Intracellular formation of calcium concretions by phagocytic cells in freshwater mussels

The gills of freshwater unionid mussels contain large accumulations of extracellular calcium phosphate concretions. Connective tissue cells located in specific areas in the gills of these animals are the site of concretion formation. The cell type responsible for concretion formation appears to differentiate from an undifferentiated precursor cell by accumulating endocytic vesicles and producing amorphous membrane-bound granules from active rough endoplasmic reticulum and Golgi apparatus. In the most mature of the cells, these granules occupy most of the cytoplasm. Concretions are initiated within the amorphous granules as observed using stereo-pair transmission electron microscopy. These connective tissue cells exhibit phagocytosis, and can ingest India ink particles and colloidal gold. Colloidal gold is internalized through an endocytic mechanism within vesicles. Gold can be observed in the amorphous granules, implying fusion of such endocytic vesicles with the amorphous granules. While the exact mechanism of concretion formation is not understood, the cellular site of formation has been identified. Further, protein synthetic activity as well as endocytic activity appear necessary for concretion production.