to the embryo, an obligatory placentotrophy from early developmental stages and the highest placental complexity known in Reptilia. All of these features converge with similar features found in eutherian mammals. Both clades evolved similar reproductive patterns and morphological features in their complex chorioallantoic placentae, which supplies all the nutrients for embryonic and fetal development.
... Sixteen species of skinks of the genus Mabuya have been described in the Americas, which occur from Mexico throughout South America (except Chile), and the West Indies (Avila-Pires, 1995;Mausfeld et al., 2002;Peters and Donoso-Barros, 1970). Mabuya species are viviparous and ovulate microlecithal eggs (1-2 mm of diameter, Blackburn and Vitt, 2002). These eggs contain only primordial yolk vacuoles and no fatty yolk platelets (Gó mez and Ramírez-Pinilla, 2004). ...
... The structure of Mabuya placentae is relatively well known, but the function of the different placental specializations has merely been inferred from the morphological complexity, suggesting that the species of this lineage have the greatest degree of placentotrophy ever seen in any reptile, and resembles that of eutherian mammals (Blackburn and Vitt, 1992). The nutritional pattern (lecitotrophy/matrotrophy) refers to the source of the nutrients invested in the embryo during its development. ...
... Further, squamates, like eutherian mammals, exhibit mechanisms to increase gestation length by changing the timing of parturition, and develop placentas from the chorioallantois and yolk sac, structures that function in gas exchange and water provision (Blackburn, 1993a;Stewart, 1993). Yet another feature found among squamates and mammals is an extreme form of placentotrophy, in which placentas supply most of the nutrients for development (Blackburn and Vitt, 1992;Flemming and Blackburn, 2003;Thompson et al., 1999). In fact, some squamate placentas have cellular specializations that are very similar to those of eutherians mammals (Blackburn and Vitt, 2002;Flemming and Branch, 2001;Jerez andRamirez-Pinilla, 2001, 2003). ...
... Yet another feature found among squamates and mammals is an extreme form of placentotrophy, in which placentas supply most of the nutrients for development (Blackburn and Vitt, 1992;Flemming and Blackburn, 2003;Thompson et al., 1999). In fact, some squamate placentas have cellular specializations that are very similar to those of eutherians mammals (Blackburn and Vitt, 2002;Flemming and Branch, 2001;Jerez andRamirez-Pinilla, 2001, 2003). ...
For over a century, research has been conducted on squamates in order to reveal how viviparity has evolved in mammals and other vertebrates. The recent proliferation of studies has yielded much information on anatomical, physiological, ecological, and evolutionary aspects, allowing a reassessment of squamates as model organisms for the study of viviparity. Strong support for the ‘‘squamate model’’ comes from phylogenetic analyses that have shown that squamates have evolved viviparity with great frequency (> 108 origins), at low taxonomic levels, and in geologically recent times. However, available data also indicate that viviparity has evolved by different chronologies and mechanisms in squamates, fishes, and mammals. Further, generalizations about squamates are difficult to make, given the diverse mechanisms by which they achieve viviparity. Thus, similarities between squamates must be demonstrated empirically, and generalizations should be based on quantitative, phylogenetic analyses of multiple lineages. Explanations for similarities between squamate clades can invoke such concepts as evolutionary constraints, exaptations, and selection pressures, and should distinguish between adaptations, correlated attributes, and features that predate viviparity. However, homocentric assumptions of an orthogenetic transformation towards the eutherian condition should be abandoned, along with untested assumptions that viviparity in squamates and mammals is similar. The value of the squamate model ultimately may lie in insights it provides into physiological problems rather than in universality of specific mechanisms that have evolved to meet those problems.
... In squamates, specializations for gas exchange across the chorioallantoic placenta include 979 decreased diffusion distance between maternal and fetal capillaries, uterine vascularity, shell 980 membrane deterioration, and modifications of both fetal and maternal blood properties (Attaway, 981 2000;Blackburn, 1998Blackburn, , 2005Blackburn & Lorenz, 2003;Blackburn & Vitt, 2002;Stewart and 982 Brasch, 2003). Absence of the eggshell may be necessary for adequate gas exchange during 983 viviparous gestation. ...
Across amniotes, squamates represent the only clade with highly variable parity modes, oviparity (egg-laying) and viviparity (live-birth). Despite this, relatively little is known about how oviparity and viviparity evolve at the genomic and physiological levels in squamates. Within the context of interdisciplinary medical, poultry science, and reproductive biology literature, I review the genomics and physiology of reproduction across five broad processes expected to change during transitions between parity modes—eggshell formation, embryonic retention, placentation, calcium transport, and maternal-fetal immune dynamics. This review is the first time that the maternal-fetal immune dynamics of squamates is considered in the context of modern medical literature, where embryos are no longer conceptualized as analogs to allografts. I offer alternative perspectives and holistic hypotheses on the genomic and transcriptomic drivers of parity mode transitions in squamates. Two new pathways through which early Lepidosaurs may have transitioned rapidly between oviparity and viviparity with no intermediate stages are presented. Overall, the physiology of reproduction illuminates the biological plausibility of highly labile parity modes in some squamate lineages, with constrained parity modes in others. Future research should be open to either possibility unless clade-specific biological evidence suggests otherwise. Rather than emphasizing the feasibility of transitions in either direction, I posit that oviparity and viviparity are relatively minor variations of a shared process.
... Beyond maternal physiology, extraembryonic tissues facilitate maternal-embryonic gas exchange in viviparous species. The chorioallantoic placenta in viviparous lizards exists along a morphological spectrum from simple to complex, with some viviparous lizards even possessing a chorioallantoic placenta similar to the true placenta of eutherian mammals which could increase maternal-embryonic gas and nutrient exchange (Blackburn & Vitt, 2002;Blackburn, 1992Blackburn, , 2000bBlackburn, , 2015Stewart, 2015 (Guillette & Jones, 1985). Similarly, uterine and placental angiogenesis has been recorded in viviparous Australian skinks (Murphy et al., 2010b;Parker et al., 2010). ...
Research focused on understanding the evolutionary factors that shape parity mode evolution among vertebrates have long focused on squamate reptiles (snakes and lizards), which contain all but one of the evolutionary transitions from oviparity to viviparity among extant amniotes. While most hypotheses have focused on the role of cool temperatures in favoring viviparity in thermoregulating snakes and lizards, there is a growing appreciation in the biogeographic literature for the importance of lower oxygen concentrations at high elevations for the evolution of parity mode. However, the physiological mechanisms underlying how hypoxia might reduce fitness, and how viviparity can alleviate this fitness decrement, has not been systematically evaluated. We qualitatively evaluated previous research on reproductive and developmental physiology, and found that (1) hypoxia can negatively affect fitness of squamate embryos, (2) oxygen availability in the circulatory system of adult lizards can be similar or greater than an egg, and (3) gravid females can possess adaptive phenotypic plasticity in response to hypoxia. These findings suggest that the impact of hypoxia on the development and physiology of oviparous and viviparous squamates would be a fruitful area of research for understanding the evolution of viviparity. To that end, we propose an integrative research program for studying hypoxia and the evolution of viviparity in squamates.
... At the other extreme lie certain highly-placentotrophic scincid lizards of Africa and South America, lizards that ovulate tiny 1 mm eggs and provide most nutrients by placental means (Blackburn et al., 1984;Flemming & Blackburn, 2003;Ramírez-Pinilla et al., 2011). Morphological studies have shown their placentas to be highly specialized for nutrient transfer (Blackburn & Flemming, 2009;Blackburn & Vitt, 2002;Flemming & Branch, 2001;Jerez & Ramírez-Pinilla, 2001;Ramírez-Pinilla et al., 2011). In fact, in some lizards, the fetal membranes show an invasive implantation into maternal tissues (Blackburn & Flemming, 2012;Vieira et al., 2007), a pattern strongly convergent on that of mammals. ...
We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates – the amniote egg presenting one “solution” that enabled the conquest of land for reproduction. This article is protected by copyright. All rights reserved.
... Among carnivores and bats with endotheliochorial placenta, the maternal uterine epithelium is either cellular or syncytial, being further displaced by fetal syncytiotrophoblasts (reviewed in [4]). A syncytial placenta of solely maternal origin is known in skinks of the genus Mabuya [160,161]. Importantly, all aforementioned syncytial placentae are initially cellular, becoming syncytial via cell fusion. In contrast, the nutritive tissue of cyclostome bryozoans mostly originates via multiplication of nuclei and cytoplasmic growth, although coenocytic fusion is potentially possible. ...
Background
Placentation has evolved multiple times among both chordates and invertebrates. Although they are structurally less complex, invertebrate placentae are much more diverse in their origin, development and position. Aquatic colonial suspension-feeders from the phylum Bryozoa acquired placental analogues multiple times, representing an outstanding example of their structural diversity and evolution. Among them, the clade Cyclostomata is the only one in which placentation is associated with viviparity and polyembryony—a unique combination not present in any other invertebrate group.
Results
The histological and ultrastructural study of the sexual polymorphic zooids (gonozooids) in two cyclostome species, Crisia eburnea and Crisiella producta, revealed embryos embedded in a placental analogue (nutritive tissue) with a unique structure—comprising coenocytes and solitary cells—previously unknown in animals. Coenocytes originate via nuclear multiplication and cytoplasmic growth among the cells surrounding the early embryo. This process also affects cells of the membranous sac, which initially serves as a hydrostatic system but later becomes main part of the placenta. The nutritive tissue is both highly dynamic, permanently rearranging its structure, and highly integrated with its coenocytic ‘elements’ being interconnected via cytoplasmic bridges and various cell contacts. This tissue shows evidence of both nutrient synthesis and transport (bidirectional transcytosis), supporting the enclosed multiple progeny. Growing primary embryo produces secondary embryos (via fission) that develop into larvae; both the secondary embyos and larvae show signs of endocytosis. Interzooidal communication pores are occupied by 1‒2 specialized pore-cells probably involved in the transport of nutrients between zooids.
Conclusions
Cyclostome nutritive tissue is currently the only known example of a coenocytic placental analogue, although syncytial ‘elements’ could potentially be formed in them too. Structurally and functionally (but not developmentally) the nutritive tissue can be compared with the syncytial placental analogues of certain invertebrates and chordates. Evolution of the cyclostome placenta, involving transformation of the hydrostatic apparatus (membranous sac) and change of its function to embryonic nourishment, is an example of exaptation that is rather widespread among matrotrophic bryozoans. We speculate that the acquisition of a highly advanced placenta providing massive nourishment might support the evolution of polyembryony in cyclostomes. In turn, massive and continuous embryonic production led to the evolution of enlarged incubating polymorphic gonozooids hosting multiple progeny.
