Echinoderms, due to their outstanding potential for regeneration, are widely used as experimental models for research in regenerative biology. One of the main problems in this field concerns identification and characterization of cells responsible for the restoration of lost body parts and organs in adult animals. In this study, we analyze the probable candidates for this role in the starfish Asterias rubens L., namely, small coelomic epithelial cells with a high nuclear-cytoplasmic ratio that have the ability to proliferate. These cells are one of several cell types common to the coelomic epithelium (CE) and coelomic fluid (CF). They are analyzed with respect to morphology, proportion in the total cell pool, dynamics after injury and distribution between CE and CF. The results of whole-mount and scanning electron microscopy provide evidence that these small cells occupy a boundary position between CE and CF. Moreover, a novel subpopulation of CE cells is identified that is enriched (up to 50 %) with small epitheliocytes capable of migrating from CE into the CF. As shown in experiments with BrdU incorporation and anti-phospho-histone H3 antibody staining, small epitheliocytes cultured on laminin retain proliferative activity for at least 1 month and can form colony-like aggregates. Two types of small proliferating cells are distinguished by their behavior in culture: some cells remain attached to the substrate and form aggregates, while others detach from the substrate during culturing. The morphology of small epitheliocytes, their proliferative activity in vivo and in vitro and the ability to migrate suggest that they possess certain properties characteristic of stem cells.

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Echinoderms, possessing outstanding regenerative capabilities, provide a unique model system for the study of response to injury. However, little is known about the proteomic composition of coelomic fluid, an important biofluid circulating throughout the animal's body and reflecting the overall biological status of the organism. In this study, we used LC-MALDI tandem mass spectrometry to characterize the proteome of the cell-free coelomic fluid of the starfish Asterias rubens and to follow the changes occurring in response to puncture wound and blood loss. In total, 91 proteins were identified, of which 61 were extracellular soluble and 16 were bound to the plasma membrane. The most represented functional terms were 'pattern recognition receptor activity' and 'peptidase inhibitor activity'. A series of candidate proteins involved in early response to injury was revealed. Ependymin, β-microseminoprotein, serum amyloid A and avidin-like proteins, which are known to be involved in intestinal regeneration in the sea cucumber, were also identified as injuryresponsive proteins. Our results expand the list of proteins potentially involved in defense and regeneration in echinoderms and demonstrate dramatic effects of injury on the coelomic fluid proteome.

Cultivation is one of the methods of modeling processes occurring in vivo. The success of cultiva tion, in particular, depends on the choice of substrate. We studied the ability of coelomocytes and coelomic epithelial cells to be attached and spread on fibronectin, laminin, polylysine, and glass. The qualitative com position of heterogeneous populations of coelomocytes and epithelial cells was determined after staining the cells with rhodamine phalloidin and DAPI, and changes in the composition of populations were evaluated in response to trauma. Seven arbitrary classes of coelomocytes have been identified, three of them being shown to participate in clot formation during primary wound reparation. There was a change in the percentage of these cells attached to specific ligands in response to trauma. In coelomic epithelium, eight arbitrary classes of cells have been identified, two of them being probable candidates for the role of progenitor cells for coelo mocytes-coelomocyte like and small epithelial cells with a high nuclear cytoplasmic ratio. Enrichment with the small cells in a population of attached coelomic epithelium cells has been revealed when seeding on laminin. Preservation of viability of epithelial cells has been shown when cultured on laminin for 2 months.

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