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Generative rules for the morphogenesis of epithelial tubes
Abstract
A finite elements model imitating the morphogenesis of smoothly curved tubular epithelial rudiments is suggested. It is based upon the experimentally proved assumption of the lateral (tangential) pressure between adjacent epithelial cells. The main idea of the model is that under a non-zero local curvature the lateral cell-cell pressure acquires the radial components which are absent under zero curvature. In the framework of the model we investigate the roles of initial geometry, the different coefficients relating the local curvatures and radial cell shifts, and of visco-elastical cell-cell linkages in the shaping process. We also employ the different temporal regimes (both periodical and constant) of the lateral pressure exerted and the different overall durations of the modelling. As a result, we get a set of biologically realistical shapes, almost all of them belonging to the same basical "trefoiled" archetype. Among the variables explored, shaping was most affected by the changes in visco-elastical coefficients, in the temporal regimes and in the overall duration of the modelling. The model shows that rather complicated and realistical shapes of epithelial rudiments can be obtained without assuming any initial regional differences inside cell layers. The model may be useful for understanding the principles underlying both genetical and epigenetical regulation of the morphogenesis.
... The same inherent tendency to form a tube or a sphere has been observed with dissociated cells of the amphibian neural plate (Townes and Holtfreter, 1955) or in vitro, where neural tube-like structures called neural rosettes form from a teratocarcinoma-derived cell line, through a morphogenetic pathway involving extracellular matrix components (Kawata et al., 1991). The fact that similar structures arise from very different developmental pathways suggests that common generative rules are used that are based heavily on generic biophysical determinants of form (Beloussov and Lakirev, 1991). ...
... For example, uniform simultaneous wedging within the width of the neural plate during primary neurulation would generate a tube having a circular cross-sectional morphology. A lack of uniform wedging or regional changes in its timing would alter the cross-sectional morphology of the neural tube (Lewis, 1947;Schoenwolf, 1982;Nagele and Lee, 1987;Beloussov and Lakirev, 1991). ...
... Regional (and species) differences in the morphogenesis of the neural tube are likely to arise from the activity of unique gene products. These molecules would directly establish the ultimate conformation of the neural tube by influencing the timing of morphogenetic events and regulating the type and magnitude of cell behaviors that occur (Beloussov and Lakirev, 1991;Steinberg, 1998). ...
Neurulation occurs during the early embryogenesis of chordates, and it results in the formation of the neural tube, a dorsal hollow nerve cord that constitutes the rudiment of the entire adult central nervous system. The goal of studies on neurulation is to understand its tissue, cellular and molecular basis, as well as how neurulation is perturbed during the formation of neural tube defects. The tissue basis of neurulation consists of a series of coordinated morphogenetic movements within the primitive streak (e.g., regression of Hensen's node) and nascent primary germ layers formed during gastrulation. Signaling occurs between Hensen's node and the nascent ectoderm, initiating neurulation by inducing the neural plate (i.e., actually, by suppressing development of the epidermal ectoderm). Tissue movements subsequently result in shaping and bending of the neural plate and closure of the neural groove. The cellular basis of the tissue movements of neurulation consists of changes in the behavior of the constituent cells; namely, changes in cell number, position, shape, size and adhesion. Neurulation, like any morphogenetic event, occurs within the milieu of generic biophysical determinants of form present in all living tissues. Such forces govern and to some degree control morphogenesis in a tissue-autonomous manner. The molecular basis of neurulation remains largely unknown, but we suggest that neurulation genes have evolved to work in concert with such determinants, so that appropriate changes occur in the behaviors of the correct populations of cells at the correct time, maximizing the efficiency of neurulation and leading to heritable species- and axial-differences in this process. In this article, we review the tissue and cellular basis of neurulation and provide strategies to determine its molecular basis. We expect that such strategies will lead to the identification in the near future of critical neurulation genes, genes that when mutated perturb neurulation in a highly specific and predictable fashion and cause neurulation defects, thereby contributing to the formation of neural tube defects.
