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Growth pulsations in hydroid polyps: Kinematics, biological role and cytophysiology
... In recent years great progress has been achieved in revealing the molecular mechanics and genetic regulation of these events. Cell movements and shape changes are mediated by contraction of actomyosin filaments associated with membrane dynamics, polymerization of microtubules (Alberts et al., 2004) and periodic changes of osmotic pressure within cells, cell vacuoles and intercellular cavities (Stern, 1984; Beloussov et al., 1993). These events, in turn, are regulated by complicated signaling pathways involving extracellular matrix, integrins, cell junctions, probably mechanosensitive ion channels and Rho machinery (Banes et al., 1995; Chrzanowska-Wodnicka & Burridge, 1996; Hunter, 2000; Balaban et al., 2001 ). ...
... This conclusion was made mostly by tracing the immediate deformations of tissues after localized dissections, as well as by other physical and geometrical methods (review: Beloussov, 1998). In lower invertebrates the main source of stresses is the osmotically driven periodic turgor pressure in cell vacuoles (Beloussov et al., 1993 ), while at the blastula stage of Echinodermata and vertebrate embryos there is a constant pressure within a blastocoel. In amphibian embryos this pressure is about 325 mOsm = 70 N/cm 2 (Wilson et al., 1989 ). ...
... 2. Self-stretching of the chick embryonic disk at the beginning of incubation is necessary for its further development (Kucera and Monnet-Tschudi, 1987). 3. Presence of a turgor pressure either within a gastral cavity (fresh-water hydra buds: Wanek et al., 1980) or within cell vacuoles (marine Hydrozoa: Beloussov et al., 1993) is necessary for growth and shape changes of the buds. It was shown also, that relaxation (even if temporary) of the internally generated mechanical stresses leads to the grave anomalies in the development of chicken embryos (Bellairs et al., 1967), eye rudiment (Coulombre, 1956) and amphibian embryos relaxed at the blastula early gastrula stages (Beloussov et al., 1993Beloussov et al., , 1994 Ermakov and Beloussov, 1998). ...
Morphomechanics is a branch of developmental biology, studying the generation, space-time patterns and morphogenetic role of mechanical stresses (MS) which reside in embryonic tissues. All the morphogenetically active embryonic tissues studied in this respect have been shown to bear substantial mechanical stresses of tension or pressure. MS are indispensable for organized cell movements, expression of a number of developmentally important genes and the very viability of cells. Even a temporary relaxation of MS leads to an increase in the morphological variability and asymmetry of embryonic rudiments. Moreover, MS may be among the decisive links of morphogenetic feedback required for driving forth embryonic development and providing its regular space-time patterns. We hypothesize that one such feedback is based upon the tendency of cells and tissues to hyperrestore (restore with an overshoot) their MS values after any deviations, either artificial or produced by neighboring morphogenetically active tissues. This idea is supported by a number of observations and experiments performed on the tissue and individual cell levels. We describe also the models demonstrating that a number of biologically realistic stationary shapes and propagating waves can be generated by varying the parameters of the hyperrestoration feedback loop. Morphomechanics is an important and rapidly developing branch of developmental and cell biology, being complementary to other approaches.
... According to the data available (Beloussov et al. 1989;Labas et al. 1992;Kazakova et al. 1994), the above-described events are based upon osmotic-contractile mechanisms, leading to periodic swelling-deswelling of intercellular vacuoles ( Fig. 3.12e, f). At the height of the extension phase, the cells oriented close to transversal positions are full of isolated swelled vacuoles containing potassium ions in the concentration abundant to that in seawater (Fig. 3.12e). ...
... During the retraction phase, the vacuoles are fused together into prolonged channels opened in external space (as indicated by equalization of their ionic content with that of external water) ( Fig. 3.12f). This concept is confirmed by GP arrest at the height of extension phase under hypotonicity of external medium or under increase of intercellular sodium transport; accordingly, GPs are arrested at the retraction phase if the opposite factors are employed (for details see Labas et al. 1992). By recent data (Nikishin and Kremnyov, in press), the proximo-distal wave of the upward cell rotations (vacuoles swelling) is arrested by blocking intercellular gap junctions, indicating thus that cell-cell contact interactions are necessary for this relay to go on. ...
