Stress-generating tissue deformations in Xenopus embryos: Long-range gradients and local cell displacements

BACKGROUND

Although the role of endogenous mechanical stresses in regulating morphogenetic movements and cell differentiation is now well established, many aspects of mechanical stress generation and transmission in developing embryos remain unclear and require quantitative studies.

RESULTS

By measuring stress-bearing linear deformations (caused by differences in cell movement rates) in the outer cell layer of blastula - early tail-bud Xenopus embryos, we revealed a set of long-term tension-generating gradients of cell movement rates, modulated by short-term cell-cell displacements much increasing the rates of local deformations. Experimental relaxation of tensions distorted the gradients but preserved and even enhanced local cell-cell displacements. During development, an incoherent mode of cell behavior, characterized by extensive cell-cell displacements and poorly correlated cell trajectories, was exchanged for a more coherent regime with the opposite characteristics. In particular, cell shifts became more synchronous and acquired a periodicity of several dozen minutes.

CONCLUSIONS

Morphogenetic movements in Xenopus embryos are mediated by mechanically stressed dynamic structures of two different levels: extended gradients and short-term cell-cell displacements. As development proceeds, the latter component decreases and cell trajectories become more correlated. In particular, they acquire common periodicities, making morphogenesis more coherent.

[Follicular cells of the amphibian ovary: origin, structure, and functions]

Formation of the follicular envelopes surrounding oocytes in the developing ovary and their subsequent morphological differentiation go hand-in-hand with succession of the steroidogenesis stages, arrest of meiosis and its maintenance, establishment of the conditions necessary for vitellogenesis, oocyte growth, and maturation. Metabolites are exchanged via gap junctions and receptor-mediated transport through the perioocytic space. The ion transport in follicular cells (FCs) regulates the plasma membrane potential, creating the conditions for efficient directed transport through gap junctions. Manifold biologically active substances accepted by follicular cells are an additional adjusting lever for regulating the state of follicle system. In this review, we have attempted to emphasize the amphibian FCs as key players in the follicle system; the more so as we have failed to find any review that would bring together the data on the origin of amphibian FCs, their morphology, as well as regulation of oocyte growth and development. As a rule, recent works in this field focus on the molecular mechanisms providing for regulation of individual stages in oocyte development. This review describes the origin and changes in the morphology of follicular cells during the development of Xenopus laevis oocyte as well as the data on their regulatory functions in vitellogenesis and their involvement in steroidogenesis, maintenance of meiotic arrest, and subsequent maturation.

[Involvement of guanine nucleotide exchange factor xLARG in epiboly of cells of the animal pole of Xenopus laevis embryos]

The development of multicellular organisms is a complicated coordinated process of the movement of groups of embryonic cells, which is controlled by many regulatory systems. At present little is known about the regulation of the earliest manifestations of the movement in the embryogenesis: epiboly and radial intercalation. The coordinators of these processes may be small GTPases of the Rho family and their activators, the factors of exchange of guanylic nucleotides. It has been shown in this work that the overexpression of the factor of exchange of guanylic nucleotides xLARG in Xenopus laevis embryos leads to an increase in the amount of the active form of xLARG. In addition, an increase in the expression of xLARG disturbs the process of radial intercalation. The data obtained suggest that xLARG is involved in maintaining the xLARG activation level necessary for the occurrence of epiboly.

[Role of cooperative cell movements and mechano-geometric constrains in patterning of axial rudiments in Xenopus laevis embryos]

The role of cooperative cell movements has been explored in establishment of regular segregation of the marginal zone of Xenopus laevis embryos into the main axial rudiments: notochord, somites and neural tissue. For this purpose, the following operations were performed at the late blastula-early gastrula stages: (1) isolation of marginal zones, (2) addition of the ventral zone fragments to the marginal zones, (3) dissection of isolated marginal zones along either ventral (a) or dorsal (b) midlines, (4) immediate retransplantation of excised fragments of the suprablastoporal area to the same places without rotation or after 90 degrees rotation, (5) pi-shaped separation of the suprablastoporal area either anteriorly or posteriorly. In experiments 1, 4, and 5, lateromedial convergent cell movements and differentiation of the axial rudiments were suppressed. In experiments 4 and 5, cell movements were reoriented ventrally, the entire embryo architecture was extensively reconstructed, and the axial rudiments were relocated to the blastopore lateral lips. In experiment 3, convergent cell movements were restored and oriented either towards the presumptive embryo midline (a), or in the perpendicular direction (b). In both cases, well developed axial rudiments elongated perpendicularly to cell convergences were formed. If the areas of axial rudiment formation were curved, mesodermal somites and neural tissue were always located on the convex (stretched) and concave (compressed) sides, respectively. We conclude that no stable prepatterning of the marginal zone takes place until at least the midgastrula stage. This prepatterning requires cooperative cell movements and associated mechano-geometric constrains.

