This paper reviews the progress of studies of echinoderm blood cells (coelomocytes), from publications mostly since 1981; particular attention is paid to recent findings on immunological function, cell culture, and molecular biology of coelomocytes. Some observations made in our own laboratory on coelomocytes of a sea cucumber (Holothuria leucospilota) are included. Six types of coelomucytes are known in echinoderms. They are the phagocyte, spherule cell, vibratile cell, crystal cell, progenitor cell, and haemocyte. These cells likely originate from mesoderm epithelium of various body parts, and their concentration is (1 to 60) x106 cells per ml of coelomic fluid. Echinoderms express both humoral and cellular immunity. Cellular immunity is carried out by phagocytes through phagocytosis, encapsulation, and cytotoxicity. Spherule cells are involved in wound healing and regeneration. Molecular studies indicate that profilin, an actin binding protein in the coelomocytes, can receive and transduce injury signals to other coelomocytes to elicit immune responses. Evidence supports the idea that the immunological system of echinoderms may be an ancestral form to that of vertebrates. Such evidence includes: the morphological and functional similarities between the echinoderm coelomocytes and vertebrate blood cells; the resemblance of echinoderm brown bodies to the multinucleated giant cells of vertebrates; the inflammation response after challenge by foreign material; and the presence of phagocytosis-regulating agent (interleukin-1-like molecule) in the coelomic fluid. Some interesting topics in the study of echinoderm coelomocytes are pointed out throughout the text and We hope that they will be useful for researchers who may want to engage in studies of echinoderm coelomocytes.
Phagocytosis, Immunology, Brown bodies, Coelomic fluid
