Phagocytes obtained from six echinoderm and four nonechinoderm species are compared in culture with respect to their ability to form syncytium. It is shown that phagocytes of all echinoderm species examined form an extensive syncytium while those of three nonechinoderm species shoe no such activity. Phagocytes of an ascidia, on the other hand, formed a less extensive type of syncytium known as a multinucleate giant cell. Kinetic analyses of the process of fusion were performed. Phagocytes of the asteroid, Asterina pectinifera, fuse more readily when transferred to culture condition in an aggregated state than as single cells. The extraordinarily fusogenic nature of echinoderm phagocyte is discussed in relation to the calcareous body wall structure of the echinoderms. © 1993 Wiley‐Liss, Inc.
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