... Among carnivores and bats with endotheliochorial placenta, the maternal uterine epithelium is either cellular or syncytial, being further displaced by fetal syncytiotrophoblasts (reviewed in [4]). A syncytial placenta of solely maternal origin is known in skinks of the genus Mabuya [160,161]. Importantly, all aforementioned syncytial placentae are initially cellular, becoming syncytial via cell fusion. In contrast, the nutritive tissue of cyclostome bryozoans mostly originates via multiplication of nuclei and cytoplasmic growth, although coenocytic fusion is potentially possible. ...
Background
Placentation has evolved multiple times among both chordates and invertebrates. Although they are structurally less complex, invertebrate placentae are much more diverse in their origin, development and position. Aquatic colonial suspension-feeders from the phylum Bryozoa acquired placental analogues multiple times, representing an outstanding example of their structural diversity and evolution. Among them, the clade Cyclostomata is the only one in which placentation is associated with viviparity and polyembryony—a unique combination not present in any other invertebrate group.
Results
The histological and ultrastructural study of the sexual polymorphic zooids (gonozooids) in two cyclostome species, Crisia eburnea and Crisiella producta , revealed embryos embedded in a placental analogue (nutritive tissue) with a unique structure—comprising coenocytes and solitary cells—previously unknown in animals. Coenocytes originate via nuclear multiplication and cytoplasmic growth among the cells surrounding the early embryo. This process also affects cells of the membranous sac, which initially serves as a hydrostatic system but later becomes main part of the placenta. The nutritive tissue is both highly dynamic, permanently rearranging its structure, and highly integrated with its coenocytic ‘elements’ being interconnected via cytoplasmic bridges and various cell contacts. This tissue shows evidence of both nutrient synthesis and transport (bidirectional transcytosis), supporting the enclosed multiple progeny. Growing primary embryo produces secondary embryos (via fission) that develop into larvae; both the secondary embyos and larvae show signs of endocytosis. Interzooidal communication pores are occupied by 1‒2 specialized pore-cells probably involved in the transport of nutrients between zooids.
Conclusions
Cyclostome nutritive tissue is currently the only known example of a coenocytic placental analogue, although syncytial ‘elements’ could potentially be formed in them too. Structurally and functionally (but not developmentally) the nutritive tissue can be compared with the syncytial placental analogues of certain invertebrates and chordates. Evolution of the cyclostome placenta, involving transformation of the hydrostatic apparatus (membranous sac) and change of its function to embryonic nourishment, is an example of exaptation that is rather widespread among matrotrophic bryozoans. We speculate that the acquisition of a highly advanced placenta providing massive nourishment might support the evolution of polyembryony in cyclostomes. In turn, massive and continuous embryonic production led to the evolution of enlarged incubating polymorphic gonozooids hosting multiple progeny.
... At the other extreme lie certain highly-placentotrophic scincid lizards of Africa and South America, lizards that ovulate tiny 1 mm eggs and provide most nutrients by placental means (Blackburn et al., 1984;Flemming & Blackburn, 2003;Ramírez-Pinilla et al., 2011). Morphological studies have shown their placentas to be highly specialized for nutrient transfer (Blackburn & Flemming, 2009;Blackburn & Vitt, 2002;Flemming & Branch, 2001;Jerez & Ramírez-Pinilla, 2001;Ramírez-Pinilla et al., 2011). In fact, in some lizards, the fetal membranes show an invasive implantation into maternal tissues (Blackburn & Flemming, 2012;Vieira et al., 2007), a pattern strongly convergent on that of mammals. ...
Evolution of the terrestrial egg of amniotes (reptiles, birds, and mammals) is often considered to be one of the most significant events in vertebrate history. Presence of an eggshell, fetal membranes, and a sizeable yolk allowed this egg to develop on land and hatch out well‐developed, terrestrial offspring. For centuries, morphologically‐based studies have provided valuable information about the eggs of amniotes and the embryos that develop from them. This review explores the history of such investigations, as a contribution to this special issue of Journal of Morphology, titled Developmental Morphology and Evolution of Amniote Eggs and Embryos. Anatomically‐based investigations are surveyed from the ancient Greeks through the Scientific Revolution, followed by the 19th and early 20th centuries, with a focus on major findings of historical figures who have contributed significantly to our knowledge. Recent research on various aspects of amniote eggs is summarized, including gastrulation, egg shape and eggshell morphology, eggs of Mesozoic dinosaurs, sauropsid yolk sacs, squamate placentation, embryogenesis, and the phylotypic phase of embryonic development. As documented in this review, studies on amniote eggs and embryos have relied heavily on morphological approaches in order to answer functional and evolutionary questions.
... Most tropical lizards lay eggs, but members of the genus Copeoglossum have live young and the placenta described from another species of South American skink is the most complex for any reptile, with similarities to the placenta of mammals 108 . The skinks I saw at INPA grew to be as wide as my thumb, but they only had blue tails when they were as small as the Tretioscincus agilis. ...
This book is about my life-long fascination with snakes and other lizards. It can be obtained from Openscienceonline <http://www.openscienceonline.com/book/newreleases/978-1-946898-49-4>.
... Within this family, viviparous skinks of the genus Mabuya exhibit extreme placentotrophy and the greatest complexity in morphology and physiology of their allantoplacenta (derived from chorionic and allantoic extraembryonic membranes) (Blackburn and Vitt, 2002;Ramírez-Pinilla, 2001, 2003;Blackburn and Flemming, 2009). Additionally, Mabuya species have a highly conservative morphology (Miralles and Carranza, 2010), early sexual maturity, long gestation period (9-10 months), and produce microlecithal eggs (1-2 mm diameter) (Blackburn et al., '84;Blackburn and Vitt, '92;Jerez and Ramírez-Pinilla, 2001). ...
Reptiles are one of the most diverse groups of vertebrates, providing an integrated system for comparative studies on metabolic, animal physiology, and developmental biology. However, the molecular data available are limited and only recently have started to call attention in the “omics” sciences. Mabuya sp. is a viviparous placentrotrophic skink with particular reproductive features, including microlecithal eggs, early luteolysis, prolonged gestation, and development of a highly specialized placenta. This placenta is responsible for respiratory exchange and the transference of all nutrients necessary for embryonic development. Our aim was to identify differentially expressed proteins in the ovary and placenta of Mabuya sp. during early, mid, and late gestation; their possible metabolic pathways; and biological processes. We carried out a comparative proteomic analysis during gestation in both tissues by sodium dodecyl sulfate polyacrylamide gel electrophoresis, two-dimensional gel electrophoresis, and matrix-assisted laser desorption/ionization. Differential protein expression in both tissues (Student's t-test P < 0.05) was related to several processes such as cell structure, cell movement, and energy. Proteins found in ovary are mainly associated with follicular development and its regulation. In the placenta, particularly during mid and late gestation, protein expression is involved in nutrient metabolism, transport, protein synthesis, and embryonic development. This work provides new insights about the proteins expressed and their physiological mechanisms in Mabuya sp. placenta and ovary during gestation.
... Based on Pincheira-Donoso et al. (2013) there are 9193 squamate species; about 1800 species may thus be matrotrophic. Morphological and experimental evidence on placentation has been recorded for species in 13 squamate families (Weekes, 1935;Bauchot, 1965;Blackburn, Vitt & Beuchat, 1984;Blackburn, 1985Blackburn, , 1993Blackburn, , 1994bBlackburn, , 1998Blackburn, , 1999bBlackburn, , 2005Blackburn, , 2014Blackburn et al., 1985;Blüm, 1986;Stewart & Blackburn, 1988;Stewart, 1992Stewart, , 1993Stewart, , 2013Lombardi, 1998;Stewart & Thompson, 1998Thompson, Stewart & Speake, 2000;Blackburn & Vitt, 2002;Jerez & Ramírez-Pinilla, 2003;Villagrán et al., 2005;Ramírez-Pinilla, 2006;Thompson & Speake, 2006;Vieira, de Perez & Ramírez-Pinilla, 2007;Leal & Ramírez-Pinilla, 2008;Blackburn & Flemming, 2009;Stewart & Ecay, 2010, and references therein). Among these, substantial placentotrophy evolved in all six subclades of a single lizard family, Scincidae (Blackburn, 2014). ...
... Based on Pincheira-Donoso et al. (2013) there are 9193 squamate species; about 1800 species may thus be matrotrophic. Morphological and experimental evidence on placentation has been recorded for species in 13 squamate families (Weekes, 1935;Bauchot, 1965;Blackburn, Vitt & Beuchat, 1984;Blackburn, 1985Blackburn, , 1993Blackburn, , 1994bBlackburn, , 1998Blackburn, , 1999bBlackburn, , 2005Blackburn, , 2014Blackburn et al., 1985;Blüm, 1986;Stewart & Blackburn, 1988;Stewart, 1992Stewart, , 1993Stewart, , 2013Lombardi, 1998;Stewart & Thompson, 1998Thompson, Stewart & Speake, 2000;Blackburn & Vitt, 2002;Jerez & Ramírez-Pinilla, 2003;Villagrán et al., 2005;Ramírez-Pinilla, 2006;Thompson & Speake, 2006;Vieira, de Perez & Ramírez-Pinilla, 2007;Leal & Ramírez-Pinilla, 2008;Blackburn & Flemming, 2009;Stewart & Ecay, 2010, and references therein). Among these, substantial placentotrophy evolved in all six subclades of a single lizard family, Scincidae (Blackburn, 2014). ...
Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy—placentotrophy—is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly
matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritivemodes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are
indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent–offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and ‘isolated’ chambers with presumably more effective nutritional relationships, and (iii) internal secretory (‘milk’) glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels—positional, structural and physiological.
... Skinks with extreme matrotrophy ovulate tiny eggs with yolk sacs that lack yolk mass and have fetal and maternal tissues in close contact which function in nutrient transfer [5,7,10,11,12,31]. These characteristics (i) may be shared, derived characters reflecting a single origin of extreme matrotrophy, (ii) may have evolved once in Neotropical Mabuyinae and separately in the ancestor of Lubuya and Eumecia or (iii) may have evolved independently in all three lineages. ...