We start from reviewing several ubiquitous approaches to morphogenesis and argue that for a more adequate presentation of morphogenesis, they should be replaced by explanatory constructions based upon the self-organization theory (SOT). The first step on this way will be in describing morphogenetic events in terms of the symmetry theory, to distinguish the processes driven either toward increase or toward decrease of the symmetry order and to use Curie principle as a clue. We will show that the only way to combine this principle with experimental data is to conclude that morphogenesis passes via a number of instabilities. The latter, in their turn, point to the domination of nonlinear regimes. Accordingly, we come to the realm of SOT and give a survey of the dynamic modes which it provides. By discussing the physical basis of embryonic self-organization, we focus ourselves on the role of mechanical stresses. We suggest that many (although no all) morphogenetic events can be regarded as retarded relaxations of previously accumulated elastic stresses toward a restricted number of metastable energy wells.
... According to the data available (Beloussov et al. 1989;Labas et al. 1992;Kazakova et al. 1994), the above-described events are based upon osmotic-contractile mechanisms, leading to periodic swelling-deswelling of intercellular vacuoles ( Fig. 3.12e, f). At the height of the extension phase, the cells oriented close to transversal positions are full of isolated swelled vacuoles containing potassium ions in the concentration abundant to that in seawater (Fig. 3.12e). ...
... During the retraction phase, the vacuoles are fused together into prolonged channels opened in external space (as indicated by equalization of their ionic content with that of external water) ( Fig. 3.12f). This concept is confirmed by GP arrest at the height of extension phase under hypotonicity of external medium or under increase of intercellular sodium transport; accordingly, GPs are arrested at the retraction phase if the opposite factors are employed (for details see Labas et al. 1992). By recent data (Nikishin and Kremnyov, in press), the proximo-distal wave of the upward cell rotations (vacuoles swelling) is arrested by blocking intercellular gap junctions, indicating thus that cell-cell contact interactions are necessary for this relay to go on. ...
An attempt is made to reconstruct the natural successions of the developmental events on the basis of a common mechanically based trend. It is formulated in terms of a hyper-restoration (HR) hypothesis claiming that embryonic tissue responds to any external deforming force by generating its own one, directed toward the restoration of the initial stress value, but as a rule overshooting it in the opposite side. We give a mathematical formulation of this model, present a number of supporting evidences, and describe several HR-driven feedbacks which may drive forth morphogenesis. We use this approach for reconstructing in greater detail the gastrulation of the embryos from different taxonomic groups. Also, we discuss the application of this model to cytotomy, ooplasmic segregation, and shape complication of tubular rudiments (taking hydroid polyps as examples). In addition, we review the perspectives for applying morphomechanical approach to the problem of cell differentiation.
... According to the data available (Beloussov et al. 1989;Labas et al. 1992;Kazakova et al. 1994), the above-described events are based upon osmotic-contractile mechanisms, leading to periodic swelling-deswelling of intercellular vacuoles ( Fig. 3.12e, f). At the height of the extension phase, the cells oriented close to transversal positions are full of isolated swelled vacuoles containing potassium ions in the concentration abundant to that in seawater (Fig. 3.12e). ...
... During the retraction phase, the vacuoles are fused together into prolonged channels opened in external space (as indicated by equalization of their ionic content with that of external water) ( Fig. 3.12f). This concept is confirmed by GP arrest at the height of extension phase under hypotonicity of external medium or under increase of intercellular sodium transport; accordingly, GPs are arrested at the retraction phase if the opposite factors are employed (for details see Labas et al. 1992). By recent data (Nikishin and Kremnyov, in press), the proximo-distal wave of the upward cell rotations (vacuoles swelling) is arrested by blocking intercellular gap junctions, indicating thus that cell-cell contact interactions are necessary for this relay to go on. ...
This book outlines a unified theory of embryonic development, assuming morphogenesis to be a multi-level process including self-organizing steps while also obeying general laws. It is shown how molecular mechanisms generate mechanical forces, which in the long run lead to morphological changes. Questions such as how stress-mediated feedback acts at the cellular and supra-cellular levels and how executive and regulatory mechanisms are mutually dependent are addressed, while aspects of collective cell behavior and the morphogenesis of plants are also discussed. The morphomechanical approach employed in the book is based on the general principles of self-organization theory.
... According to the data available (Beloussov et al. 1989;Labas et al. 1992;Kazakova et al. 1994), the above-described events are based upon osmotic-contractile mechanisms, leading to periodic swelling-deswelling of intercellular vacuoles ( Fig. 3.12e, f). At the height of the extension phase, the cells oriented close to transversal positions are full of isolated swelled vacuoles containing potassium ions in the concentration abundant to that in seawater (Fig. 3.12e). ...
... During the retraction phase, the vacuoles are fused together into prolonged channels opened in external space (as indicated by equalization of their ionic content with that of external water) ( Fig. 3.12f). This concept is confirmed by GP arrest at the height of extension phase under hypotonicity of external medium or under increase of intercellular sodium transport; accordingly, GPs are arrested at the retraction phase if the opposite factors are employed (for details see Labas et al. 1992). By recent data (Nikishin and Kremnyov, in press), the proximo-distal wave of the upward cell rotations (vacuoles swelling) is arrested by blocking intercellular gap junctions, indicating thus that cell-cell contact interactions are necessary for this relay to go on. ...
