The digestive cells of the hepatopancreas in Aplysia depilans (Mollusca, Opisthobranchia): ultrastructural and cytochemical study

Digestive cells are the most abundant cell type in the digestive diverticul a of Aplysia depilans. These are tall columnar or club shaped cells, covere d with microvilli on their apical surface. A large number of endocytic vesi cles containing electron-dense substances can be found in the apical zone, but the presence of many heterolysosomes of large diameter is the main feat ure of these cells. Glycogen particles and some lipid droplets were also ob served. Peroxisomes with a circular or oval profile were common, but crysta lline nucleoids were not detected in them, although a dense spot in the mat rix was observed in a few cases. These organelles were strongly stained aft er cytochemical detection of catalase activity. The Golgi stacks are formed by 4 or 5 cisternae, with dilated zones containing electron dense material . Arylsulphatase activity was detected in the Golgi stacks and also in lyso somes. Cells almost entirely occupied by a very large vacuole containing a residual dense mass seem to be digestive cells in advanced stages of matura tion. The observation of semithin and ultrathin sections indicates that the se very large vacuoles are the result of a fusion among the smaller lysosom es, Some images suggest that the content of these large vacuoles is extrude d into the lumen of the digestive diverticula, (C) 2000 Harcourt Publishers Ltd.