Most mammals and approximately 20% of squamates (lizards and snakes) are viviparous, whereas all crocodilians, birds and turtles are oviparous. Viviparity evolved greater than 100 times in squamates, including multiple times in Mabuyinae (Reptilia: Scincidae), making this group ideal for studying the evolution of nutritional patterns associated with viviparity. Previous studies suggest that extreme matrotrophy, the support of virtually all of embryonic development by maternal nutrients, evolved as many as three times in Mabuyinae: in Neotropical Mabuyinae (63 species), Eumecia (2 species; Africa) and Trachylepis ivensii (Africa). However, no explicit phylogenetic hypotheses exist for understanding the evolution of extreme matrotrophy. Using multilocus DNA data, we inferred a species tree for Mabuyinae that implies that T. ivensii (here assigned to the resurrected genus Lubuya) is sister to Eumecia, suggesting that extreme matrotrophy evolved only once in African mabuyine skinks. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
... In this regard, his work is an early forerunner of a contemporary per-spective. Viviparity is now known to have evolved independently in more than 100 squamate lineages (Blackburn, 2006(Blackburn, , 2015a, some of which have placentas that have converged strongly on eutherian mammals (Blackburn, 2015b;Blackburn and Flemming, 2012;Blackburn and Vitt, 2002). Decades before Haacke's work, a placental relationship had been described in a viviparous lizard from Italy (Studiati, 1855); however, being written in a pre-Darwinian milieu, it did not offer an evolutionary interpretation. ...
As viewed from a modern perspective, Wilhelm Haacke’s 19th century research on Australian lizards of the genus Tiliqua made significant contributions to our understanding of viviparity and placentation in squamate reptiles. He recognized the close structural relationship between the fetal membranes and the uterus through which maternal support of the embryos is accomplished. He also described the placental vasculature, the vestigial nature of the eggshell, and the lack of an egg tooth in viviparous embryos, and challenged the traditional concept of “ovoviviparity.” In recognizing that viviparity had evolved independently in reptiles and mammals, Haacke’s work foreshadowed the outpouring of research on comparative placentation of the past 50 years.
... Pregnancy lasts 8-12 months, and most of the placental nutrient provision occurs in the last 3 months of gestation (Vitt and Blackburn,'83). The chorioallantoic placenta exhibits multiple specializations for nutrient provision (Blackburn and Vitt, 2002). One is the placentome, a region at the dorsal pole of the conceptus where radiating folds of the uterine mucosa interdigitate with chorioallantoic tissue (Fig. 3A). ...
Current studies on fetal membranes of reptiles are providing insight into three major historical transformations: evolution of the amniote egg, evolution of viviparity, and evolution of placentotrophy. Squamates (lizards and snakes) are ideal for such studies because their fetal membranes sustain embryos in oviparous species and contribute to placentas in viviparous species. Ultrastructure of the fetal membranes in oviparous corn snakes (Pituophis guttatus) shows that the chorioallantois is specialized for gas exchange and the omphalopleure, for water absorption. Transmission and scanning electron microscopic studies of viviparous thamnophine snakes (Thamnophis, Storeria) have revealed morphological specializations for gas exchange and absorption in the intra-uterine environment that represent modifications of features found in oviparous species. Thus, fetal membranes in oviparous species show morphological differentiation for distinct functions that have been recruited and enhanced under viviparous conditions. The ultimate in specialization of fetal membranes is found in viviparous skinks of South America (Mabuya) and Africa (Trachylepis, Eumecia), in which placentotrophy accounts for nearly all of the nutrients for development. Ongoing research on these lizards has revealed morphological specializations of the chorioallantoic placenta through which nutrient transfer is accomplished. In addition, African Trachylepis show an invasive form of implantation, in which uterine epithelium is replaced by invading chorionic cells. Ongoing analysis of these lizards shows how integration of multiple lines of evidence can provide insight into the evolution of developmental and reproductive specializations once thought to be confined to eutherian mammals.
... Eggs with intact maternal oviducts wrapped were excised from three females in late gestation. The tissues were stabilized in Bouins' Fluid, dehydrated in increasing concentrations of ethanol, cleared in xylene, and embedded in paraffin (Blackburn and Vitt, 2002). The tissues were sectioned transversely with regard to the long axis of the uterus. ...
In this study we observed the embryonic development processes and compared the weight of eggs with neonates to
identify the form of nutritional provisioning in Eremias multiocellata. We also used light microscopy to determine
the features of chorioallontoic placenta during late stages of gestation. Neonatal dry weight was approximately
10.3% smaller than egg dry weight. Chorioallantoic membrane and uterine epithelium exhibited an indirect apposition
without any fusion. No specialized region, placentome, has been found for nutrition transfer between these
membranes. The chorioallontoic placenta of E. multiocellata is consistent with the simplest and most common
type, Weekes type I. We infer that E. multiocellata is predominately lecithotrophic, meaning that nourishment for
embryonic development is primarily supplied by the yolk. This kind of embryonic nutrition confirms the pattern
found in most squamates with a simple placenta. This simple placenta adds anatomical evidence to understand the
weak immune interaction between matrix and fetus in E. multiocellata. Finally, we discussed the implications for
interpreting the evolution of viviparity and placentation.
... Based on Pincheira-Donoso et al. (2013) there are 9193 squamate species; about 1800 species may thus be matrotrophic. Morphological and experimental evidence on placentation has been recorded for species in 13 squamate families (Weekes, 1935;Bauchot, 1965;Blackburn, Vitt & Beuchat, 1984;Blackburn, 1985Blackburn, , 1993Blackburn, , 1994bBlackburn, , 1998Blackburn, , 1999bBlackburn, , 2005Blackburn, , 2014Blackburn et al., 1985;Blüm, 1986;Stewart & Blackburn, 1988;Stewart, 1992Stewart, , 1993Stewart, , 2013Lombardi, 1998;Stewart & Thompson, 1998Thompson, Stewart & Speake, 2000;Blackburn & Vitt, 2002;Jerez & Ramírez-Pinilla, 2003;Villagrán et al., 2005;Ramírez-Pinilla, 2006;Thompson & Speake, 2006;Vieira, de Perez & Ramírez-Pinilla, 2007;Leal & Ramírez-Pinilla, 2008;Blackburn & Flemming, 2009;Stewart & Ecay, 2010, and references therein). Among these, substantial placentotrophy evolved in all six subclades of a single lizard family, Scincidae (Blackburn, 2014). ...
Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.
© 2015 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
... For example, despite placentae arising from homologous tissues in every known viviparous squamate, there are extensive morphologic differences in placentae. These morphologic differences are most noticeable in species that have evolved complex placentation, and include a vessel-dense elliptical area in Eulamprus tympanum (Murphy et al., 2011), interdigitating regions of the chorioallantois and uterus in Niveoscincus species (Stewart and Thompson, '94, 2009b;Stewart et al., 2009b;Wu et al., in press), invasive placentae in Trachylepis ivensi (Blackburn and Flemming, 2012), and elaborate placentome structures in Pseudemoia species (Stewart and Thompson, '96, '98;Adams et al., 2005), Chalcides chalcides (Blackburn,'93b;Blackburn and Callard,'97), and Mabuya species (Blackburn and Vitt, 2002;Jerez and Ram ırez-Pinilla, 2003;Ram ırez-Pinilla et al., 2006), as well as mechanisms of nutrient transport (Brandley et al., 2012;Griffith et al., 2013b). The diversity in placental structures in squamates, but lack of morphological diversity in the features responsible for egg production and oviparity strongly supports an oviparous ancestor for the squamates. ...
To understand evolutionary transformations it is necessary to identify the character states of extinct ancestors. Ancestral character state reconstruction is inherently difficult because it requires an accurate phylogeny, character state data, and a statistical model of transition rates and is fundamentally constrained by missing data such as extinct taxa. We argue that model based ancestral character state reconstruction should be used to generate hypotheses but should not be considered an analytical endpoint. Using the evolution of viviparity and reversals to oviparity in squamates as a case study, we show how anatomical, physiological, and ecological data can be used to evaluate hypotheses about evolutionary transitions. The evolution of squamate viviparity requires changes to the timing of reproductive events and the successive loss of features responsible for building an eggshell. A reversal to oviparity requires that those lost traits re-evolve. We argue that the re-evolution of oviparity is inherently more difficult than the reverse. We outline how the inviability of intermediate phenotypes might present physiological barriers to reversals from viviparity to oviparity. Finally, we show that ecological data supports an oviparous ancestral state for squamates and multiple transitions to viviparity. In summary, we conclude that the first squamates were oviparous, that frequent transitions to viviparity have occurred, and that reversals to oviparity in viviparous lineages either have not occurred or are exceedingly rare. As this evidence supports conclusions that differ from previous ancestral state reconstructions, our paper highlights the importance of incorporating biological evidence to evaluate model-generated hypotheses. J. Exp. Zool. (Mol. Dev. Evol.) 00B: 1-11, 2015. © 2015 Wiley Periodicals, Inc.
© 2015 Wiley Periodicals, Inc.
... While the ancestral state of Squamata is generally considered to be oviparity, viviparity has evolved numerous times in different squamate lineages (Blackburn 1999). In some skinks, this has been taken to extremes, with a few species exhibiting placental hypertrophy and essentially mammal-like embryonic development and pregnancy (Blackburn & Vitt 2002;Brandley et al. 2012;Murphy et al. 2012). ...
Viviparity has putatively evolved 115 times in squamates (lizards and snakes), out of only ~ 140 origins in vertebrates, and is apparently related to colder climates and other factors such as body size. Viviparity apparently evolves from oviparity via egg-retention, and such taxa may thus still have the machinery to produce thick-shelled eggs. Parity mode is also associated with variable diversification rates in some groups. We reconstruct ancestral parity modes accounting for state-dependent diversification in a large-scale phylogenetic analysis, and find strong support for an early origin of viviparity at the base of Squamata, and a complex pattern of subsequent transitions. Viviparous lineages have higher rates of speciation and extinction, and greater species turnover through time. Viviparity is associated with lower environmental and body temperatures in lizards and amphisbaenians, but not female mass. These results suggest that parity mode is a labile trait that shifts frequently in response to ecological conditions.
... The evolution of the amniote egg, with its extraembryonic membranes influenced many features of early embryogenesis and provided the setting in which the mammalian placenta evolved. However, in the context of late growth of the embryo, the complexities and uncertainties regarding evolution of the mammalian chorioallantoic placenta do not call into question the overall role of the placenta in nourishing the embryo during a prolonged period of embryonic/fetal growth (Blackburn and Vitt, 2002; Thompson et al., 2002). Importantly, placental support of growth provides a means of further extending a growth period that was already a major part of the growth of chordates with yolk-rich eggs. ...
Life forms range enormously in size, but as yet, we have little understanding of the mechanisms that determine the size of a cell or the size of an organism. Not only do we have little understanding of the mechanisms, but there is also a lack of appreciation of what is represented by growth control. Until recently, growth control was equated with the control of proliferation (Raff 1996; Su and O’Farrell 1998). However, the simple observations that cells can grow to different sizes, and that cells can divide without growth to produce larger numbers of smaller cells, suggest that the processes ought to be considered separately. It is recognized that growth should be considered in terms of increase in mass rather than increase in cell number. To many investigators, this appears to be a formalism, because they are so familiar with growth situations in which the two go hand in hand. The formalism becomes much more concrete when examining growth and cell proliferation in metazoans. It turns out that there is an extraordinary and fundamental segregation of organism growth and cell proliferation in the life histories of most metazoans.
