ИСТИНА

Investigation of Hydractinia development has very long history. However, descriptions of embryonic stages taken from different sources are sometimes contradict to each other, and Hydractinia embryogenesis is still poorly characterized. This lack of information is very problematic as Hydractinia is a conventional model for stem cell biology. To fill this gap, we have created the time-table of Hydractinia embryogenesis. All developmental stages have been characterized using in vivo observations, histology and electron microscopy. We characterized the pattern of early cleavage in Hydractinia and found that active cell movements and the rate of cytokinesis are the factors responsible for cleavage variability. We described embryonic morphology and ultrastructure at the set of developmental stages which have never been characterized. These are the stages between early morula and preplanula. At these stages we have detected high level of morphological variability that does not affect further development. Gastrulation in Hydractinia is uncoupled from axis patterning, which is a very unusual situation in animals in general. Despite gastrulating in an apolar fashion, the Hydractinia embryo transforms into a properly patterned, anterior-posterior polarized larva. We have found absolutely unique morphological landmarks of the anterior and posterior regions. Identification of the posterior half becomes possible at the early preplanula stage. The future posterior half at this stage is the most morphologically disordered region of an embryo. Moreover, in situ hybridization shows that this disordered region expresses Wnt3, the molecular marker of Hydractinia larva posterior pole. It seems that an exact position of the posterior pole becomes strictly determined later in development. We have described succession of stages leading to the formation of planula larva. The main morphological events at these stages are: elongation of anterior-posterior axis and ordering of the shape of the posterior half. Both processes are based on the active intercalating movements of ectoderm cells.