L. V. Beloussov

Tension-dependent collective cell movements in the early gastrula ectoderm of Xenopus laevis embryos

Abstract Ventral ectodermal explants taken from early gastrula embryos of Xenopus laevis were artificially stretched either by two opposite concentrated forces or by a distributed force applied to the internal explant’s layer. These modes of stretching reflect different mechanical situations taking place in the normal development. Two main types of kinematic response to the applied tensions were detected. First, by 15 min after the onset of concentrated stretching a substantial proportion of the explant’s cells exhibited a concerted movement towards the closest point of the applied stretching force. We define this movement as tensotaxis. Later, under both concentrated and distributed stretching, most of the cell’s trajectories became reoriented perpendicular to the stretching force, and the cells started to intercalate between each other, both horizontally and vertically. This was accompanied by extensive elongation of the outer ectodermal cells and reconstruction of cell-cell contacts. The intercalation movements led first to a considerable reduction in the stretch-induced tensions and then to the formation of peculiar bipolar ”embryoid” shapes. The type and intensity of the morphomechanical responses did not depend upon the orientation of a stretching force in relation to the embryonic axes. We discuss the interactions of the passive and active components in tension-dependent cell movements and their relations to normal morphogenetic events.

Mechanical stresses in embryonic tissues: patterns, morphogenetic role, and involvement in regulatory feedback

Publisher Summary This chapter discusses how to detect and locate mechanical stresses in developing embryos and presents some maps of the stresses based on amphibian embryos. The chapter discusses some attempts to estimate the range of absolute stress values. The morphogenetic effects of artificial stretching and relaxation of stress in embryonic tissues are also discussed. The chapter discusses the role of “surface tension (ST)-like’’ and quasi-elastic components in generating and maintaining stresses. One of the simplest and biologically reasonable ways to detect the presence and location of mechanical stresses is to dissect or incise embryonic tissues and trace their deformation immediately after dissection under conditions that prevent the tissue from contracting. Rough maps of mechanical stresses can be constructed by making a series of precisely located incisions during successive developmental stages of amphibian embryos. At the subcellular level ST-like phenomena may be identified with internalization-resorption (an increase in ST) of cell membrane subunits or the opposite, externalization-insertion (a decrease in ST as far as a negative ST, or surface compression). The first process is usually expressed as endocytosis and the second as exocytosis.

The role of external tensions in differentiation of Xenopus laevis embryonic tissues

Explants extirpated from Xenopus laevis embryos at the early gastrula stage were placed on pieces of hydrophilized latex film which were then either stretched or remained intact. In explants cultivated on the intact films most cells emigrated out of the explants and remained undifferentiated, whereas the explants on the films stretched for 10 min or more developed a normal set of rudiments. In the explants of suprablastoporal zone stretched perpendicularly to the cranio-caudal direction, the axial organs were oriented in the direction of stretching. In the stretched explants, unlike the intact ones, a system of microfilament-associated intercellular contacts was formed within a few minutes.

Asymmetrical rotations of blastomeres in early cleavage of gastropoda

SummaryThe movements of blastomere surfaces marked with carbon particles during cytokinesis of the Ist–IVth cleavage divisions in the eggs of the gastropodsLymnaea stagnalis, L. palustris, Physa acuta and Ph. fontinalis have been studied by time-lapse cinematographic methods. The vitelline membrane was removed with trypsin. At 2- and 4-cell stages shifts of nuclei have also been studied.Symmetrical as well as asymmetrical surface movements (in respect to the furrow plane) have been revealed. Symmetrical surface movements at the beginning of cytokinesis consist mainly in contraction of the furrow zone and in expansion of the more peripheral regions; between these there is a stationary zone. After the end of cytokinesis the furrow region expands.Considerableasymmetrical surface movements have also been observed in all four divisions. From anaphase until the end of cytokinesis each of the two sister blastomeres rotates with respect to the other in such a way, that if viewed along the spindle axis, the blastomere nearest to the observer rotates dexiotropically in a dextral species and laeotropically in a sinistral species (primary rotations). After the completion of cytokinesis the blastomeres may rotate in a reverse direction. The latter rotations are less pronounced in the IInd and IIIrd divisions and most pronounced in the IVth division. Blastomeres with the vitelline membrane intact retain a slight capacity for primary rotations. In normal conditions nuclei of the first two blastomeres shift mainly laeotropically in dextral species, but dexiotropically in sinistral species, being carried along by the reverse surface rotations.The invariable primary asymmetrical rotations of blastomeres seem to be the basis of enantiomorphism in molluscan cleavage. They are assumed to be determined by an asymmetrical structure of the contractile ring carrying out the cytokinesis.

Morphogenesis as a macroscopic self-organizing process.

We start from reviewing different epistemological constructions used for explaining morphogenesis. Among them, we explore the explanatory power of a law-centered approach which includes top-down causation and the basic concepts of a self-organization theory. Within such a framework, we discuss the morphomechanical models based upon the presumption of feedbacks between mechanical stresses imposed onto a given embryo part from outside and those generated within the latter as a kind of active response. A number of elementary morphogenetic events demonstrating that these feedbacks are directed towards hyper-restoration (restoration with an overshoot) of the initial state of mechanical stresses are described. Moreover, we show that these reactions are bound together into the larger scale feedbacks. That permits to suggest a reconstruction of morphogenetic successions in early Metazoan development concentrated around two main archetypes distinguished by the blastopores geometry. The perspectives of applying the same approach to cell differentiation are outlined. By discussing the problem of positional information we suggest that the developmental pathway of a given embryo part depends upon its preceded deformations and the corresponding mechanical stresses rather than upon its static position at any moment of development.