... As shown by modeling (Beloussov and Lakirev, 1991), these very same forces are sufficient for transforming even quite smooth initial shapes into much more complicated and realistic ones, the shell's elasticity and the patterns of pressure pulsations being the main ordering parameters (Fig. 4). Is there any need for PI in this construction? ...
... Shapes a-d and e -h are arranged from left to the right proportionally to the growth pulsations' parameters (the extension/retraction ratios per pulsation). As a result, without using any kind of a positional information gradient a family of rather realistic shapes is obtained (fromBeloussov and Lakirev, 1991). ...
Two alternative versions of interpreting the developmental events are discussed. The first of them regards the development as a set of highly specific steps each of them being caused by a unique special force, or an “instruction”. By this version, nothing outside the rigidly determined chain of events is presented, and the ultimate aim of a researcher is in making a list of specific instructions. The second version is centered around the notion of an extended spatio-temporal continuum (morphogenetic field). Any developmental trajectory is now considered to be the function of this continuum’s geometry in Euclidean and/or phase space. Within the context of such an alternative we review the classical embryological data related to inductive phenomena and embryonic regulations. The contours of a morphogenetic field theory are sketched.
... Among all developmental biologists, Lev Beloussov accomplished the most toward mapping tensions in the surfaces of living embryonic cells (Beloussov and Lakirev, 1991;Beloussov et al., 1997;Beloussov, 1998;Beloussov, 2008Beloussov, ,2012. One of his methods was to poke small circular holes in cell surfaces and measure the gaping around each pin prick. ...
A major step forward for developmental biology will be accomplished when someone figures out how to extend the concept of homeostasis to apply to shapes, in the sense of geometric properties of cells, tissues and organs. I propose that the biggest obstacle to this forward step is that biological researchers are not yet familiar with the properties of tensor variables, as compared with scalars. This key difference is that tensor properties can and usually do have different amounts in different directions, whereas scalar properties cannot vary with direction. Examples of tensor variables include stress, strain, curvature, permeability, and stiffness. Examples of scalar properties include chemical concentrations, osmotic pressure, hydrostatic pressure, adhesiveness and electrical voltage. Even D'Arcy Thompson treated mechanical tension (which is the classic example of a tensor variable) as if it were a scalar constant. This greatly reduced the number of geometric shapes that he could explain as being directly produced by forces. For example, in order to generate cylinders, surface contractions need to be twice as strong in one direction as compared with the perpendicular direction. Unless surface contractions vary with direction, only spheres can be generated.
Another example of not distinguishing tensors from scalars is the use of suction pipettes to measure stresses of cell surfaces (for example, during cytokinesis). This method of measurement inescapably lumps together directional components of two different tensors (tension and stiffness) as if they were one scalar. Yet another obstacle was that certain scientists argued persuasively, but mistakenly, that attractor basins were evidence of minimization of thermodynamic free energy.
Chemical concentrations have no special ability to generate gradients. Neither do any other scalar variables. As will be discussed below, repeated local equilibration of any quantitative variable will generate at least as good a gradient as diffusion can. It is misguided to think of scalars as being in any sense more quantitative than tensors. In fact, tensor variables can convey more information than chemical gradients, often faster and with less vulnerability to disturbance.
... It has been discussed that organogenesis and neurulation in particular depend on generic biophysical determinants of form acting in epithelial rudiments, such as cell-adhesion-generated tissue surface tensions, gravitational effects, viscosity and elasticity (Newman & Comper 1990;Foty et al. 1996;Forgacs et al. 1998;Dias et al. 2014; reviewed by Beloussov & Lakirev 1991;Colas & Schoenwolf 2001;Kondo 2014). At the cellular level neurulation depends on a combination of internal and external factors that shape the neural plate. ...
