Ultrastructural studies of regenerating spines of the sea urchin Strongylocentrotus purpuratus. II. Cell types with spherules.

The fine structure of spherulecytes, cell types with large, intracellular membrane-bound vacuoles termed spherules, was investigated in regenerating tips of spines of the sea urchin Strongylocentrotus purpuratus. Two categories of cell types were observed: red spherulecytes and colorless spherulecytes. Red spherulecytes were represented by a single cell type, the eleocyte, while colorless spherulecytes consisted of three morphologically distinct cell types termed morula cells, granulocytes, and vacuolecytes. Eleocytes and morula cells were distributed in both the epidermis and dermis, while granulocytes and vacuolecytes were present only in the dermis. After processing for light and electron microscopy, the spherules of eleocytes typically appeared empty, having lost their content of the red pigment, echinochrome. In contrast, the spherules of morula cell, granulocytes, and vacuolecytes enclosed a variety of granular and other material. The cell types reported in this paper resembled, to various degrees, spherulecytes in the coelomic fluid of echinoids described by other investigators.