Cytological definition of the poriferan stylocyte: A cell type characterized by an intranuclear crystal

The stylocyte (Gr. stylos ; pillar) of Corvomeyenia carolinensis Harrison (Spongillidae), a previously undescribed proiferan cell type, was examined using phase contrast microscopy, histochemistry and electron microscopy. The stylocyte, an anucleolate amoebocyte, is characterized by a rhomboidal intranuclear crystal. The crystal, lacking an investing membrane, is embedded directly into the nucleoplasm. It is homogenous with no demonstrable crystalline subunits. Histochemical studies suggest that the crystal is proteinaceous, containing no DNA or RNA. Cytoplasmically, the stylocyte contains promienent homogenous smooth membrane‐bound inclusions which contain high levels of neutral (PAS‐positive) and polycarboxylated mucopolysaccharides but low levels of glycogen and no significant phosphatase activity. The granular endoplasmic reticulum is poorly developed. Correspondingly, with various histochemical methods, little or no cytoplasmic RNA is demonstrated. Because electron microscopic studies of C. carolinensis indicate the probable absence of viral inclusions in the sponge and because the crystal contains no histochemically demonstrable nucleic acid, the evidence appears to suggest that the crystal neither represents an assemblage of mature virus units nor a virus‐induced structure. The stylocyte cell type may play a role in nutrient cycling in C. carolinensis with the crystal acting either as a site of protein storage or as an excretory product.