ИСТИНА

It is possible to distinguish at least two types of morphogenetic processes in embryogenesis using as a key feature the main mode of cell behavior. Morphogenesis of the first type goes on via changes in cell shape occurring in a coordinated fashion, directed by cell interactions based on signalling molecules or on the external mechanical forces acting on the epithelial cells (Leptin, Roth, 1994). Epithelial morphogenesis in the embryonic tissues of high metazoans is normally based upon cooperative cell-to-cell interactions (Belintsev et al., 1987). Endoderm invagination in echinoderms or neural tube formation during neurulation of amphibians are the best examples of such a morphogenesis. Changes in the shape of each individual cell affect the mechanical state of all neighboring cells and thereby provoke their active re-shaping. In this case, epithelial sheet morphogenesis can be considered as a side effect of a coherent self-reinforcing shaping of individual cells. Very often (but not always), epithelial cells undergoing the morphogenesis form a spatial unfolding - continuous spatial series of cell shapes corresponding to the succession of changes in the shape of a single cell (Cherdantsev, 2006). Morphogenesis of the second type is based on the active autonomous reshaping of single cells. In Drosophila gastrulation, prospective mesoderm cells constrict their apices and become bottle cells in an apparently stochastic order (Sweeton et al., 1991). It seems that all mesodermal cells independently follow their developmental program and independently undergo the cell shape changes (Leptin, Roth, 1994). Cnidaria are the simple metazoan animals. A striking feature of this phylum is a great variety of morphogenetic processes that can be observed in the different cnidarian species during gastrulation. Gastrulation of cnidarians provides good examples of the both types of morphogenetic processes described above. Formation of the epithelial ectoderm from non-epithelial cells is the major event in the gastrulation of the colonial hydroid Dynamena pumila (Kraus, 2006). This process is based upon cooperative cell-to-cell interactions. Each cell that joins forming epithelial sheet fragment gradually changes its shape in concert with the changes in the shape of neighboring cells and in the shape of an entire fragment. The sea anemone Nematostella vectensis gastrulates by invagination of a pre-endodermal plate consisting of cells undergoing epithelial – mesenchymal transition (EMT) and acquiring the “bottle” shape (Kraus, Technau. 2006). It seems that EMT spreads other the region of presumptive endoderm. The spreading of EMT is an extremely variable process: the embryos differ from each other in the geometry of pre - endodermal plate and in the number of cells involved. Thus, it is possible to conclude that this process is based upon cooperative cell-to-cell interactions. Gastrulation in the colonial hydroid Clytia hemispaerica has been described as an ingression of the presumptive endoderm cells from the specialized oral territory established by the action of maternally localized determinants (Metschnikoff, 1886; Momose, Houliston, 2007). Single bottle cells or very small groups of bottle cells are stochastically interspersed among ordinary epithelial cells. It means that EMT never spreads over the entire oral territory, and formation of bottle cell in Clytia is an autonomous process. In order to find general rules governing the morphogenetic movements based upon coherent and autonomous cell behavior, it would be very interesting to analyze a set of the developmental parameters using a biomechanical models of cnidarian gastrulation. Belintsev B.N., Beloussov L.V., Zaraisky A.G., 1987. Model of pattern formation in epithelial morphogenesis. J. Theor. Biol. 129: 369-394. / Cherdantsev V.G., 2006. The dynamic geometry of mass cell movements in animal morphogenesis. Int. J. Dev. Biol. 50: 169-182. / Kraus Y., Technau U., 2006. Gastrulation in Nematostella vectensis occurs by invagination and immigration: an ultrastructural study. Dev. Genes Evol. 216: 119–132. / Kraus Yu. A., 2006. Morphomechanical programming of morphogenesis in cnidarian embryos. Int. J. Dev. Biol. 50: 267-275. / Leptin M., Roth S., 1994. Autonomy and non-autonomy in Drosophila mesoderm determination and Morphogenesis. Development 120: 853-859. / Metschnikoff E., 1886. Embryologische studien an Medusen. Ein Beitrag zur genealogie der Primitiv-organe. Alfred Holder, Vienna / Momose T., Houliston E., 2007. Two oppositely localised Frizzled RNAs as axis determinants in a cnidarian embryo. PLoS Biol 5(4): e70. doi:10.1371 / journal.pbio. 0050070 / Sweeton, D., Parks, S., Costa, M., Wieschaus, E., 1991. Gastrulation in Drosophila: the formation of the ventral furrow and posterior midgut invaginations. Development 112: 775-789.