There is a current lack of consensus on whether the ancestral parity mode was oviparity (egg‐laying) or viviparity (live‐birth) in amniotes and particularly in squamates (snakes, lizards, and amphisbaenids). How transitions between parity modes occur at the genomic level has primary importance for how science conceptualises the origin of amniotes, and highly variable parity modes in Squamata. Synthesising literature from medicine, poultry science, reproductive biology, and evolutionary biology, I review the genomics and physiology of five broad processes (here termed the ‘Main Five’) expected to change during transitions between parity modes: eggshell formation, embryonic retention, placentation, calcium transport, and maternal–fetal immune dynamics. Throughout, I offer alternative perspectives and testable hypotheses regarding proximate causes of parity mode evolution in amniotes and squamates. If viviparity did evolve early in the history of lepidosaurs, I offer the nucleation site hypothesis as a proximate explanation. The framework of this hypothesis can be extended to amniotes to infer their ancestral state. I also provide a mechanism and hypothesis on how squamates may transition from viviparity to oviparity and make predictions about the directionality of transitions in three species. After considering evidence for differing perspectives on amniote origins, I offer a framework that unifies ( i ) the extended embryonic retention model and ( ii ) the traditional model which describes the amniote egg as an adaptation to the terrestrial environment. Additionally, this review contextualises the origin of amniotes and parity mode evolution within Medawar's paradigm. Medawar posited that pregnancy could be supported by immunosuppression, inertness, evasion, or immunological barriers. I demonstrate that this does not support gestation or gravidity across most amniotes but may be an adequate paradigm to explain how the first amniote tolerated internal fertilization and delayed egg deposition. In this context, the eggshell can be thought of as an immunological barrier. If serving as a barrier underpins the origin of the amniote eggshell, there should be evidence that oviparous gravidity can be met with a lack of immunological responses in utero . Rare examples of two species that differentially express very few genes during gravidity, suggestive of an absent immunological reaction to oviparous gravidity, are two skinks Lampropholis guichenoti and Lerista bougainvillii . These species may serve as good models for the original amniote egg. Overall, this review grounds itself in the historical literature while offering a modern perspective on the origin of amniotes. I encourage the scientific community to utilise this review as a resource in evolutionary and comparative genomics studies, embrace the complexity of the system, and thoughtfully consider the frameworks proposed.
Viviparity is the reproductive pattern in which females gestate eggs within their reproductive tract in order to complete their development and give birth to live offspring. Within extant sauropsids, only the Squamata (e.g. snakes, lizards and amphisbaenians) evolved viviparity, representing 20% of the existing species. The genus Plestiodon is represented by 43 species and is one of the most widely distributed genera of the Scincidae in Mexico. The goal of this research has been to determine the placental morphology and ontogeny during gestation in the lizard Plestiodon brevirostris. Specimens were dissected in order to obtain the embryonic chambers and the embryos were categorized to carry out the correlation between the development stage and the placenta development. The embryonic chambers were processed using the conventional histological technique for light microscopy. The identified embryonic stages were 4, 29, 34, 36 and 39. A thin eggshell surrounds the egg in early developmental stages; however, this structure is already absent in the embryonic hemisphere during the developmental stage 29. The results revealed that P. brevirostris is a lecithotrophic species, but a close maternal-fetal relationship is established by tissue apposition. Ontogenically, the placental types that form in the embryonic hemisphere are the chorioplacenta, choriovitelline placenta, and chorioallantoic placenta; whereas the omphaloplacenta is formed in the abembryonic hemisphere. The structure of the chorioallantoic placenta in P. brevirostris suggests that it may play a role during gas exchange between the mother and the embryo, due to the characteristics of the epithelia that comprise it. The structure of embryonic and maternal epithelia of the omphaloplacenta suggests a role in the absorption of the eggshell during gestation and possibly in the transport or diffusion of some nutrients. In general, it is evident that ontogeny and placental characteristics of P. brevirostris match those of other species of viviparous lecithotrophic scincids. This article is protected by copyright. All rights reserved.
Herein we studied the reproductive biology of a viviparous lizard (Mabuya dorsivittata) from the Wet Chaco region (northeastern Argentina) and compared the results with other populations from the Espinal (central Argentina) and the Atlantic Forest (southeastern Brazil), and with other Neotropical species of Mabuya to better understand the possible causes of its reproductive phenotype variation. Males and females of M. dorsivittata from the Wet Chaco exhibited associated, seasonal, and annual reproductive cycles. Spermatogenic activity related positively to a lengthening photoperiod reaching maximum activity in late spring (December). Females displayed an extended gestation period of 11 months, from mid-summer (February) to late spring or early summer (December) when births occur. Embryonic development was associated with temperature and historical rainfall. Litter size ranged from 3 to 8 (mean = 5.3 ± 1.3 SD) and increased with body size and body mass of females. Fat-body mass varied seasonally and was inversely correlated with spermatogenesis and to embryonic development. Females were larger in body size and interlimb length, and smaller in head length than males. We observed interpopulational differences in minimum body size, litter size, and timing of birth, probably as a result of phenotypic plasticity, genetic divergence or both.
By the 1890s, placental arrangements had been documented macroscopically in lizards and fishes, but placental studies on such species lagged far behind research on mammals. In 1891, the biologist Ercole Giacomini (at the University of Siena, Italy) published the first histological analysis of a reptile placenta. Focusing on a placentotrophic lizard (Chalcides chalcides) with a morphologically complex placenta, Giacomini documented the histological and cellular bases for placental nutrient transfer and gas exchange. In conjunction with a follow-up study in 1906, he demonstrated that placental structure is correlated with function and can vary dramatically between related species. Giacomini's work was highly influential in showing that placentation in lizards had converged evolutionarily on that of mammals, while establishing reptile placentology as a highly promising area for future research.
Ultrastructure of the placental tissues from redbelly watersnakes (Nerodia erythrogaster) was analyzed during late pregnancy to provide insight into placental development and function. Examination of the chorioallantoic placenta with transmission electron microscopy reveals that chorionic and uterine epithelia are extremely attenuated but intact and that the eggshell membrane is vestigial and lacks a calcareous layer. These features minimize the interhemal diffusion distance across the placenta. Scanning electron microscopy reveals that fetal and maternal components of the placentas are richly vascularized by dense networks of capillaries. Although the yolk sac omphalopleure has largely been replaced by chorioallantois by late gestation, it retains patches of yolk droplets and regions of absorptive cells with microvilli and abundant mitochondria. Transmission electron microscopy reveals that yolk material is taken up for digestion by endodermal cells. As yolk is removed, allantoic capillaries invade to occupy positions just beneath the epithelium, forming regions of chorioallantoic placentation. Ultrastructural features indicate that the chorioallantoic placenta is specialized for gas exchange, while the omphalallantoic ("yolk sac") placenta shows evidence of functions in yolk digestion and maternal-fetal nutrient transfer. Placental features of this species are consistent with those of other thamnophines, and are evolutionarily convergent on snakes of other viviparous clades.
The placenta has had the most dynamic evolutionary history of all mammalian organs. It has undergone massive shifts in anatomy, physiology, and the way in which uterine and fetal tissue interact with one another during pregnancy. The human placenta is arguably the best studied amongst mammals, yet much about its function during pregnancy is not understood. The purpose of this paper is to outline the evolutionary history of the placenta, and to point out major gaps in the current state of knowledge. I also propose novel theoretical, experimental, and computational approaches that are likely to provide insight into the normal process of placentation and the role the placenta plays in the great obstetrical syndromes.
Viviparity in squamate reptiles is widely recognized as having evolved convergently from oviparity more than 100 times. However, questions persist as to whether reversals from viviparity back to oviparity have ever occurred. Based on a theoretical model, a recent paper (Pyron and Burbrink, 2014) has proposed that viviparity is ancestral for squamates and that viviparity-oviparity reversals have far outnumbered origins of viviparity in reproductive history. Close examination of this analysis reveals features that cast doubt on its plausibility, notably the requirement of repeated, sequential transformations back and forth between these reproductive modes, as well as numerous, uncounted evolutionary transformations that have produced inaccurate estimates of parsimony. Evidence derived from studies of anatomy, physiology, and developmental biology strongly supports the inference that oviparity is ancestral for squamates and has given rise to viviparity on numerous occasions. Biological data provide important insights into the likelihood of evolutionary transformations, and deserve to be incorporated fully into future analyses of the evolution of reproductive modes. J. Exp. Zool. (Mol. Dev. Evol.) 9999B: XX-XX, 2015. © 2015 Wiley Periodicals, Inc.
© 2015 Wiley Periodicals, Inc.
In placentotrophic viviparous reptiles, pregnant females deliver nutrients to their developing fetuses by diverse morphological specializations that reflect independent evolutionary origins. A survey of these specializations reveals a major emphasis on histotrophy (uterine secretion and fetal absorption) rather than hemotrophy (transfer between maternal and fetal blood streams). Of available hypotheses for the prevalence of histotrophic transfer, the most promising derives insights from the theoretical parent-offspring conflict over nutrient investment. I suggest that histotrophy gives pregnant females greater control over nutrient synthesis, storage, and delivery than hemotrophic transfer, reflecting maternal preeminence in any potential parent-offspring competition over nutrient investment. One lizard species shows invasive ovo-implantation and direct contact between fetal tissues and maternal blood vessels, potentially conferring control over nutrient transfer to the embryo. Future research on squamates will benefit from application of parent-offspring conflict theory to the transition from incipient to substantial matrotrophy, as well as by testing theory-derived predictions on both facultatively and highly placentotrophic forms. J. Exp. Zool. (Mol. Dev. Evol.) 9999B: 1-17, 2015. © 2015 Wiley Periodicals, Inc.
© 2015 Wiley Periodicals, Inc.
Natural-history studies represent the observational stage of the scientific method, and the single greatest discovery in biological sciences, evolution by natural selection, was based largely on a vast amount of natural-history information collected by Charles Darwin. I briefly review natural-history observations that I have made during my career that led to discoveries in life-history theory, placentation in New World Mabuya that rivals that of eutherian mammals, social behavior in the North American clade of five-lined skinks, and the relationship of ecological traits of lizards globally to their evolutionary history (phylogeny). Gifted collaborators provided the intellectual interplay that led to these discoveries, and they certainly deserve as much credit as I do. I briefly comment on what I consider to be frontiers in herpetology that involve combining phylogenetic hypotheses with natural-history data. In a final comment, I encourage those among us who are able to spend extended time periods in the field to collect as much natural-history data as possible, because these data describe reality, and, as theories and phylogenies evolve, the kinds of basic data that led to Darwin's theory of evolution by natural selection can be applied again and again.