Neurulation is defined as a process of neural tube closure. Recent reports suggested that upon completion of this process the major factors of neurulation remain in force at least until the central canal of the neural tube is formed. Hence, an idea has been put forward to define the two periods of neurulation: early neurulation corresponds to the period of neural tube closure and late neurulation corresponds to the period of formation of the central canal. These ideas are discussed in a context of neural tube defects that may affect late neurulation and result in distention of the central canal.
... Hence, the change of shape and form (the morphogenesis of its own!) which is thought to be a consequence of these primary patterns, was not considered. However other approaches have considered the geometric changes due to morphogenetic movements of epithelial surfaces [Gierer, 1977; Odell et al., 1981; Mittenthal and Mazo, 1983; Belintsev et al., 1987; Beloussov, 1991]. Morphogenesis, as distinct self-organising processes, requires effective nonlinear feedback between its dynamic components [Beloussov et al., 1994]. ...
We approach the problem of how the information encoded in linear DNA molecule becomes translated into a three-dimensional form from position of Pattern-Form Interplay Models. The characteristic feature of these models is the existence of feedback loops from (bio)chemical pattern formation to modeling embryo form changes. In accordance with the model the system is open and changes in a pattern give rise to changes in form and these changes in form (surface geometry) cause further pattern changes, and so on.
Spontaneous pattern formation takes place in the model as primary and secondary bifurcations of nonlinear parabolic PDEs describing reaction-diffusion systems with imposed gradient. We briefly review the main results of previous works and consider the phenomenon of axis tilting as a case of symmetry breaking via secondary bifurcations. The axis tilting bifurcation occurs as a consequence of position-dependency of diffusion coefficients. The explicit demonstration of this phenomenon in Pattern-Form Interplay Models is believed to be new.
... The geometric shape of the substrate upon which cells reside has crucial implications for their future behavior (Chen et al., 1997(Chen et al., , 1998Huang and Ingber, 2000); this geometry is an ideal example of a signal that cannot be described by genetic or proteomic profiling alone. Additional physical properties that can serve similar functions include mechanical properties of tissues Beloussov and Grabovsky, 2007;Beloussov and Lakirev, 1991;Beloussov et al., 2000Beloussov et al., , 1997Brodland et al., 1994;Discher et al., 2005;Savic et al., 1986), ultraweak photon emission Popp, 2003), and bioelectrical gradients (Levin, 2007b(Levin, , 2009(Levin, , 2011a. ...
Establishment of shape during embryonic development, and the maintenance of shape against injury or tumorigenesis, requires constant coordination of cell behaviors toward the patterning needs of the host organism. Molecular cell biology and genetics have made great strides in understanding the mechanisms that regulate cell function. However, generalized rational control of shape is still largely beyond our current capabilities. Significant instructive signals function at long range to provide positional information and other cues to regulate organism-wide systems properties like anatomical polarity and size control. Is complex morphogenesis best understood as the emergent property of local cell interactions, or as the outcome of a computational process that is guided by a physically encoded map or template of the final goal state? Here I review recent data and molecular mechanisms relevant to morphogenetic fields: large-scale systems of physical properties that have been proposed to store patterning information during embryogenesis, regenerative repair, and cancer suppression that ultimately controls anatomy. Placing special emphasis on the role of endogenous bioelectric signals as an important component of the morphogenetic field, I speculate on novel approaches for the computational modeling and control of these fields with applications to synthetic biology, regenerative medicine, and evolutionary developmental biology.
... They concluded that several rules of cell behavior operate simultaneously during neurulation in the frog. A similar approach was pursued by Beloussov and Lakirev (1991), who modeled an epithelium as a shell containing movable elements. In this model, the radial displacement of each element depends on the resultant force acting on that element. ...