[Molecular mechanisms of germ cell line determination in animals]

In the present work, principles of formation of germ line cells are reviewed. Germ line cells separate themselves from the rest of the embryo at the early stages of embryogenesis. In certain animal groups, formation of precursors of germ cells occurs by induction by surrounding cells. However, for most animal taxons, formation of primordial germ cells (PGCs) is determined by inheritance of certain maternal determinants--the so-called germ plasm. It is formed by mitochondria, electron-dense granules with the complex structure, and maternal RNAs and proteins necessary for formation of germ line. In Xenopus, the source of material for germ plasm is a mitochondrial cloud, which also specifically binds and transports to the vegetal pole maternal RNAs important for PGC formation. Cis elements determining the transport of these RNAs are usually located in the 3' untranslated region of RNA, and their function is mediated by binding of trans acting protein factors. In addition to a specific localization of certain macromolecules in germ plasm, special status of germ line cells is provided by degradation of RNA and protein components of germ plasm in somatic cells, silencing of transcription in PGCs until advanced stages of embryogenesis, and specific regulation of RNA translation in somatic and germ cells. In this review, we also briefly discuss results obtained by authors regarding the properties of a novel component of Xenopus germ plasm, namely maternal RNA germes, and encoded protein.

[Statistical and frequency-amplitude characteristics of ultra-weak emissions of loach eggs and embryos under normal conditions and on their optic interactions. II. Changes in characteristics of ultra-weak emissions upon optic interaction of groups of embryos of different ages. ]

We compared the characteristics of ultraweak emissions from groups of loach embryos of different ages in the presence or absence of optic interaction. The percentage of zero values of emission gradually increased during the first hour of optic interaction. The number and height of rare big pulses estimated by the value of kurtosis increased in parallel. In addition, the correlation between the Fourier spectra of optically interacting samples decreased at a higher rate than in the absence of optical contact. Just after the 1-hour optic interaction was terminated, the number of high pulses decreased in a younger interacting group and increased in the older one and the farther away the partner groups were in developmental stages, the more pronounced these differences were. Measurements of the Fourier spectra after long-term (12-22-hour) optic interactions have shown that an "exchange" of autocorrelation characteristics of the spectra took place among the samples: the sums of autocorrelation coefficients were inverted in the vast majority of cases, often with an "overshoot" or, at least, were smoothed over with reference to the control samples. We conclude that the previously described effects of optic interactions between groups of loach embryos of different ages could be due to changes in the frequency spectra of their ultraweak emissions.

[A quantitative study of regional and stage specific reaction of African clawed frog embryonic tissues on mechanical stress]

Residual deformation of fragments of the embryonic tissues preserved after relaxation of the stretching force serve as a criterion of active redistribution of their cells caused by this stretching. We measured residual deformations of the Xenopus laevis ventral and dorsal ectoderm at the early gastrula and lateral ectoderm at the late gastrula-early neurula after stretching of varying time and force. While the samples responded to moderate (up to 40%) short-term stretching as elastic bodies (residual deformations were absent), residual deformation appeared in the early gastrula tissues after 30-60-min stretching, which were more pronounced in the ventral tissues than in the dorsal ones. On the contrary, a contractile reaction developed in the late gastrula-early neurula tissues in response to 60-min stretching, which almost relaxed residual deformation within 20 min after unloading. A conclusion was drawn that gastrulation and neurulation proceed under the conditions of relaxing and nonrelaxing mechanical tensions, respectively. Mechanical bases and morphogenetic role of the described reactions is discussed.

[Statistical and frequency-amplitude characteristics of ultra weak emissions of the loach eggs and embryos under the normal conditions and during their optic interactions. I. Characteristics of ultra weak emission in normal development and the optic role of egg envelope]

Ultraweak emissions of groups comprising several dozens of unfertilized and fertilized loach eggs, embryos, larvae, and their egg envelopes were measured on a photomultiplier tube. The envelopes absorbed the light from external sources but readily gave it back in the absence of embryos. We carried out statistical and frequency-amplitude analyses of ultraweak emissions and studied the autocorrelation structure of their frequency spectra. The frequencies of signals with different intensity underwent regular changes during development. Cascades of short-term (< or = 1 ms) flashes timed (during cleavage) to furrowing were a characteristic element of ultraweak emission. The Fourier spectra of developing embryos had pronounced frequency-amplitude peaks and higher, than in unfertilized eggs and inanimate samples, mutual correlation during successive time intervals. Stage-specific translational symmetry of the frequency spectra of ultraweak emissions was demonstrated, which suggests the presence in groups of embryos of a coordinated system of harmonic oscillators. The latter underwent regular changes during development. The measurement of ultraweak emissions represents a unique non-invasive method of analysis of these oscillators.

[Cytomechanical control of morphogenesis]

The role of mechanically strained state of cells and multicellular structures in morphogenesis regulating in vertebrate embryos is discussed. Regular changes in patterns of mechanical strain during embryonic development are described. Artificial relaxation of mechanical strain performed on definite developmental stages and retension of embryonic tissues in arbitrary directions considerably affects morphogenesis and cell differentiation patterns. Cytomechanical models of morphogenesis are reviewed and a concept of hyperrestoration of mechanical strain as a possible driving force of morphogeneiss is suggested.