Although viviparity has evolved many times in the animal kingdom, it remains relatively uncommon—scorpions and therian mammals being rare examples of entirely viviparous major taxa. Viviparity is a specialised form of intra-species parasitism which biases parental investment towards fertilised eggs, temporally spreads that investment, and also temporarily protects offspring from many selection pressures. Importantly, the mammalian viviparity appeared at a relatively late stage in the process of vertebrate evolution. Because of this, viviparity was ‘superimposed’ on complex pre-existing cardiovascular, respiratory, metabolic and immune systems, and has altered them dramatically. Also, pregnancy has exerted pervasive effects on gene expression in mammals, including genetic imprinting, X inactivation, sex determination, and the ectopic expression in the extra-embryonic membranes of many genes previously expressed in the gonads, brain, pituitary and immune system. Finally, although lactation probably pre-dated viviparity in mammalian evolution, the two have co-evolved as alternative strategies of offspring nutrition ever since.
In many species of snakes, particularly viperids from temperate regions, produc-tion of offspring by individuals occurs on a less-than-annual schedule. Accord-ingly, acquiring sufficient energy and nutrient reserves for reproduction in females often requires more than a single active season. This is termed capital mode. Yet, in some instances, annual reproduction occurs under conditions where foraging success is high and environmental factors are compliant. This is termed income mode. Here, we addressed the hypothesis of annual versus less-than-annual reproduction from a long-term radio-telemetric study involving female western diamond-backed rattlesnakes Crotalus atrox from a population of the Sonoran Desert in Arizona. From 2001 to 2008, 16 of 20 radio-telemetered females produced 36 litters, which 32 were informative in addressing the hypothesis of reproductive frequency. In 14 females, litters were produced on a biennial or at-least-biennial (Zbiennial) cycle. However, seven females demonstrated annual reproduction, of which several had previously reproduced on a biennial or greater cycle. Because our study was non-experimental, we were unable to unambiguously identify specific proximate factors that contributed to the shift in annual reproduc-tion. Nonetheless, we established that greater annual rainfall was significantly correlated with shifts to annual reproduction. Based on other studies, we hypothesize that increased rainfall was causally linked with increases in rodent densities and the foraging success of female C. atrox, which in turn is linked to reproduction. We describe, moreover, several characteristics of female C. atrox that appear to facilitate the potential for annual reproduction. In long-lived species, such as C. atrox, our research underscores the necessity to follow individuals for extended periods to gain insights on reproductive cycles not captured by point sampling methods, such as short-term field studies or reliance on museum specimens.
Among squamate reptiles, gravid females commonly are said to be able to resorb infertile and malformed eggs from their oviducts. This pattern, if it existed, would allow females to recycle nutrients from abortive attempts at reproduction, and to increase lifetime reproductive potential by modulating reproductive effort according to environmental circumstances. However, a review of the literature reveals that evidence for oviductal egg resorption is weak, and does not preclude other fates for abortive eggs (egg retention or expulsion). Furthermore, for the oviduct to resorb eggs would require that it have the functional properties of the digestive tract, properties that may be incompatible with its several reproductive functions. Future work should not assume oviductal egg resorption in squamates without definitive evidence that the eggs are not simply aborted or retained by females following absorption of water.
Phylogenetic analyses of reproductive patterns have clarified several aspects of the evolution
of viviparity in reptiles and mammals, and offer useful comparisons to that pattern in fishes. In amniotes, viviparity, placentation, and matrotrophy have been assumed to evolve as three successive stages in a continuous, unilinear transformation, with yolk sac placentas originating prior to chorioallantoic placentas. However, the phylogenetic distribution of reproductive characters in mammals indicates that obligative uterine matrotrophy actually evolved under conditions of oviparity, and that viviparity and placentation evolved subsequently. In contrast, in lizards and snakes, viviparity frequently has evolved simultaneously with placentation and incipient matrotrophy, apparently in accord with a punctuated equilibrium model of evolutionary change. In squamates (and quite possibly, mammals), placentas formed from the chorioallantois and yolk sac have evolved simultaneously. Phylogenetic analysis indicates that reptilian viviparity has originated independently in over 100 separate lineages, often at low taxonomic levels and in geologically recent times. At least four of these clades subsequently have evolved substantial matrotrophy. Mammals, squamates and fishes have evolved viviparity and matrotrophy convergently, but apparently by different historical sequences and different anatomical and physiological specializations. These findings reveal the difficulties in making evolutionary generalizations about viviparity across higher taxonomic boundaries, and the importance of employing quantitative phylogenetic analyses.
Reproduction in most vertebrates requires that the female construct an egg containing all of the nutrients needed to sustain development and that she deposit that egg in an environment where it can develop and hatch. Like the seed of a plant, a newly-laid egg can be viewed as a compact package of nutrients and energy, housed with detailed genetic intructions on their use, and provided with a protective covering. The success of egg-laying reproduction, i.e., “oviparity,” in terrestrial environments is revealed by its presence in such amniotes as birds, monotremes, turtles, tuataras, crocodilians, and most squamates (i.e., lizards, amphisbaenians, and snakes). Nevertheless, developing eggs that have been laid on land can be subject to the vicissitudes of the terrestrial environment—temperature extremes, thermal fluctuations, dehydration, flooding, and predation by animals, fungi, and bacteria that may be genetically programmed to exploit egg nutrients for their own growth and reproduction.
The viviparous lizard Mabuya bistriata was studied in two lowland tropical forest sites in Amazonian Brazil with additional data taken on museum specimens. These diurnal lizards are active primarily during mid- to late morning on fallen logs or low on tree trunks. They are heliothermic averaging 32.9 f0.98 C in body temperatures. Prey include orthopterans, spiders, eruciform larvae, termites, and other invertebrates. Prey differences between the two sites most likely reflect differences in prey availability associated with tropical seasonality in rainfall.
Mabuya bistriata reach sexual maturity by the end of the first year of life, and females produce their first brood at an age of one yr. Females have larger body size than males, and males have larger head size than females. Females ovulate ova 1 mm in diameter and 0.47 mg dry mass. A chorioallantoic placenta forms through apposition of the chorioallantois to the uterine mucosa. During gestation, dry and wet mass increase by over 47,400% and 74,700%, respectively, with virtually all nutrients for development supplied by the female. Gestation lasts 9-12 months. Brood size varies from 2-9, and females with near-term embryos are heavier and wider than nongravid females suggesting a potential cost to reproduction related to the effect of clutch mass and volume on locomotor abilities.
Quantitative analysis of the composition of eggs and their sibling neonates in the viviparous natricine snake Thamnophis ordinoides revealed that yolk provided the principal source of organic nutrition but that embryos received a substantial allotment of inorganic nutrients from the placentas. The placental provision of water and sodium equaled or exceeded yolk supplies, and placental transport accounted for23% of neonatal calcium composition. There was no difference between egg and newborn quantities of total phosphorus or total potassium, whereas neonates contained less total magnesium than eggs. The mode of embryonic nutrition in this species is characterized as predominantly lecithotrophic, yet placental nutrient provision contributes significantly to embryonic nourishment. Placental transport of sodium and embryonic uptake of water was greater in recently ovulated eggs that contained relatively low levels of sodium and water respectively. Thus, placental sources compensated for low yolk provision. Placental transport of calcium was independent of yolk calcium content and correlated positively with neonatal calcium content. This pattern of provision, in which placental sources determine neo-natal content independent of egg content, has been described as facultative placentotrophy. A similar embryonic nutritional pattern was recognized previously in another predominantly lecithotrophic natricine snake.
Review of the literature on reptilian placental terminology reveals that confusion associated with the assignment of terms to different placental organs is unwarranted. Reptilian placentae can be assigned easily to one of four distinct structural types defined by the extraembryonic membranes that are involved in placental formation. The terms "allantoplacenta" and "chorioallantoic placenta" have consistently been applied to the organ formed by the chorioallantoic membrane and adjacent uterine epithelium. Some confusion, however, has been associated with the assignment of terms to regions of the yolk sac that participate in placentation. We recognize two distinct types of yolk sac placentation: 1) the "choriovitelline placenta," defined as the apposition of the vascularized trilaminar omphalopleure (ectoderm, mesoderm, endoderm) and the uterine epithelium; and 2) the "omphaloplacenta," which consists of the non-vascular omphalopleure (ectoderm, endoderm) of the isolated yolk mass and associated structures, in apposition to the uterine epithelium. The "omphalallantoic placenta," constituting the fourth structural category, forms as the outer allantoic membrane becomes apposed to the inner margin of the omphaloplacenta. Based upon these definitions, both chorioallantoic and choriovitelline placentation occur among reptiles, marsupials and eutherians, whereas omphaloplacentation and omphalallantoic placentation are unique to squamates.
The tropical Brazilian skink Mabuya heathi is viviparous, producing ova which increase in wet mass by 53 800 % and in dry mass by 38 400 % during gestation. Ovulation occurs during October-January, rapid growth in embryos takes place during June-October, and parturition occurs in September-November, between 9 and 12 months after ovulation. Female and male reproduction is cyclic and is synchronous between the sexes. Brood size is 2-9 (i = 5.0 + 0.1) and is significantly correlated to female size. Fat bodies of females are largest when embryos are small, and decrease in mass as embryos increase in size. Testes in males are largest during September-February, associated with a decrease in fat body size. Mabuya heathi is the second species of lizard in the caatinga herpetofauna shown to be strongly cyclical in reproduction and to reproduce only once per year.
Examination of late-stage placental material of the lizard Chalcides chalcides from the Hubrecht Laboratorium (Utrecht, The Netherlands) reveals several cytological and histological specializations that appear to have been superimposed over a morphological pattern that is typical for squamates. The chorioallantoic placenta is highly vascularized and consists of a single mesometrial placentome and a generalized paraplacentomal region, both of which are epitheliochorial. The placentome is deciduate, and contains deeply interdigitating folds of hypertrophied uterine and chorioallantoic tissue. Chorionic epithelium lining the placentome comprises enlarged, microvilliated cells, a small proportion of which are diplokaryocytes. The placentomal uterine epithelium is not syncytial and consists of enlarged cells bearing microvilli. The yolk sac placenta is a true omphaloplacenta (sensu stricto), being formed by juxtaposition of uterine tissues to an avascular, bilaminar omphalopleure. Epithelium of the omphalopleure is stratified and is hypertrophied into papillae that project into detritus of the uterine lumen. The omphalopleure is separated from the yolk sac proper by a yolk cleft that is not confluent with the exocoelom and is not invaded by the allantois. Neither an omphalallantoic placenta nor a true choriovitelline placenta is present in late gestation. Morphologically, the mature placentae of C. chalcides are among the most specialized to have been described in reptiles, reflecting the substantial maternal-fetal nutrient transfer that occurs in this species. © 1993 Wiley-Liss, Inc.