This review deals with biomechanical aspects of growth (mass change), remodeling (property change), and morphogenesis (shape change) in living systems. The emphasis is on theoretical models, but relevant experimental results also are discussed. As an aid to the reader, the fundamental biological terms and concepts are defined for the general problem and for each specific topic. At the outset, the processes involved in growth, remodeling, and morphogenesis are described and placed within the context of the evolution of species. Next, some of the analytical methods used in biomechanical models for these processes are presented. Then, applications of these and other techniques to specific systems are discussed, beginning at the cellular level and proceeding upward to the tissue and organ levels. At the cellular level, modeling and experimental studies are reviewed for cell division, cell movement, and pattern formation, and then morphogenetic mechanisms for epithelia (cell sheets) are discussed. At the tissue and organ levels, the musculoskeletal and cardiovascular systems are considered. Several models are described for growth, remodeling, and morphogenesis of bone, and mainly experimental results are examined in the cases of skeletal muscle, the heart, and arteries. Specific topics for the cardiovascular system include hypertrophy, residual stress, atherosclerosis, and embryonic development. Finally, some future research directions are suggested.
In modern science, a most adequate conceptual framework for treating the behaviour of complex dynamic systems is given by the theory of self-organization (e.g., Prigogine, 1980). The developing organisms may be definitely attributed to self-organizing entities by a number of criteria and, above all, by their capacity for spontaneous breaks of the symmetry order. We define those breaks of macroscopical symmetry as spontaneous which do not imply any definite macroscopical causes (dissymmetrizators), let they be located outside or inside the embryo. As is well established by descriptive and experimental embryology, such symmetry breaks are taking place not only at the level of a visible morphology, but also within the phase space of the developmental potencies. The latter means that embryonic development is always associated with a progressive narrowing and specification of the morphogenetical potencies initially delocalized throughout embryonic space.
Both for an experienced and for a naive observer the development of a living sample, be it plant or animal, looks, first of all, as a regular succession of complicated changes in the shapes and mutual arrangement of its parts; such a succession is usually defined as a morphogenesis while its components as morphogenetic processes. Invaginations, evaginations and the bending of epithelial layers, condensations of freely moving mesenchymal cells, as well as the changes in shapes and overall proportions of the large masses of almost immobile plant cells may serve as the examples of morphogenetic processes. As was shown by the molecular biology within several last decades, all of these processes are based upon a highly regulated motile activity of the molecular and supramolecular components of the living cells. In the first approximation, all of these processes may be considered as mechanical, what means that they are associated with the production of mechanical forces and changes in space positions of the material constituents.
Neurulation is traditionally defined as the process of closure of the neural tube. New data have shown that the major driving forces of neurulation continue to operate with the closure of the neural tube, at least until the central canal of the neural tube has formed. Owing to this, the paper proposes to distinguish two periods of neurulation. According to these notions, early neurulation corresponds to the period of closure of the neural tube, and late neurulation corresponds to the period of formation of the central canal. Examples of neural tube defects that affect late neurulation are discussed.
Any theoretical construction in morphological modeling is useful only when it can be linked to the practice. Any formalism is not optimal for describing the processes of morphogenesis, if it is not comparable with the shape of tissue structures. Thus, it is necessary to find the best approximation for the correct comparison of the experimental and theoretical results. Proposed in this paper, the use of test functions for genetic algorithms, evolutionary programming, and swarm optimization for the approximation of the morphogenesis of cellular structures and their models is a mathematical step towards the implementation of the thesis of the analyzed article author (Gradov O.V., 2011), deduced not precise enough. There are other ways of analytical approximation for this case, but they have no fundamental differences in terms of their ease of use in mathematical biology. Achieved in this way comparability of morphometric and model histogenetic-morphogenetic data can be used in mathematical and morphological analysis and modeling in histology and embryology.
We take a fresh look at diatoms, and find a reasonable visual match of some valve and girdle contours to various kinds of
buckling phenomena, describable as corrugations, pleats and folds, and Bessel functions, such as describe the vibrations of
a round musical drum. Buckling of raphes and costae, considered as buckling of columns, may account for some of their features.
The matches we find will have to be tested by finite element method, computer simulation, time sequence microscopy, micromanipulation,
and direct measurement of the physical (constitutive) properties of the materials from which a diatom constructs itself. The
genome may set up boundary conditions that lead to mechanical instabilities and buckling, rather than controlling these diatom
patterns in any direct way. This investigation also led us to propose that there is an electric potential in the silicalemma
that may account for the generally straight course of growth of costae.
