ИСТИНА

Staurozoans are cnidarians with a sessile medusa stage. Jellyfishes spawn their gametes into seawater. Fertilized egg cleaves, gastrulates and eventually forms preplanula which hatches out from the fertilization membrane. Preplanula elongates and develops planula. These common events of a cnidarian life cycle have though a peculiar characteristic: the embryo and larva have extremely low number of cells (Wietrzykowski, 1912; Berrill, 1949; Otto, 1976). Here we show how low cell number effects the morphogenetic pathways providing embryogenesis and larva development. The work was done on the staurozoan Lucernaria quadricornis (Müller, 1776). To trace cell dynamics we employed vital observations, fluorescent labelling, and light and electron microscopy. Lucernaria zygote undergoes equal holoblastic cleavage and develops stereoblastula consisting of about 30 blastomers. Stereoblastula as well as all further developmental stages is solid. During gastrulation, a group of blastomers ingresses from the surface inside the embryo, thus establishing endoderm, while the blastomers remaining at the surface represent ectoderm. Endoderm cells being rounded and arranged without any certain pattern right after gastrulation, become coin-shaped and start to line up along the long axis as preplanula develops. Reshaping and rearrangement of the endoderm cells make preplanula elongated, though the number of cells does not increase significantly and is equal to approx 50. Preplanula hatches and then gradually transforms into planula, all gastroderm cells of which are heavily vacuolated and aligned one after another. Epiderm cells form uneven epithelium with gaps. The number of epiderm and gastroderm cells is about 50 and 16-22, respectively. Thus, morphogenetic events in the early development of Lucernaria involve rearrangement and reshaping of cells rather than movement of cell sheets followed by multiple divisions. This strategy is reminiscent of morphogenetic processes occurring in the embryogenesis of other animals with low cell number embryos. This work was supported by RFBR grant # 15-04-08214.