Regular patterns of mechanical stresses are perfectly expressed on the macromorphological level in the embryos of all taxonomic groups studied in this respect. Stress patterns are characterized by the topological invariability retained during prolonged time periods and drastically changing in between. After explanting small pieces of embryonic tissues, they are restored within several dozens minutes. Disturbance of stress patterns in developing embryos irreversibly breaks the long-range order of subsequent development. Morphogenetically important stress patterns are established by three geometrically different modes of cell alignment: parallel, perpendicular, and oblique. The first of them creates prolonged files of actively elongated cells. The second is responsible for segregation of an epithelial layer to the domains of columnar and flattened cells. The model of this process, demonstrating its scaling capacities, is described. The third mode which follows the previous one is responsible for making the curvatures. It is associated with formation of “cell fans,” the universal devices for shapes formation due to slow relaxation of the stored elastic energy.
... The entire shape-forming process roughly resembles the process of glass-blowing, in as much as both are effectively regulated by the pattern of pressure pulses: sudden brief pressure pulses produce narrow tubular structures while more prolonged blows lead to the creation of wide vessels with flattened bottoms. In hydroids the patterns of the pressure pulses are both species-and stage-specific and can be also modified experimentally ( Beloussov et al., 1993). 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). Figure 4. Modeling of hydranth's morphogenesis regarded as a function of a pulsating hydrostatic pressure within an epithelial layer. ...
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.
... The frequency of these pulsations matches the frequency of growth pulsations and differs from pulsation frequency in subsequent zones. It has been established by previous research that GT pulsations are caused by regular changes in the volume of epidermal MECs (Beloussov et al. 1989(Beloussov et al. , 1993(Beloussov et al. , 1997Schierwater et al. 1992). Epidermal cells abut the perisarc tube at one end, and the 'axial shaft' formed by the thickened gastrodermis at the other end. ...
In many hydroids the movement of hydroplasm inside the colony is generated by the pulsation of coenosarc walls. We hypothesized that the stolon coenosarc is not only morphologically but also functionally non-homogeneous, and tested this hypothesis using time-lapse video microscopy to study rhythmic patterns – transverse pulsations, longitudinal pulsations and hydroplasmic flows – in the five distal internodes of the stolon in Gonothyraea loveni, which is commonly used in laboratories to study growth and morphogenesis. Growth (apical) and transverse (lateral) pulsations of the coenosarc are rhythmic to varying degrees. Growth pulsations and longitudinal oscillations of the distal part of the growing tip (GT) have the most regular rhythm. In the proximal part of the GT and in the coenosarc of the terminal internode behind the GT, transverse pulsations have a different period, about double that of growth pulsations. The highest amplitude of transverse pulsations is observed in the terminal segment of the stolon and the following segment between the 1st and 2nd upright stems. Longitudinal oscillations of the coenosarc tube are clearly discernible throughout all five internodes, even though they subside with distance from the GT. Thus, the zone of intercalary growth spreads over several internodes of the stolon and is not limited to its distal segment. Hydroplasmic flows are rhythmical. Their velocity generally increases with distance from the GT, but can differ significantly between adjacent internodes, which points to the important role of local subcurrents. There are six morphofunctional zones in the hydroid stolon.
... Among medusae, such as Cassiopea (Scyphozoa, Figure 1), the spontaneous rhythmic pulsations of the medusa bell present a behavior pattern in which the influence of certain sense organs and the overall control of the nervous system have been investigated [85,86]. Moreover, growth pulsations, that is, successive rhythmic extensions and retractions of the shoot and stolon growing tips, are thought to be essential for growth and morphogenesis of colonial hydroid polyps (Hydrozoa, Figure 1) [87][88][89]. Another impressive behavior among cnidarians is the 'wedding dance', or courting behavior exhibited by some cubozoan medusae [90]. ...
The nervous systems of cnidarians, pre-bilaterian animals that diverged close to the base of the metazoan radiation, are structurally simple and thus have great potential to reveal fundamental principles of neural circuits. Unfortunately, cnidarians have thus far been relatively intractable to electrophysiological and genetic techniques and consequently have been largely passed over by neurobiologists. However, recent advances in molecular and imaging methods are fueling a renaissance of interest in and research into cnidarians nervous systems. Here, we review current knowledge on the nervous systems of cnidarian species and propose that researchers should seize this opportunity and undertake the study of members of this phylum as strategic experimental systems with great basic and translational relevance for neuroscience.
