ИСТИНА

To a great extent, embryonic shape is a product of epithelial sheets morphogenesis occurring as response to unbalanced mechanical stresses that is based on coordinated cell shape changes and cell movements (Davidson, 2012). The role of mechanical cues modulating the morphogenesis deserves an increasing interest. However, we know very little about the evolutionary basal and essential reactions of epithelial cells to mechanical stimuli. Comparative analysis of cell behavior in basal metazoans and in bilaterians may be instructive in understanding how these reactions evolved. The cnidarian representative, Hydractinia echinata, is a model organism for developmental biology (Plickert et al., 2012). Gastrulation in Hydractinia proceeds by mixed delamination and is completely uncoupled from axis patterning, as the gastrulating embryo lacks any morphological sign of the primary body axis. However, there is absolutely unique landmark of embryonic polarity that we described as a gradient of order. The maximum of order coincides with the future anterior pole of the larva, and the most morphologically disordered region of an embryo is the future posterior pole. Posterior half of an embryo is covered by multiple ectodermal folds with bottle cells located on the concave side of the fold. Formation of bottle cells and ectodermal folds has no known function in Hydractinia development, and we propose that these are the side effects of rapid cell proliferation in the ectoderm leading to an increase of planar compression in epithelial sheet. We assume that compressed ectodermal cells start to reduce apex area in order to reduce compression and restore normal tension. Gradual constriction of apices of neighbouring cells automatically leads to macromorphological consequence – bending of the epithelium and formation of folds (Keller, Shook, 2011). Of note, Wnt3 transcripts formed a counter gradient to the gradient of order and can facilitate the epithelial morphogenesis in the posterior region. Experimental support for our assumptions can be derived from the data obtained on the explants of the amphibian embryo. We have shown that the explant of the blastocoel roof from amphibian gastrula actively react to mechanical compression produced by an ectopically applied force by apical constriction cells acquiring the bottle shape (Kremnyov et al., 2012). Interestingly, reaction of cells depended on the explant origin. This indicates that cells, which are strictly programmed to perform important morphogenesis (such as formation of neural folds), are able to resist accidental mechanical stimuli. In conclusion, we have shown that epithelial cells respond to mechanical cue in a manner of self-organization, as in both model systems this response had no developmental function. The capability of regulation of unbalanced mechanical stresses by active cell shape changes is an intrinsic characteristic of metazoan’s epithelial cell. This ability can be tuned in evolution under imposed developmental constraints.