The cells found in the coelom (coelomocytes) of Strongylocentrotus purpuratus, the purple sea urchin, and Strongylocentrotus franciscanus, the red sea urchin, can be maintained for long periods in hanging drops. The morphological changes, clotting mechanisms, and in vitro reactions undergone by the various coelomocyte types can be followed with ease. The four cell types present are phagocytic leukocytes, vibratile cells, red spherule cells, and colorless spherule cells. On contact with a surface, the phagocytic leukocytes change from bladder amoebocytes to flattened syncytial cells. Within the liquid they form permanent aggregations composed of cells which do not lose the plasma membranes to form plasmodia. Other cell types are caught passively in the aggregations. Apparently only the phagocytic leukocyte is involved in cellular clotting, and extracellular gelling of the coelomic fluid was not observed. A special lysing cell type which initiates clotting was not found despite suggestions in the literature that cell aggregation in echinoids is dependent upon such a mechanism. Under adverse conditions small aggregations of cells will form plasmodia but in large clots only the surface cells form a syncytial membrane while plasma membranes of the interior cells appear to remain intact. It has been though that the function of vibratile cells is to mix the fluid and cells of the capacious and nonmuscular echinoid coelom. In hanging drops, vibratile cells are not efficient in mixing the fluid or in moving cells from one place to another. However, both in vivo and in vitro they will release a portion of a mucoid compound present in their cytoplasm when stressed by intrusion of fluid from the gut into the coelom or hanging drop. The temporary semigel thus formed may operate in limiting spread of gut fluid or other foreign material through the coelom. © 1969.
