Ultrastructural and histochemical study of the salivary glands of Aplysia depilans (Mollusca, Opisthobranchia)

The digestive system of the sea hare, Aplysia depilans, includes a pair of ribbon-shaped salivary glands. A central duct and a large blood vessel run close to each other along the length of these glands and both are surrounde d by a layer of muscle cells. Three cell types form the glandular epitheliu m: granular cells, vacuolated cells and mucocytes. The granular cells posse ss cilia and spherical secretion granules, located primarily in the apical region. The granules of immature cells have a low electron density and are mainly formed by neutral polysaccharides with small amounts of proteins. Th e granules of mature cells are larger, have a high electron density and are mainly formed by proteins with lower amounts of neutral polysaccharides. T ransition stages between immature and mature granular cells are observed. T he vacuolated cells are large and frequently pyramidal in shape, but after the application of histochemical techniques almost all vacuoles remain unco loured. The numerous vacuoles contain flocculent material in a clear backgr ound and the mitochondria possess large crystalline structures in the matri x. A pyramidal shape is also typical of the mucocytes, which are filled wit h vesicles containing granular masses surrounded by a network of secretion material. These large cells are strongly stained by Alcian blue, revealing the presence of acidic mucopolysaccharides. This is the first ultrastructur al study of the salivary glands in opisthobranch gastropods.