From the very beginning of their evolutionary history the primitive organisms have been confronted with the numerous constraints of external environment, and among them the mechanical ones, associated with the increase of intracellular osmotically‐driven hydrostatic pressure. In order to withstand it, the organisms had to develop a set of stress‐dependent reactions directed to the decrease of cell surface tension. Due to robustness of these reactions, they could led to the development of a surface pressure. The main idea of this paper is that such an initially adaptive reaction may create a basis for a large category of morphogenesis which result in the formation of structurally stable and biologically realistical shapes out of geometrically simple configurations. The attempt is made to embrace a whole set of stress‐dependent reactions by a principle of hyperrestoration, suggesting a tissue tendency to “hyperrestore” its initial mechanical stresses after any disturbances. The evolutionary formation of a Vertebrate body plan is considered as a chain of such reactions. The cognitive and, in particular, the self‐cognitive character of organic evolution expresses itself in the genetical and epigenetical memorization and reinforcement of evolutionary ancient and standard reactions to external and internal constraints.
This is a review of studies on morphogenesis carried out at the Department of Embryology, Moscow State University, over the
past 30 years. The main direction of studies has been to reveal and describe the properties of self-organizing fields of mechanical
stresses in developing embryos.
A model of the development of living organisms is presented which couples the geometry of the middle surface of a (closed) epithelial surface to a “morphogen”, and the morphogen in turn is affected by the geometry of the surface in a closed system of two partial differential equations. A number of “desiderata” are set out for the identification of a suitable morphogen. Four parameters are involved in the model, which are assumed to be under genetic control. The morphogen is pictured as an adhesive molecule on the cell surface, and a suggestion is made for a particular molecular manifestation of this morphogen. A simple picture of gastrulation is presented as an example of the formalism.
Ce travail de recherche a eu comme objectif principal la conception d'un modèle numérique aux éléments finis donnant une représentation réaliste des mouvements de l'embryon de la Drosophila Melanogaster. Les simulations de trois mouvements durant la phase de gastrulation de l'embryon ont été realisées soit individuelles soit simultanées, ce qui jusqu'à présent, n'a jamais été proposé, constituant ainsi une contribution originale de cette étude. La thèse est composée de quatre chapitres. Le premier fournit une brève mais assez complète description du contexte dans lequel ce travail se situe. Le processus complexe de l'embryogénèse chez la Drosophila Melanogaster est presenté en se focalisant sur les trois mouvements morphogénetiques qui seront ensuite simulés numériquement: l'invagination du sillon ventral, la formation du sillon céphalique et l'extension de la bande germinale. Chaque événement est décrit du point de vue biologique et mécanique, ce qui permet donc de mettre en avant les aspects les plus intéressants des différents mouvements. Une revue des plus récents travaux est aussi proposée. Dans le deuxième chapitre on présente les outils analytiques pour l'analyse du problème dans son intégrité. Etant donnée la complexité du système biologique, plusieurs hypothèses ont été introduites pour simplifier l'approche numérique utilisée. Seul le mésoderme est modélisé comme un milieu continu dans un espace tridimensionel par un ellipsoïde épais régulier de 500 µm de longueur. La méthode de la décomposition du gradient de déformation, dont quelques interprétations alternatives sont présentées, permet de coupler les déformations passives et actives subies par chaque point matériel du milieu. L'équilibre mécanique est écrit à partir du Principe des Puissances Virtuelles: les forces internes du système sont donc prises en compte avec les conditions aux limites. Dans notre cas particulier celles-ci sont fondamentales pour obtenir des configurations finales réalistes et comprennent le contact entre le mésoderme et la membrane vitelline externe et le pression exercée par le yolk sur la surface interne du mésoderme. Les propriétés mécaniques des tissus embryonnaires ne sont