Allantoplacentae in lizards and snakes form during the evolution of viviparity through apposition of the chorioallantois and a vestige of the shell membrane to the uterine lining. Generalized squamate allantoplacentae are epitheliochorial, diffuse, adeciduate, and highly vascular, accomplish maternal-fetal gas exchange, and possibly transfer small quantities of organic and inorganic nutrients. Placental gas exchange presumably is enhanced by the thinning of tissues lying between fetal and maternal capillaries, a progressive increase in placental vascularity, and in some species, by differences in oxygen affinity of fetal and maternal blood. A few saurian genera (e.g., Chalcides, Mabuya, and Pseudemoia) include species with specialized placentae that transfer large quantities of nutrients. Specializations of the allantoplacentae of these lizards include interdigitating, hypertrophied uterine and chorioallantoic tissues, and enlarged absorptive chorionic epithelia. South American Mabuya are further specialized by the presence of chorionic areolae and a distinctive placentome. Weekes' 1935 classification of placental morphotypes is reviewed, and a few minor modifications are proposed, in addition to recognition of a fourth morphotype. Phylogenetic analyses suggest that placental organs have originated on more than 100 occasions among squamate reptiles, and indicate that three separate lineages have converged on substantial placentotrophy through the evolution of specialized histotrophic placentae. © 1993 Wiley-Liss, Inc.
Obplacental giant cells are large (less than or equal to 210 microns) polyploid cells that appear in the stroma of the pregnant uterus of the rabbit following ovoimplantation. Histological examination of a complete developmental series indicates that obplacental giant cells arise from trophoblastic knobs that have traversed the uterine epithelium during early implantation. During maturation, the cells undergo a massive (approximately 6,000%) increase in volume and penetrate deeply into the uterine stroma and myometrium, where they often become associated with blood vessels and smooth muscle cells. Giant cells at mid-gestation contain one or two large nuclei with prominent nucleoli and appear to be amitotic. They are rich in Golgi complexes, RER, SER, and cortically distributed cytoplasmic filaments, and contain intracellular canaliculi lined by microvilli. Giant cells vary with respect to the occurrence of lipid droplets, phagocytotic inclusions, lysosomal structures, and electron-dense granules. Immunocytochemistry demonstrates that the giant cells exhibit intermediate filaments related to cytokeratin and vimentin, but are negative for desmin and for an endothelial cell marker, Factor VIII-related antigen. The cells are positive for cytokeratin from their inception, but only become vimentin-positive between Days 12 and 15 of pregnancy, a change seemingly related to their detachment from epithelial tissue to take on an independent existence. Our findings indicate that the giant cells originate from obplacental trophoblast and, at maturity, exhibit cytoskeletal characteristics of isolated epithelial cells, as well as a complement of organelles suggestive of synthetic activity.
Available evidence provides little support for a recent proposal that the term “trophoblast” be applied solely to eutherian mammals. Arguments for such a restricted usage are based on a dichotomous interpretation of therian reproduction that underestimates the developmental, structural, and functional diversity of trophoblastic tissues occurring within the infraclass Eutheria. The occurrence of developmental patterns that are phenotypically intermediate between those of commonly studied eutherians and metatherians suggests that blastocyst development is not fundamentally different in marsupials and eutherians.
The trophoblast of marsupials accomplishes most or all of the major functions of the eutherian trophoblast, including maternal-fetal physiological exchange, implantation, contribution to placental membranes, steroid metabolism, and possibly, immunological protection of the conceptus. Furthermore, application of the term “trophoblast” to marsupials is consistent with present and past usage, as well as with the original definition and etymological derivation of the term. Therefore, we recommend that the term “trophoblast” continue to be applied in a functional-morphological sense to the appropriate extraembryonic tissues of marsupials. Such use of functional (rather than taxonomic) criteria for application of this term avoids biasing interpretations of mammalian reproductive evolution.
1. Inner cell mass (ICM) and trophoblast tissue were isolated from 3½-day post-coitum mouse blastocysts that were homozygous for different electrophoretic variants of the enzyme glucose phosphate isomerase (GPI). Blastocysts were reconstituted from these tissues, transferred to pseudo-pregnant recipients and allowed to develop to the early somite stage.
2. The embryo plus membranes and trophoblast were dissected and typed separately for GPI.
3. Contamination of trophoblast with maternal decidual tissue was quantified.
4. The trophoblast of the implanted embryos was almost exclusively of the trophoblastdonor GPI type. The embryos plus membranes were mainly of the ICM-donor type but most also showed a substantial proportion of trophoblast-donor type.
5. It is argued that the ICM controls trophoblast proliferation by inhibiting giant cell transformation of adjacent trophoblast cells rather than through making a significant cellular contribution.
Marsupials differ from most other mammals in their method of reproduction, in that they have chosen, in an evolutionary sense, to develop lactation rather than placentation for the nurture of their young. The neonate is therefore born with a mixture of advanced and embryonic characters, and yet is readily accessible within the pouch, providing a unique system for the study of the ontogeny of various physiological and endocrinological parameters. Marsupials are therefore ideal animals for research into mammalian reproductive physiology. The results of this exciting new research are summarized in this book by two of the foremost workers in the field. Individual chapters analyse the genetic and hormonal control of sexual differentiation, male and female reproductive structures and their functions, the role of the corpus luteum in the oestrous cycle and pregnancy, the hormonal control of embryonic diapause and the role of the marsupial placenta in the development of the embryo. This book is more than just a straightforward review of marsupial reproduction for its detailed analyses and broad comparative coverage will attract mammalogists and reproductive physiologists with a wide range of research interests.
Recently ovulated eggs and newborn of two species of viviparous reptiles, Gerrhonotus coeruleus (Sauria: Anguidae) and Nerodia rhombifera (Serpentes: Colubridae), were analyzed for organic (nitrogen, lipid) and inorganic (total ash, sodium, potassium, calcium, magnesium) content. Eggs contain 41%-48% water, 46%-56% organic matter, and 3%-5% inorganic salts. Total protein (based on measurement of nitrogen) is higher in egg yolk than total lipid. Analysis of the composition of newborn indicates that eggs of both species gain water and lose dry mass during development. Most of the dry mass loss is a reduction in amount of lipid, presumably due to catabolism. There was no substantial change in total nitrogen in newborn compared to yolk. There was evidence of an extravitellogenic source of mineral ions in both species. Newborn G. coeruleus have a substantially higher ash content than recently ovulated eggs. Although total ash was not higher, newborn N. rhombifera contain more sodium and potassium than eggs. Transplacental transfer of inorganic material occurs in both species but to a greater degree in Gerrhonotus than Nerodia.
The extraembryonic or fetal membranes of vertebrates play an important functional role in the nutrition, respiration, excretion, and protection of the embryo and fetus during prenatal life. They are auxiliary structures which develop in continuity with the tissues of the embryo proper, and both embryo and fetal membranes are derived from the same three basic germ layers (ectoderm, mesoderm, and endoderm). The fetal membranes are transitory structures which persist for only a relatively brief period during the ontogeny of the individual; nevertheless, their functional differentiation is essential for the normal development of the embryo during prenatal life. The functional life of the fetal membranes is terminated at the time of birth or hatching; they may become partially resorbed into the body of the newborn, or, more commonly, they become disrupted and degenerate.
The biomechanical concept of the vertebrate tendon consists of a complex of homogenous fibers oriented parallel to the main axis of the tendon, implying the direction of the fibers corresponds to the lines of forces. However, some observations are inconsistent with this model. 1) In several multimotor musculo-tendinous systems, e.g., the human m., triceps surae complex, the constituents are spirally organized, so that the fiber direction deviates from the ideal axis. 2) High resolution MRI demonstrated that the tendons of the human mm. flexores and extensores digitorum are not homogenous. In the present study MR images of the tendons of the human mm. gastrocnemius and mm. flexores digitorum were used in a 3-D reconstruction of tendon architecture, by smoothing a digitized set of data of the outlines of fascicles and fibers, using a computer program developed by the second author. These parameters were smoothed using bicubic spline technics. transition into visual graphics was accomplished by MATLAB tools application. Most striking in our results is the occurrence of coil in the tendons. In the Achilles tendon spiralization is caused by the fascicles, whereas coiling in the finger flexor tendons originates from the helicoidal arrangement of the fibers. Coil progressively increases peripherally and also in the proximal direction. These findings may throw new light on the visco-elastic properties of tendons.
Newborn size in the viviparous snake Virginia striatula is determined by female size, egg size, and placental nourishment and is correlated inversely with current fecundity. Of the 2 modes of embryonic nutrition used, lecithotrophy and placentotrophy, an increase in vitellogenic nourishment results in a reduction in current fecundity, whereas placental nourishment does not affect current fecundity. Newborn composition is determined by 2 mechanisms: one is correlated with female size, the other regulated by the developing embryo. -from Author
Vertebrate reproductive modes, which are characterized by egg and development type, have been compared and, in some cases, re-defined. Reproductive strategies regulate reproductive effort. Reproductive modes and strategies have been selected in species history: more anciently and with more conservative characters the former; more recently and with some flexibility in response to environmental changes the latter.
Semelparity is the expression of entire reproductive effort in a single event in the life cycle; iteroparity, in several events. In iteroparous species living in uniform, aseasonal environments, continuous reproduction can be also found. However, in fluctuating environments, seasonality concentrates reproduction only in the most favourable period of the year (seasonal reproduction). Several mechanisms (i.e. refractoriness) can block reproductive processes in unfavourable periods, avoiding energy waste. Different strategies such as male sperm storage, delayed fertilization, embryonic diapause, have been selected in seasonal breeders, optimizing reproductive effort.
In all vertebrate classes, except birds (all oviparous), oviparity and viviparity may co-exist in a genus or a species. Viviparity has been attained many times independently in evolutionary history of vertebrates, as an adaptive response to particular environments (i.e. acquatic environment, cold climate, deserts). Egg retention and ovoviviparity (aplacental viviparity) are considered transitional to placental viviparity; internal fertilization, modifications in oviducal structures and secretions, evolution of a corpus luteum secreting progesterone, are prerequisite for evolution to viviparity.
Iteroparity, seasonal reproduction and viviparity have allowed species continuity and individual survival to be combined in reproductive effort.
1.1. Inorganic ion content of developing follicles and of whole eggs and separated embryos and yolk sacs of the viviparous lizard, Sphenomorphus quoyii has been measured.2.2. There is a net increase in calcium, sodium and potassium in whole eggs during gestation. Magnesium and phosphorus content remains constant.3.3. The additional ions are incorporated into the developing embryo.4.4. Calcium content of the yolk is compared with that of the fowl and other species of reptile.