... Cellular proliferation, migration, and differentiation create preconditions for emergenece the next growing tip, but further on do not affect the realization of the morphogenetic cycle by the growing tip. Growth pulsations, i.e., recurrent proximalldistal waves of contractions and relaxation of epii and gass trodermal cells of growing tip (Beloussov, 1961b; Wytt tenbach, 1968 Wytt tenbach, , 1969 Wyttenbach et al., 1973; Beloussov et al., 1980 Beloussov et al., , 1993 Zarayskiy et al., 1984), are the basis of its morr phogenetic activity. This activity is inherent only in growing tips, and it is their integral feature. ...
The morphogenetic approach is applied to analyze the diversity of spatial organization of shoots in thecate hydroids (Cnidaria, Hydroidomedusa, Leptomedusae). The main tendencies and constraints of increased evolutionary complexity in thecate hydroids colonies are uncovered.
The laboratory is engaged in morphomechanics—the study of self-organization of mechanical
forces that create the shape and structure of the embryonic primordia. As part of its work, the laboratory
described pulsating modes of mechanical stresses in hydroids, identified and mapped mechanical stresses in
the tissues of amphibian embryos, and studied morphogenetic reorganization caused by the relaxation and
reorientation of tensions. The role of mechanical stresses in maintaining the orderly architectonics of the
embryo is shown. Mechano-dependent genes are detected. Microstrains of embryonic tissues and stress gradients
associated with them are described. A model of hyper-recovery of mechanical stresses as a possible
driving force of morphogenesis is proposed.
We start from demonstrating that macroscopic embryonic “designs” are mechanically stressed and that the formation of embryonic structures of different geometry is owed to the partial and controlled stress relaxation. Next, we suggest a model of morphomechanical feedbacks giving rise to the regular structure of mechanical stress fields (stress hyperrestoration, or HR model). With the use of this model, different biologically realistic shapes and types of development can be reproduced. The origin and role of nonmechanical factors in specifying the results of morphomechanical feedbacks are briefly discussed. In general, we emphasize a role played by the upper level of embryonic organization, characterized by a definite mechanogeometry, in regulating developmental events.
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.
We define the active memory of a developing system as the capacity to build its future from the reminiscences about its own immediately preceded finite past. We review several examples of active developmental memory, manifesting themselves in the morphomechanic reactions and in the properties of an ultraweak photon emission. By our suggestion, an active memory is an essential temporal component of the morphogenetic fields.
Cnidarians are primitive multi-cellular animals whose body is constructed of two epithelial layers and whose gastric cavity
has only one opening. Most cnidarians are colonial. Colonial hydroids with their branched body can be regarded as a model
for the whole phylum and are the most-studied cnidarian group with respect to developmental biology. Their colonies are constructed
by repetition of limited number of developmental modules. The new modules are formed in the course of activity of terminal
elements—growing tips of stolons and shoots. The growing tips of cnidarians, in contrast to those of plants, lack cell proliferation
and drive morphogenesis instead by laying down and shaping the outer skeleton and formation of new colony elements. Cell multiplication
takes place proximally to the growing tips. Branching in colonial hydroids happens due to the emergence of the new growing
tip within the existing structures or by subdivision of the growing tip into several rudiments. Marcomorphogenetic events
associated with different variants of branching are described, and the problems of pattern control are discussed in brief.
Less is known about genetic basis of branching control.
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.
Two spatially separated processes underlie the growth and morphogenesis in hydroids (Cnidaria, Hydroidomedusa): (1) growth
pulsations of the terminal growing tips and (2) cell proliferation and migration in more proximal parts of the colony soft
tissues. Growing tips are morphogenetic elements of the colony that provide for the colony elongation and morphogenesis. In
thecate hydroids (subclass Leptomedusae) with highly integrated colonies and monopodial shoot growth, the initiation of the
lateral branches and hydranth rudiments looks like a periodic splitting of the growing tip into two or more rudiments. Published
descriptions and proposed models of this process assume that the splitting results from the formation of the furrows running
into the tip from its apical surface. In this study on a Sertulariidae species, we demonstrate that the visible process of
the tip splitting into several rudiments begins in its proximal part. At the same time, the inner ridges are initiated at
the skeleton lateral surfaces surrounding the growing tip. These ridges develop and grow along the proximodistal axis. Eventually,
the opposite ridges fuse, which splits the tip into several rudiments. We propose that the tip splitting into several rudiments
is impossible without the spatial regulation of the outer skeleton formation. This process explains many species-specific
properties of the shoot spatial organization in thecate hydroids such as the partial or complete fusion of the zooid skeleton
with the shoot stem skeleton, deflection of the distal parts of the zooid skeleton from the shoot stem axis, etc. The revealed
mechanisms considerably supplements and corrects the models describing morphogenesis in colonial hydroids.