pas faciles à déterminer expérimentalement. Une approximation a été faite pour ce qui concerne la loi de comportement du mésoderme qui a été modélisé comme un matériau de Saint-Venant linéaire, élastique et isotrope. Notre choix étant en contraste avec le modèle hyperélastique qu'on retrouve souvent en littérature, une comparaison entre les deux matériaux est proposée tout en considérant les avantages et les limitations de notre démarche. La méthode de la décomposition du gradient de déformation a été auparavant testée sur des cas géométriquement très simples dont la solution analytique peut être facilement calculée et validée par les résultats obtenus à partir des simulations numériques. Le troisième chapitre peut être divisé en deux parties distinctes. Dans la première, grâce à la description paramétrique de l'ellipsoïde qui représente l'embryon, on calcule les expressions analytiques des positions intermédiaires où on voit apparaître les déformations actives responsables de chaque mouvement morphogénétique. Les gradients de déformation active sont donc couplés avec les gradients passifs pour obtenir la déformation finale. La deuxième partie du chapitre concerne l'analyse des résultats pour les simulations individuelles des événements. Pour la simulation de l'invagination du sillon ventral une étude paramétrique a été conduite pour évaluer l'influence de certains paramètres sur la configuration finale. Pour la simulation de l'extension de la bande germinale les résultats ont été comparés avec les données expérimentales. En particulier on s'est intéressé à l'analyse des contraintes mécaniques (les pressions et les contraintes de cisaillement) induites au niveau du pôle antérieur où un chemin de mécanotransduction aurait lieu et conduirait à l'expression du twist, un gène normalement exprimé seulement dans la partie ventrale de l'embryon. Pour conclure, d'autres géométries que celle de l'ellipsoïde ont été utilisées pour les simulations de l'invagination du sillon ventral et de l'extension de la bande germinale. Ces nouvelles représentations de l'embryon permettent de prendre en compte deux aspects intéressants: d'un côté l'arrondissement des deux pôles, de l'autre l'aplatissement de la partie dorsal par rapport à la partie ventrale. Le dernier chapitre du manuscrit introduit la simulation simultanée des trois mouvements qui a été mise en place pour deux raisons principales. Tout d'abord le fait que les événements analysés se produisent l'un après l'autre lors du développement de l'embryon. Deuxièmement, les résultats obtenus pour les simulations individuelles sont très encourageants et ont permis aussi de confirmer plusieurs hypothèses avancées par les biologistes; d'où l'intérêt de coupler les mouvements pour permettre une vision encore plus réaliste de cette phase importante de la gastrulation chez l'embryon de la Drosophila Melanogaster. Deux méthodes différentes ont été testées. La première, la plus intuitive et simple, permet de combiner les gradients de déformation active de chaque mouvement et ne requiert pas de manipulations supplémentaires des équations précédemment trouvées, tout en prenant en compte le déphasage réel entre les événements. Cette approche ne pose pas de problèmes quand seulement les deux sillons sont couplés, alors que l'introduction de l'extension de la bande germinale donne lieu à quelque limitations. Une nouvelle démarche est donc proposée, plus rigoureuse et précise, qui nous a permis de considérer certains aspects importants pas encore développés d'un point de vue théorique.
One of the most promising trends in modern developmental and cell biology, recently defined as , or , is directed towards revealing the role of mechanical stresses, chemomechanical transduction and active stress responses of cells antissues of developing embryos. We review here the results obtained in this field by our research group and compare them with those from other labs. Our studies relate to the buds of hydroid polypes and to amphibian embryos. We describe the space-temporal patterns of mechanical stresses in these species, analyze their morphogenetical role and the tissue responses to the experimental modulations of stress patterns. In hydroid polypes we explore also the molecular events involved in mechanochemical coupling. A model, linking the passive mechanical stresses with the active stress-responses of embryonic tissues is suggested. We consider these investigations as a first approach to a developing embryo as to an .
