Immunocytochemical localization of a calcium-binding phosphoprotein in hemocytes of heterodont bivalves.

Calcium-binding phosphoprotein particles are the most abundant extracellular proteins in the hemolymph of heterodont bivalves, and granular hemocytes are the most abundant cells in the same fluid. In this study, the hemocytes of Rangia cuneata were examined ultrastructurally and probed with anti-phosphoprotein IgG to demonstrate that the granulocytes are a probable source of the hemolymph phosphoprotein. The granulocyte cytoplasm is laden with large vesicles containing an amorphous homogenous matrix and variable numbers of electron-dense particles; the latter are ultrastructurally similar to the extracellular phosphoprotein. The vesicle particles and matrix are related forms of the hemolymph phosphoprotein as evidenced by heavy gold labeling when Lowicryl sections were sequentially treated with rabbit-anti-phosphoprotein IgG and colloidal gold-anti-rabbit IgG. The vesicles may be the loci for posttranslational phosphorylation and eventual secretion of the calcium-binding phosphoprotein, or alternatively the vesicles may be digestive structures which degrade internalized phosphoprotein.