An electron-microscope study of periostracum formation in some marine bivalves. II. The cells lining the periostracal groove

The histology and ultrastructure of the epithelial cells of the outer and middle mantle folds of the bivalves Mytilus edulis (L), Cardium edule (L), Macoma balthica (L) and Nucula sulcata Bronn are described. The cells lining the inner face of the outer fold exhibit a prominent granular endoplasmic reticulum, and Golgi complexes which are concerned with the elaboration of granules and vesicles eventually incorporated into the periostracum. A gradual reduction in the protein synthetic apparatus occurs towards the tip of the fold. Within the cells, it is proposed that the ovoid inclusion bodies are lysosomes and that they control the rate of secretion. The cells of the middle fold are cuboidal in appearance. Those of M. edulis and N. sulcata exhibit prominent granules, whereas those of C. edule and M. balthica possess vesicles. The cells of M. edulis differ from the others in possessing stout bundles of filaments, which occupy large areas of the cell and constitute a cell web. The cells of the epithelium in all cases do not appear to be implicated in periostracum formation.