[Tensotaxis--a collective movement of embryonic cells up along the gradients of mechanical tensions]

We have examined the active collective movement of ectodermal cells from early gastrula of Xenopus laevis towards the point source of stretching, using techniques of videomicroscopy and scanning electron microscopy. We define this mode of cell movement as tensotaxis. This movement begins near the source of tension 5-10 min after the beginning of stretching and is spread in a relay fashion to more distant cells. As a result, a considerable fraction of observed cells more towards the source of stretching over a considerable territory at a rate of 0.6-3 mu/min. Subsequently, these movements are replaced by cell intercalation roughly oriented in the direction transverse to that of tissue stretching. It is proposed that tensotaxis is initiated by asymmetric deformation of the embryonic tissue due to the concentration (focusing) of a stretching force and contains both passive and active components. Data are presented supporting the view that, during normal development, tensotaxis may determine the movement of embryonic cells towards the blastopore and can also participate in other morphogenetic processes.

Studies in developmental cytomechanic

One of the most promising trends in modern developmental and cell biology, recently defined as <<cyto>>, or <<morphomechanics>>, 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 <<active solid body>>.

[The morphogenetic reactions of the ectoderm in the early gastrula of the clawed toad to mechanical stretching]

The ventral ectoderm explants of the Xenopus laevis early gastrula were stretched on an elastic substrate 1.5- to twofold for 30 min, then fixed in the stretched state and incubated for up to 6 h. The stretching of explants generated their two-phase active reaction, which was estimated using the criterion of eccentricity of cells and whole explants developed in this study. At the first phase, during the external force application, the explant cells were elongated towards its stretching to a markedly greater extent than the whole explant: eccentricity of the whole cells exceeded that of the whole explants. Cell processes elongated in the same direction appeared, cell contacts and apical cytoskeleton structures developed, and mechanical tensions increased. At the second phase, within 5 h after the stretching, the shape of individual cells, rather than the whole explant, returned to the initial one (approximately isotropic), mechanical tensions decreased, and visible morphogenesis of the explant proceeded: it acquired a complex shape with longitudinal groove and polar thickenings. We conclude that intercalary movements of the cells are present at the second phase of the active explant reaction and morphogenesis is realized at the phase of relaxation of mechanical tensions.

Biomechanical feedback in morphogenesis, as exemplified by stretch responses of amphibian embryonic tissues

We explore the idea that morphogenetical processes may be self-regulated by the biomechanical feedback established between the active stress-generating devices and the passive stresses of stretching and (or) compression, these feedback directed towards hyperrestoration (restoration with overlapping) of the initial stress values. As an example, a stretch-induced behaviour of the pieces of ventral ectoderm of Xenopus laevis early gastrulae is considered. By stretching the explants in 1.3-1.7 times, we induced several active poststretching cell responses, including further autonomous elongation of an explant in the stretch direction and contraction in the perpendicular direction, as well as more complicated shape changes. At the cellular level, these responses were associated with the return of the stretched cells to isodiametrical shapes and with the production of extensive cell protrusions along the stretch direction. As shown by dissections, the stretch-induced tissue tensions were considerably diminished in the poststretching period. The results obtained are discussed within the framework of the hyperrestoration hypothesis.

[Mechanically dependent heterotopias of the axial rudiments in clawed toad embryos]

Relaxation of tangential tensions was achieved in Xenopus laevis embryos at the stage of early gastrula by introducing radial cuts and insertion of ventral tissue wedges from the embryos at the similar stage. Velocity and direction of movement of the cellular material was compared in the normal and operated embryos using vital marking methods. We observed the formation of complexes of axial organs in atypical positions: after dorsomedial cuts, along the medial line and in the lateral blastopore lips; after anteroposterior (animal-vegetal) cuts, also on the ventral side; after lateral cuts (incisions), on the contralateral side of embryos. Ultrastructure of cells from the ventral region, which participate in the formation of neural plate, resembles that of the neural plate of the normal embryo as early as 4 h after surgery. We interpret these heterotopias of axial rudiments as the result of overtensions in relaxed tissues. The presence of overtensions was confirmed by tissue separation after local incisions. In all experimental series, axial rudiments were elongated in the direction of maximal overtension. Movements of the cellular material both in the normal and in the operated embryos were directed towards maximal tension.

[Intercellular relay interactions in explants of amphibian embryonic tissues. I. Intercellular relay interactions in normal explant morphogenesis]

In the explants of lateral mesoderm, together or without the overlaying ectoderm, taken from the neurula-early tail bud of amphibian embryos (Xenopus laevis, Rana temporaria) within 5-30 min, several types of cytological transformations have been observed, some of these being: formation of a microfilamentous layer under the naked explant surface, morphological polarization, and unilateral spreading of cells. These processes are described at the light and electron microscopy levels. It is suggested that these may be of relay character, that is, each transformed cell induces the same transformation in the adjacent one. The cytological transformations are correlated with metabolic ones described elsewhere.