1.1. The Eastern water skink is a viviparous lizard with a simple placenta and a large yolk sac.2.2. Changes in wet, dry and ash weights of developing follicles and of the egg and egg components during gestation were analysed.3.3. The total wet weight of the whole egg (embryo, yolk plus embryonic fluids and extraembryonic membranes) increases significantly during development.4.4. This increase is due to uptake of water and inorganic ions from the mother.5.5. Both the water and inorganic ions are incorporated into the developing embryo.6.6. Lipid is the principal organic material metabolized and it is estimated that there is sufficient lipid in the yolk for embryonic growth and metabolism. Relatively little protein is metabolized.7.7. The results show that embryos take up water and some inorganic ions from the mother but there is no evidence to suggest a dependence on the mother for organic nutrients.
In the scincid lizard Chalcides chalcides, females ovulate small ova and supply most of the nutrients for development by placental means. The yolk is enveloped precocially by extraembryonic ectoderm and endoderm during the gastrula stage, establishing a simple bilaminar yolk sac placenta. The shell membrane begins to degenerate at this time, resulting in apposition of extraembryonic and maternal tissues. A true chorioplacenta has developed by the early pharyngula stage, as has a choriovitelline placenta and the first stages of an omphaloplacenta. Although the choriovitelline membrane disappears rapidly, the omphaloplacenta spreads to occupy the entire abembryonic pole. The yolk cleft is not confluent with the exocoelom, and no omphalallantoic placenta develops. By the limb-bud stage, an allantoplacenta has been established, with a mesometrial placentome composed of interdigitating ridges of chorioallantois and uterine mucosa. The discovery of five distinct placental arrangements in this species, three of which are transitory and two of which have not previously been recorded in reptiles, emphasizes the need for accounts that specify ontogenetic stages and the precise identity and composition of squamate placental membranes. Contrary to previous interpretations, the pattern of extraembryonic membrane development in C. chalcides is evolutionarily conservative, despite the presence of a reduced yolk mass and cytological specializations for nutrient transfer. Our observations indicate that substantial placentotrophy can evolve in squamates without major modifications of morphogenetic patterns. J Morphol 232:35–55, 1997. © 1997 Wiley-Liss, Inc.
Development of the extraembryonic membranes and their structural alignment in the formation of the four placental categories that occur in Virginia striatula is similar to that of other Serpentes. The vascularized trilaminar omphalopleure contacts the uterine epithelium early in development to form the choriovitelline placenta, which subsequently extends over the surface of the yolk to its limit at the margin of the isolated yolk mass. As in other squamates, the isolated yolk mass is separated from the yolk sac by the growth of intravitelline cells into the yolk. The bilaminar omphalopleure (ectoderm, endoderm) of the isolated yolk mass contributes the fetal epithelium of the omphaloplacenta. During formation of the omphidoplacenta, the allantois makes its initial contact with the chorion dorsal to the embryo. As the allantois expands, the chorioallantoic placenta gradually replaces the choriovitelline placenta. The terminal placental stage is defined by an extensive chorioallantoic placenta and an omphalallantoic placenta associated with the isolated yolk mass. Although similar in most aspects to the omphalallantoic placentae of other snakes, a secondary yolk cleft develops in V. striatula. This structure, which separates the outer allantoic membrane from the isolated yolk mass, has not been described in other squamates. The choriovitelline placenta and most of the surface of the chorioallantoic placenta are characterized by close vascular apposition, whereas the omphaloplacenta and omphalallantoic placenta feature uterine and fetal epithelial cell hypertrophy in the absence of fetal vascular support. A narrow zone within the chorioallantoic placenta, lying on either side of the omphalallantoic placenta, contains hypertrophied epithelial cells with supporting capillaries on both fetal and maternal aspects.
Paraffin sections of an ontogenetic series of embryos of the viviparous lizard Gerrhonotus coeruleus and the oviparous congener G. multicarinatus reveal that although general features of the development of the chorioallantoic and yolk sac membranes are similar, differences are evident in the distribution of the chorioallantoic membrane in late stage embryos. An acellular shell membrane surrounds the egg throughout gestation in both species although the thickness of this structure is much reduced in G. coeruleus over that of G. multicarinatus. The initial vascular membrane to contact the shell membrane in both species is a trilaminar omphalopleure (choriovitelline membrane) composed of ectoderm, mesoderm of the area vasculosa, and endoderm. This transitory membrane is replaced by the vascularized chorioallantois as the allantois expands to contact the inner surface of the chorion. Prior to the establishment of the chorioallantois at the embryonic pole, a membrane begins to form within the yolk ventral to the sinus terminalis. This membrane, which becomes vascularized, extends across the entire width of the abembryonic region and isolates a mass of yolk ventral to the yolk mass proper. The outer membrane of the yolk pole is a nonvascular bilaminar omphalopleure (chorionic ectoderm and yolk endoderm). In G. multicarinatus the bilaminar omphalopleure is supported internally by the vascularized allantoic membrane, whereas in G. coeruleus the allantois does not extend beyond the margin of the isolated yolk mass and the bilaminar omphalopleure is supported by the vascularized intravitelline membrane. Both the chorioallantoic placenta (uterine epithelium, chorionic ectoderm and mesoderm, and allantoic mesoderm and endoderm) and the yolk sac placenta at the abembryonic pole (uterine epithelium, chorionic ectoderm, and yolk sac endoderm) persist to the end of gestation in G. coeruleus.
A unique type of reptilian allantoplacenta was described by Weekes [1930] (Proc. Linn. Soc. N.S.W. 55:550–576) from a single embryonic stage of the Tasmanian skink, Niveoscincus ocellatus (as Lygosoma (Liolepisma) ocellatum). She also assigned N. metallicus to this placental category but did not provide a description. Here we provide a description of allantoplacentation and yolk sac placentation of N. metallicus. The allantoplacenta is regionally differentiated and differs from other reptilian allantoplacentae by the presence of a zone of hypertrophied chorionic epithelial cells in apposition to uterine blood vessels which are contained within ridges formed from uterine epithelial cells. This zone is located dorsolaterally along the long axis of the egg at the upper margin of the yolk sac. In contrast, the cells of the chorionic epithelium dorsal to the embryo are smaller and the uterine blood vessels are not contained in ridges. The definitive yolk sac placenta is an omphaloplacenta. The bilaminar omphalopleure of the omphaloplacenta consists of an outer layer of cuboidal or columnar cells. Cells of the uterine epithelium of the omphaloplacenta are cuboidal or columnar in shape and are supported by uterine blood vessels. © 1994 Wiley-Liss, Inc.
The extraembryonic membranes of amniote vertebrates are shared structures that have a brief, yet substantive role in the ontogeny of the organism. The yolk sac, which is the first of these structures to develop, has an important function in regulation and mobilization of nourishment to embryos of oviparous species and additionally contributes to maternal-fetal exchange in many viviparous species. Yolk sac placentation has been described in all major taxa of viviparous amniotes. The yolk sac of viviparous reptiles contributes to three categories of placentation, the choriovitelline placenta, which also occurs in marsupial and eutherian mammals, and the omphaloplacenta and omphalallantoic placenta, which occur only among squamate reptiles (lizards and snakes). The fetal contribution to the omphaloplacenta includes a structure termed the isolated yolk mass, which forms in all Squamata independent of reproductive mode, and is produced by a morphogenesis of extraembryonic mesoderm that is unique to this reptilian order. For some viviparous squamates, the allantois contributes to the inner margin of the isolated yolk mass and this structural arrangement constitutes the omphalallantoic placenta. The isolated yolk mass is bordered externally by a bilaminar omphalopleure. Epithelial cells of the bilaminar omphalopleure of the omphaloplacenta and omphalallantoic placenta are hypertrophied. The apposed uterine epithelium likewise contains enlarged cuboidal or columnar shaped cells and the underlying lamina propria is richly vascularized. This epithelial cell structure suggests that the region of the isolated yolk mass is a site of placental specialization in viviparous reptiles. Further, since these features of omphaloplacentation or omphalallantoic placentation commonly occur in species without apparent morphological specialization in other areas of fetal-uterine contact, it is likely that the bilaminar omphalopleure of the isolated yolk mass is evolutionarily the initial site of placental specialization among viviparous reptiles. © 1993 Wiley-Liss, Inc.
A prominent model for the evolution of placentation among Reptilia is based on placental structure among species in the Eugongylus group of Australian lygosomatine skinks. We studied the development of the extraembryonic membranes of an oviparous species, Bassiana duperreyi, and a viviparous species, Pseudemoia entrecasteauxii, within this taxonomic group. We observed differences in the timing of development of shared features and in the structure of extraembryonic membrane epithelia in the two species. In the viviparous species, there is earlier vascularization of the yolk sâc and increased vascular support for the abembryonic yolk sac splanchnopleure. Structural differences between species result in partitioning of the egg into two distinct hemispheres and produce epithelia which appear functionally histotrophic in both the chorioallantoic membrane and the bilaminar omphalopleure of the viviparous species. We propose that the evolution of placentation in P. entrecasteauxii involved a combination of heterochrony and structural innovation. Further, because our interpretation of placental structure of this species provides new information relevant to placental function, we propose a revision of a classic model for the evolution of placentation among Reptilia. This model predicts specific relationships among reproductive characteristics and thus is testable by comparative analysis among other species within the Eugongylus group of Australian skinks. © 1996 Wiley-Liss, Inc.
S ummary
This is a reveiw of reptilian placentation and other phenomena in the reptilian reproductive cycle concerned with viviparity.
There are three distinct types of placentation so far decribed for reptiles:‐
Type (i). A simple where the partial degneration of maternal and embryonic epithelium allows for a close apposition of maternal and embryonic blood streams. The lizards with this type of placents have no apparent reduction in the yolk‐content of thier eggs at the time of ovulation. These Lizards are Lygosoma (Hinulia) quoyi, L. (Hemiergis) quadridigitatum, Tiliqua scincoides, t. nigrolutea , and from Giacomini's decription Chalcides ocellatus , as well as the snakes Denisonia superba and D.suta.
Type (ii.). A simple type where the maternal capillaries are raised into small folds. Ther grooves between the folds are lined with glandular epithelium, but the cdapillaries themselves are exporsed at the surface of the folods. The underlying chorionic ectoderm may be thickened and glandular. No early stages were available for and examination of the yolk‐content of the eggs. The lizards with this type of placenta are Lygosoma (Liolepisma) pretiosum, L. (L.) ocellatum, and L. (L.) metallicum.