The formation of the neural tube (neurulation) depends on the physical properties of the cells and tissues both inside and outside the neural plate. One such important physical property is cell adhesion. Theoretical and biological evidence support a role for cell adhesion in neurulation, but few specific cell adhesion molecules have been identified during this process. Ephrin-A5 and Integrin alpha6 are two of the known genes encoding cell adhesion molecules that are likely to be directly involved in neurulation because neural tube defects result when they are knocked out in mice. Yet it remains unclear how they can act on the cell and tissue behaviors of neurulation, because their domains of expression in neurulating tissues have not been reported. We report here the detailed pattern of expression of these two cell adhesion molecules in the chick embryo throughout the stages of neurulation at the mRNA and protein level. We show that Ephrin-A5 and Integrin alpha6 are differentially expressed in the ectoderm, outside and inside the neural plate, respectively, and that they are both restricted to neurulation at cranial (brain) levels. We discuss the potential contribution of this differential expression to the cell adhesion mechanisms involved in cranial neurulation and anencephaly.
We present a biomechanical model of morphogenesis highlighting the extensive formative capacities of stressed networks with a very simple initial geometry. They consist of a restricted number of kinematically independent elements exerting a pressure to each other and increasing thus the local curvatures. The pressure is applied as a series of periodic impulses and is opposed by a constant quasi-elastic resistance force. Single elements can be also regarded as the half wave-lengths of the undulations determined by the mechanical properties of a given body. All of the model parameters are assumed to be evenly spread throughout a body (no prepatterns are implied). On the other hand, the model parameters can be associated with genetic factors. Thus, our model relates to as yet unsolved problem of genetic regulation of shape formation. We classify the modeled shapes according to their symmetry orders and compare them with the ancient Echinodermata and with Arthropods. Possible evolutionary and developmental implications are discussed.
Growth pulsations (GP) in hydroid polyps are associated with changes in vacuolar patterns which can be imitated by altering external osmolarity. With the use of X-ray spectroscopy we measured the elemental contents in the vacuoles and cytoplasm of the growing tips of a hydroid polyp, Podocoryne carnea, under various tonicity conditions. Under hypertonic condition which arrested the samples at the retraction phase of normal GP, the elemental content within the vacuolar compartment appeared to be similar to that of the external medium, confirming our previous conclusion about the dehermetization of the vacuolar compartment under these conditions. Under hypotonical condition which arrested samples at the extension GP phase (vacuoles isolated) element ratio data displayed an obvious bimodality. At least one of the data groups could be characterized by a significant increase in the concentrations of sodium and potassium, as related to Cl, Ca and Mg, and in comparison to the same ratios in hypotonical samples and those in the external medium. We suggest that under hypotonical conditions the isolated vacuolar compartment is formed by influx of sodium and potassium ions. These cations are accompanied by anions other than chloride. Potassium appears to be transferred into the vacuoles from the cytoplasm while the sodium derives from the external environment.
1. Growth, morphogenesis and cell movements were studied in Obelia loveni, O. geniculata and Dynamena pumila with the use of time-lapse cinematography, visual observations of vitally stained objects and by histological techniques.
2. Growth pulsations with the period around 14min and the amplitude around 15 μm exist in Dynamena pumila and with the period 5–8 min and amplitude up to 5 μm in Obelia loveni. It was demonstrated that the rhythm of growth pulsations does not coincide with the rhythm of periodical contractions of the proximal part of coenosarc.
3. The distalwards movements of individual cells in the ectoderm of growing stems and hydranth rudiments are described. A considerable variability in the rates of movements of ectodermal cells has been demonstrated.
4. Different kinds of cell reorientations in developing rudiments are described. As a rule, they precede the alterations of growth directions or of rudiment shapes.
5. The mechanisms involved in deformations of epithelial layers are discussed.
6. The possibility of the existence of passive, elastico-plastic structures in the deforming epithelial sheets is suggested.