We propose a model which imitates the morphogenesis of several species of the hydroid polyps and permits the derivation of
the geometry (surface curvature) of each developmental stage from that of a preceding stage. The model is based upon two experimentally
verified assumptions. First, neighboring cells are assumed to compress each other laterally in a regular and species-specific
pulsatile manner. It is this pressure, and/or an active cell reaction to it, which changes the curvature of a cell layer.
Secondly, cell layers are assumed to have quasi-elastic properties tending to smooth out their curvature. With our model,
the different pulsatile patterns of cell-cell pressure are reproduced and the elasticity parameters are modulated. The main
principles of the model can also be used for interpreting the morphogenesis of other groups of animals. A suggested model
emphasizes the self-organizing properties of a “stressed geometry” of embryonic rudiments.
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 .
Several alternative properties which we define as deterministic or field ones are formulated and analyzed in their relations to the realms of morphogenesis and biophoton emission. In spite of all the differences between these two groups of events both of them share the properties of non-additivity, delocalization, self-focusing and several others which we relate to the field phenomena. To a large extent, the field properties of the biological systems are associated with a set of oscillations of different time periods. We suggest that even such deterministic events as, for example, a ligand-receptor coupling are acting, within an activated cell, as the switches and/or modulators of its field properties.
The colonies of thecate hydroids are covered with a chitinous tubelike outer skeleton, the perisarc. The perisarc shows a species-specific pattern of annuli, curvatures, and smooth parts. This pattern is exclusively formed at the growing tips at which the soft perisarc material is expelled by the underlying epithelium. Just behind the apex of the tip, this material hardens. We treated growing cultures of Laomedea flexuosa with substances we suspected would interfere with the hardening of the perisarc (L-cysteine, phenylthiourea) and those we expected would stimulate it (dopamine, N-acetyldopamine). We found that the former caused a widening of and the latter a reduction in the diameter of the perisarc tube. At the same time, the length of the structure elements changed so that the volume remained almost constant. We propose that normal development involves a spatial and temporal regulation of the hardening process. When the hardening occurs close to the apex, the diameter of the tube decreases. When it takes place farther from the apex, the innate tendency of the tip tissue to expand causes a widening of the skeleton tube. An oscillation of the position at which hardening takes place causes the formation of annuli.
A model is proposed which imitates the morphogenesis of several species of the lower invertebrate animals, the hydroid polyps and permits the derivation of the geometry (surface curvature) of each developmental stage from that of the preceding stage. The model is based upon two experimentally verified assumptions. First, neighbouring cells are assumed to compress each other laterally in a regular and species-specific pulsatile manner. It is this pressure, and/or an active cell reaction to it, which changes the curvature of a cell layer. Secondly, cell layers are assumed to have quasi-elastic properties tending to smooth out their curvature. With our model, the different pulsatile patterns of cell-cell pressure are reproduced and the elasticity parameters are modulated. As a result, within a large zone of parameter values (a so-called "morphogenetic zone", MZ) realistic shapes of the rudiments are reproduced. The main principles of the model can also be used for interpreting the morphogenesis of other groups of animals. A suggested model emphasizes the self-organizing properties of a "stressed geometry" of embryonic rudiments.
A deep (although at the first glance naïve) question which may be addressed to embryonic development is why during this process quite definite and accurately reproduced successions of precise and complicated shapes are taking place, or why, in several cases, the result of development is highly precise in spite of an extensive variability of intermediate stages. This problem can be attacked in two different ways. One of them, up to now just slightly employed, is to formulate robust macroscopic generative laws from which the observed successions of shapes could be derived. Another one, which dominates in modern embryology, regards the development as a succession of highly precise 'micropatterns', each of them arising due to the action of specific factors, having, as a rule, nothing in common with each other. We argue that the latter view contradicts a great bulk of firmly established data and gives no satisfactory answers to the main problems of development. Therefore we intend to follow the first way. By doing this, we regard developing embryos as self-organized systems transpierced by feedbacks among which we pay special attention to those linked with mechanical stresses (MS). We formulate a hypothesis of so-called MS hyper-restoration as a common basis for the developmentally important feedback loops. We present a number of examples confirming this hypothesis and use it for reconstructing prolonged chains of developmental events. Finally, we discuss the application of the same set of assumptions to the first steps of egg development and to the internal differentiation of embryonic cells.
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.
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