Type (iii.). The most specialized placenta so far described for reptiles, where the uterine wall, over a distinct elliptical area beneth the main longitudinal utering blood‐vessels, is raised into a series of folds filled with capillaries and lined with thickened glandular epithelium and is underlain by an elliptical embryonic area of much thickened chorionie ectoderm. The Lizards with this type of placements have a reduced yolk‐content at the time of ovualion. Thes lizards are Chalcides tridactylus, Lygosoma (Liolepisma) entrecasteauxi, and L. (L.) weekesæ.
It is suggested that (α) in reptiles with and obvious reduction in the yolk‐content of their eggs at the time of ovulation, and its invaribale accompaniment by a speciallized folded glandular area of allanto‐placentation, the function of this specialized placenta, is nutrition; that (b) the funciton of the yolk‐sao placenta is possibly to supply water to the developing egg: and that (c) the function of the simple placenta, so consistently present in lizards with eggs with an apparently yolk‐content, is respiration.
The condition of the large yolk‐laden egg held tighly in a stretched utersu is obviously the most primitive expression of viviparity among repitles. The condition of a samll egge with a reduced yolk‐content is more advanced, and is obviously but a step behind the yolkless egg, such as occures among the eutherian mammals. Hence it is suggested that the function of fixation, although one of the first concerns of the trophoblast of the small mammalian egg to‐day, has probably originated secondariyly to some other funciton, presumably respiration.
It is suggested that with a decrease in the yolk‐content of the egg the yolk‐sac placenta would gradually lose significance as a water absorbing organ, but the virture of its extra‐embryonic circulation, a placenta to funciton for respirtion or nutrition, or both, could be evolved, and it is conceivable that the mammalian yolk‐sac placenta could have been evolved along lines similar to theses.
The specialized placenta. type (iii.) is considered to have evolved from the simple conditon described as type (ii.), and it is thought that the placenta in ungulates (the mammalian non‐deciducte plancenta) may have had origin form some such simple type as type (i.), through a stage in placentation similar to type (iii.), which Giacomini likened to a cow's cotyledon in early stages of development.
The facts are condsidered to justify the conclusion that among reptiles placentation has arisen independently development of similar types of placentas is common.
It ws found that the proportion of viviparous to oviparous reptiles on the Great Dividing Range and the inland plain of south‐easten Australia was extradinarily high, and that the number of oviparous species only approached the number of viviparsous speices on the lower slopes of the Dividing Range, and on the coastal plain.
In the Pyrenees and the French Alps by far the majority of lizards found at high altitudes were viviparous. the oviparous species were practically confined to the mountaind slopes up to about 3000‐4000 feet above sea‐level.
It is suggested that the failure of oviparous species to establish themselves at 4000 feet and over above sea‐level amy be caused by cold interfering with development of the eggs in the nest.
All viviparous speices found on, or recorded from, the inland plain, the mountain slopes, ro the coastal plain of south‐eastern Australia also occur at least 4000 feet above sea‐level on the Great Dividing Range.
Two viviparous speices with teh most highly specialized allanto‐placeta and reproductive cycle yet recorded for reptiles in Australia, and only equlled by the lizard C. tridactylus in Italy, are restricted to 4000‐7000 feet above sea‐level.
Therefore, it is suggested that in south‐eastern Australia, at least, the factros determining the adoption of viviparity may be either (α) definitely associated with the altitudes to which these lizards are restricted or (b) may work most efficently under the conditons existing at such altitudes.
Cold is suggested as the most likely external factor associated with high altitudes that may influence either directly or indircectly the adoption of viviparity.
The placental memebranes and uterus of the garter snake, Thamnophis sirtalis, were studied using histological, histochemical, electron microscopic, dye transfer, and radioisotopic techniques. The conceptuses are completely enclosed throughout gestation by a transparent shell membrane which is produced by glandular epithelia in the uterine segment of the oviduct.
Both chorio-allantoic and omphalo (yolk-sac) placentation are observed in this snake. The growth of the extra-embryonic mesoderm takes place in a manner peculiar to placental reptiles, and results in the isolation of the omphaloplacenta from the yolk-sac wall. On the basis of morphology, enzyme histochemistry, and phagocytosis of Trypan blue particles, this structure is interpreted as a site of histiotrophic absorption.
The chorio-allantoic region of placentation is simple in structure. Fetal and maternal capillaries are closely apposed but always separated by layers of uterine and chorionic epithelium and the thin shell membrane. The placental membranes of the garter snake are similar in many respects to those of other live-bearing snakes, but less specialized than most lizard placentate.
Isotopically labelled sodium and glycine are passed to the fetus following maternal injection, the latter at least apparently via the omphaloplacenta. The permeability to iron and phospate is extremely low. On the basis of these results and the selective transfer of certain dyestuffs, it appears that the shell membrane functions as a dialyzing membrane.
The composition of egg yolks and neonates of the viviparous lizard, Pseudemoia pagenstecheri, one of the most placentotrophic reptiles studied to date, are described. Neonates (43.3 ± 5.2 mg) have twice the dry mass of the initial eggs (22.0 ± 1.9 mg). The protein content of neonates (29.1 ± 1.1 mg) is more than twice that of eggs (12.2 ± 1.1 mg), while the energy content (908.1 ± 107.4 J) is 1.6 times higher than that of the egg (565.0 ± 42.9 J). The energy densities of eggs (27.5 kJ g−1) and neonates (23.1 ± 0.3 kJ g−1) are similar to the energy densities of eggs and neonates of oviparous species. The total ash per neonate (4.1 ± 0.4 mg) is three times greater than that of the egg contents (1.4 ± 0.2). Neonates contain significantly more calcium, sodium and potassium, but not magnesium, than do eggs. Thus, the placenta has a quantitatively important role in supplying nutrients for the embryo. The proportions of triacylglycerol (66%), phospholipid (19%), and free cholesterol (5%) in the eggs are similar to those in eggs of birds and crocodilians, but the proportion of cholesteryl esters (7%) is much higher in eggs of P. pagenstecheri. The proportion of docosahexaenoic acid in the egg phospholipid is relatively low (1.4%) but rises to 5.4% in the neonate. The eggs contain vitamin E (mainly in the form of -tocopherol) and vitamin A, but no detectable carotenoids. The overall composition of the eggs is not substantially different from that of oviparous species, suggesting that the small egg size relative to neonate size is a result of a reduction in egg size rather than modification by omission of some nutrients from the yolk. The pattern of placental nutrient provision of P. pagenstecheri contains both an obligate and a facultative component suggesting that enhancement of offspring quality through facultative placentotrophy is a general characteristic of placental reptiles independent of pattern of embryonic nutrient provision.
From microphotometric studies of the Raman spectrum of diamond, the distribution of intensity in it has been derived. The
spectra themselves exhibit much observable fine structure, including several sharply-defined and intense lines. The intensity
curve also exhibits steep drops to zero near the frequency shifts 2176 and 2665 cm.−1 These facts are discussed and it is shown that the features mentioned, as also the appearance of second-order Raman spectra
generally with crystals, are irreconcilable with the Born-Karman theory of lattice vibrations.
While yolk is generally the primary source of embryo nutrients in squamates, numerous species supplement this with facultative placentotrophy. We argue that facultative placentotrophy should have selective importance relevant to offspring fitness. In the skink Niveoscincus metallicus, the size of ovulated eggs is unrelated to maternal size but large females produce offspring that are larger than is necessary for survival, providing evidence for facultative placentotrophy. We discuss the circumstances in which facultative placentotrophy might be used to supplement the nutritional support provided by yolk and obligate placentotrophy in this species, and present summary data from experiments designed to investigate these circumstances. Clutch reduction by oviduct removal had no effect on neonate mass or snout-vent length, indicating that the number of embryos does not influence allocation of maternal resources once gestation has commenced. Manipulation of maternal basking opportunity in combination with food intake during pregnancy suggested that an important role of facultative placentotrophy is the optimization of embryonic fat reserves. This hypothesis was supported by the observation that larger neonates have larger abdominal fat bodies. These reserves presumably facilitate survival in the relatively short pre-hibernatory period available to newborn animals. Our data indicate that they also play a vital role in maintaining pre-natal condition if birth is delayed by adverse weather, a common circumstance in this species. In such circumstances the yolk has been used up and the placental membranes have degenerated. Experimental induction of premature ovulation of eggs with reduced yolk, achieved by injecting females with FSH, was followed by fertilization using stored sperm. Gestation length was greatly reduced and the resulting neonates were all ≤75% normal birth mass, with two of the six births being stillborn. Thus facultative placentotrophy does not appear to be a means of compensating for a poor yolk supply. We suggest that facultative placentotrophy in N. metallicus is not a transitional stage en route to greater reliance on obligate placentotrophy, but a uniquely squamate adaptation that provides flexibility in embryonic nutrition, and optimizes offspring fitness in an unpredictable temperate climate.
Thesis (doctoral)--Universiteit Utrecht, 1933.
Use of the term "trophoblast" in descriptions of therian (marsupial and eutherian) mammals has caused confusion because of misinterpretations of blastular homologies and because of imprecise application in functional versus ontogenetic-phylogenetic senses. Marsupials follow the plan of early development characteristic of noneutherian amniotes. Eutherians, in contrast, are unique in the early determination of presumptive embryonic versus extraembryonic cells through formation of inner cell mass versus trophoblastic (or trophectodermal) tissues, respectively. No cellular unit of the eutherian blastula is recognizable unequivocally as the homologue of a specific part of the protodermal marsupial blastula; progressive deletion of innovative but phylogenetically older ontogenetic steps probably figured importantly in the evolution of eutherian early embryogenesis. Because of marked differences in mode of formation and in cellular fates, homology of the blastocoel between marsupials and eutherians is questioned. It is suggested that use of the term "trophoblast" be restricted to eutherians in discussions of ontogenesis or phylogenesis, and be deemphasized in the functional sense (i.e., fetal-maternal exchanges) for marsupials, in favor of the more appropriate tissue terms of "choriovitelline" and "chorioallantoic" membranes. Integral to the origin of the eutherian style of embryogenesis was the evolution during Cretaceous time of neomorphic, extraembryonic tissues (i.e., trophoblast) having physiological properties that allowed the unique combination of intimate apposition of fetal and maternal tissues and circulatory systems, along with sustained, active morphogenesis. Marsupials have not achieved such a combination.
Throughout most of normal pregnancy 15 to 20 per cent of the cells of the trophectodermal epithelium of the cow, deer, goat and sheep placentomes were found to be binucleate when a counting technique suitable for electron microscopy was used. Of this binucleate cell population about one-fifth were discovered to be migrating up to and across the microvillar junction at all stages of pregnancy. This migration appears to serve at least two functions: the transfer of the characteristic granules of the fetal binucleate cell to the maternal circulation; and the formation of the placentomal syncytium bounding the maternal connective tissue in sheep and goat.