The fine structure and innervation of gill lamellae in Anodonta.

Gill lamellae of a bivalve Anodonta woodiana lauta (v. Martens) were observed by electron microscopy. The Anodonta gill wall consists of a single layer of epithelial cells, its basal lamina and the underlying connective tissue layer. It was confirmed that there is no true endothelium in the vessels and that the connective tissue layer of the vessel wall is therefore in direct contact with the blood. Cells of a specific type referred to as "trabecular cells" lie in the blood lacunae. These cells closely resemble the pillar cells of fish gills, but show certain fundamental differences. Characteristic features of the trabecular cells are (1) an elongated cell body which lies across the vascular lumen and attaches to the vessel wall by means of the tips of their long processes, (2) two types of myofilaments (thick and thin) in the cytoplasm, (3) external dense plaques at the cell surface which are associated with the insertion of myofilaments into the cell membrane, (4) direct contact between the cell surface and the blood except at the regions where the cell is covered by external plaques and connective tissue fibrils. These facts suggest that the Anodonta trabecular cell is not analogous with the pillar cell of fish gills but rather with muscle cells which show a specific morphological modification and a peculiar relationship to the vessel wall due to the absence of the endothelium. These cells are assumed to regulate blood flow within the gill vessels. As to the permeability of the wall of Anodonta gill vessels, junctional complex consisting of an intermediate and a septate junction between adjacent gill epithelial cells probably plays the main role as a barrier between the blood and the surrounding water. The basal lamina underlying the gill epithelium is assumed to act as a coarse permeability barrier. Numerous nerve endings of unknown function are observed in the gill epitheliu. It is strongly suggested, however, that they are associated with the additional function of the Anodonta gill lamellae as a food-sorting device.