ИСТИНА

Aurelia aurita is a representative scyphozoan demonstrating complex life cycle with an alternation of the polyp and medusa stage. Strobilation (transverse fission of the polyp body) is a process by which scyphozoan polyp produces multiple medusa larvae (ephyrae). Unfortunately, there is an obvious lack of data concerning the morphogenetic bases of this process. Using in vivo observations, histology, electron microscopy and immunohistochemistry we studied the developing ephyra from originating of the ephyra anlage to liberating of the ephyra from the polyp. We found that the timing of ephyra development is highly variable, but each individual ephyra goes through a conserved sequence of developmental stages. This allowed us to create a time-table of ephyra development. Surprisingly, from the morphogenetic point of view, the endoderm overcomes ectoderm in the beginning of the ephyra formation. A transverse endodermal fold lying beneath the smooth ectoderm is the first morphological sign of ephyra anlage. At the next developmental stage, ectoderm adjusts itself to the shape of this fold by forming of the transverse constriction. In the beginning, this constriction looks like a unilateral indentation. The indentation spreads around the polyp body outlining the border of the ephyra disk. Thus, we can detecttheleadingandtheretarding halves of the developing ephyra, the asymmetry being clearly visible at almost all stages of the ephyra formation. Enlargementand morphogenesis of the ephyra disk start soon after the completion of circular constriction. It seems that these processes are based on a series of cell shape changes as well as on the intercalation of cells in both cell layers. Our preliminarily data allow us to suppose the involvement of localized cell proliferation into morphogenesis of the ephyra. Differentiation of the disk margin into the typical ephyra lappets proceeds simultaneously with the disk enlargement.