Neuro-mesodermal patterns in artificially deformed embryonic explants: a role for mechano-geometry in tissue differentiation.

The mutual arrangement of neural and mesodermal rudiments in artificially bent double explants of Xenopus laevis suprablastoporal areas was compared with that of intact explants. While some of the bent explants straightened or became spherical, most retained and actively reinforced the imposed curvature, creating folds on their concave sides and expanding convex surfaces. In the intact explants, the arrangement of neural and mesodermal rudiments exhibited a distinct antero‐posterior polarity, with some variability. In the bent explants, this polarity was lost: the neural rudiments were shifted towards concave while the mesodermal tissues moved towards the convex side, embracing the neural rudiments in a horseshoe‐shaped manner. We associate these drastic changes in neuro‐mesodermal patterning with the active extension and contraction of the convex and concave sides, respectively, triggered by the imposed deformations. We speculate that similar events are responsible for the establishment of neuro‐mesodermal patterns during normal development. Developmental Dynamics 239:885–896, 2010.

The role of tensile fields and contact cell polarization in the morphogenesis of amphibian axial rudiments

SummaryThe role of stretching-generated tensile stresses upon the organization of axial rudiments have been studied. Pieces of the dorsal wall ofXenopus laevis andRana temporaria embryos at the late gastrula stage were rotated through 90°, transplanted into the field of neurulation tensions of another embryo and replaced by ventral tissues already insensitive to inductive influences. The axial rudiments which developed from rotated and transplanted dorsal tissues oped from rotated and transplanted dorsal tissues almost completely reorientated according to the tensile patterns in adjacent host tissues. Some of the donor cells changed their presumptive fates in accordance with their new positions in the host tensile field. Transplanted ventral tissues were involved in the morphogenetic movements specific for the dorsal regions and imitated some typical dorsal structures. In the regions without pronounced tensions the structure of transplanted axial rudiments was chaotic. It is suggested that the organization of the axial structures is established and maintained by tensile fields created by uniformly polarized cells. Cell polarization can be transmitted by contact from host to donor tissues. The specificity of this propagating process and its morphogenetical role is discussed.

From observations to paradigms; the importance of theories and models. An interview with Hans Meinhardt

Hans Meinhardt received his PhD in physics from the University of Cologne at 1966. For a postdoctoral fellowship, he went to the European High Energy Laboratory CERN in Geneva where he joined a group working on the leptonic decay of the Xi-minus particle. One of his duties was to perform computer simulations to optimize the complex experimental setup--a skill which turned out to be helpful later on. In 1969 he switched to biology and joined the department of Alfred Gierer at the Max Planck Institute for Developmental Biology (formerly Virus Research) in Tubingen. His interest was focused on mechanisms of biological pattern formation. Using computer simulations as a tool, he developed models for essential steps in development. Most fascinating for him was the possibility to recapitulate and to reconstructusing the computer the genesis of structures where no structures were before and to see how these emerging structures become subsequently further refined. In addition to the interaction with Alfred Gierer and his group working on hydra development, the Max-Planck Institute as a whole provided a very stimulating environment. In the seventies, the work of Klaus Sander on gradients in early insect development was highly influential. Collaboration with Martin Klinger in the eighties revealed that the pigmentation patterns on tropical sea shells are convenient to study highly dynamic patterning processes. The variability and the asthetic beauty of these patterns turned out to result from the chaotic nature of the underlying reactions. Mechanisms deduced from shell patterns became a key to understand other developing systems such as orientation of chemotactic cells or phyllotaxis. Officially Hans Meinhardt retired at the end of 2003. At present he works on refinements and extensions of models which account for the different modes of embryonic axis formation in different phyla from an evolutionary point of view.

Relocations of cell convergence sites and formation of pharyngula-like shapes in mechanically relaxed Xenopus embryos

Influence of the relaxation of mechanical tensions upon collective cell movements, shape formation, and expression patterns of tissue-specific genes has been studied in Xenopus laevis embryos. We show that the local relaxation of tensile stresses within the suprablastoporal area (SBA) performed at the early-midgastrula stage leads to a complete arrest of normal convergent cell intercalation towards the dorsal midline. As a result, SBA either remains nondeformed or protrudes a strip of cells migrating ventralwards along one of the lateral lips of the opened blastopore. Already, few minutes later, the tissues in the ventral lip vicinity undergo abnormal transversal contraction/longitudinal extension resulting in the abnormal cell convergence toward ventral (rather than dorsal) embryo midline. Within a day, the dorsally relaxed embryos acquire pharyngula-like shapes and often possess tail-like protrusions. Their antero-posterior and dorso-ventral polarity, as well as expression patterns of pan-neural (Sox3), muscular cardiac actin, and forebrain (Otx2) genes substantially deviate from the normal ones. We suggest that normal gastrulation is permanently controlled by mechanical stresses within the blastopore circumference. The role of tissue tensions in regulating collective cell movements and creating pharyngula-like shapes are discussed.

Morphomechanics of Development

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. 1.1 Deterministic Approaches to Development: Expectations and Impediments 1.1.1 Lessons from Embryonic Regulations Please take a look at Fig. 1.1, displaying development of sea urchin embryo from a non-fertilized egg (Fig. 1.1a) up to a free-swimming larva (Fig. 1.1l, m). This is a textbook example of embryonic development, known for long ago in great details. Let us put a naïve question: Why just such a succession is taking place at all and why it is reproduced for innumerable set of generations? Obviously, our first suggestion will be that within any stage embryo, a certain set of “causes” is embedded providing its transition to the next stage. How large should be such a set? It is easy to see that as the development proceeds, the structure of embryo becomes ever more complicated: some structures not seen before are emerged. So-called