Cell orientation and intercellular changes accompanying growth pulsations (GP) have been studied in the marine hydroids Obelia longissima, O. loveni, and Dynamena pumila with the use of time-lapse filming, mechanography, optical microscopy, and electron microscopy. The extension phase of the GP was correlated with an increase in cell volume and the rotation of cells to a transverse orientation. Tip cells return to an oblique orientation during the retraction phase via a rapid (about 1 μm/second) distalward sliding of external cell poles. In most samples, a proximodistal wave of transversad cell rotations was observed within a period of 30–90 μm/minutes. In contrast, return to the oblique orientation is almost synchronous. The extension GP phase is correlated with extensive cell vacuolization and the retraction phase with fusion of these vacuoles into elongated channels opening into external space.
The extension phase was stabilized in hypotonic medium, isotonic medium with increased NaCl concentration, and by ionic transport inhibitors (which increase cytoplasmic concentration of Na+ and Cl−). GP are arrested in the retraction phase in hypertonic medium, isotonic medium with decreased Na+ and Cl− concentration, and by inhibitors whose decreasing cytoplasmic concentrations of Na+ and Cl− arrest GP in the retraction phase. These data point out the participation of osmotic mechanisms in the regulation of GP.
The form of multicellular animals and their organs is mainly defined by the curvature of cell layers. They are boundaries for solid tissues; and some organs and organisms consist mainly of distinct cell layers. The form of adult organisms results from a complex interplay of tissue evagination, growth patterns, production of and interaction with extracellular material, and other effects; but the rudiments and basic features of the forms produced can often be traced back to processes of evagination or invagination of nearly flat cell sheets at defined location in the course of embryogenesis. Though there has been considerable progress in the study of intercellular interactions and intracellular features affecting cell form, it is still not known which molecular mechanisms are the main determinants of tissue morphogenesis. A theory of biological form should be based on physical-chemical parameters which are interpretable by molecular mechanisms, but at the same time permit derivation of evagination and form of multicellular tissues. This paper is an attempt to provide a framework for such a physical theory.
An extracellular matrix (ECM) lies between the upper and lower epithelial layers of the wing imaginal discs of moths. Organization and composition of this extracellular matrix, as revealed by staining with ruthenium red, tannic acid, and alcian blue, changes in concert with levels of hormones in the haemolymph. The ECM of the wing imaginal disc is an environment for cellular movements. Reorganization of the matrix and increase in ecdysteroid level is coupled with the proximal----distal migration of tracheal cells as well as the distal----proximal outgrowth of sensory neurons.
The behaviour of bottle cells in normal and microsurgically altered gastrulae and in cultured explants of Xenopus laevis was analysed, using time-lapse micrography, scanning electron microscopy (SEM) and cell tracing with fluorescein dextran amine (FDA). The results shed new light on the function of bottle cells. Bottle cells forming in vivo show a predominantly animal-vegetal apical contraction and a concurrent apical-basal elongation, whereas those forming in cultured explants show uniform apical contraction and remain rotund. Bottle cells forming in embryos with fewer subblastoporal cells contract more uniformly than those in normal embryos and release of normal bottle cells from supra- and subblastoporal cells results in immediate loss of the bottle shape. These results, and an analysis of the effects of bottle cell formation on the shapes and movements of surrounding tissues, show that unique shape of bottle cells and their probable function in development are not intrinsic properties but result from a modulation of the effect of a uniform and intrinsic apical contraction by the geometric and mechanical properties of the surrounding tissue. Mechanical simulations of bottle cell formation, using the finite element method, suggest how the site of bottle cell formation and the thickness and stiffness of adjacent tissues might change the effects of their formation. These results and FDA marking of prospective bottle cells and the adjacent deep mesodermal cells suggest that bottle cells function during their formation to initiate the involution of the prospective mesodermal mantle. Later they respread to deepen the archenteron and to form its peripheral wall.
It was previously thought that bud elongation was caused by cell division in the budding zone. Studies on hydra show, however, that reorientation and repolarization of tissues, and morphogenetic movement, are the principal mechanisms.
Embryos, Genes and Evolution
- R A Kaufman
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