Fine structure of sacciform cells in the epidermis of the brown trout, Salmo trutta

The fine structure of a cell type in the epidermis of brown trout, Salmo trutta L., is described. Its cytological features are compared with those of other sacciform cells reported in several species of teleost fish. This cell type also has a highly electron-dense cytoplasm with numerous Golgi systems and extensive rough endoplasmic reticulum. However, surrounding the large central vacuole there are no peripheral vesicles—'bubbles'—which are characteristic features in non-salmonid teleosts. Instead of these, in the vacuole there are cytoplasmic intrusions of circular cross-sections forming a mesh in the cortical zone. Inside these intrusions there are granules which are interpreted as ribosomes trapped by fusion of the vacuole and reticulum membranes. The way in which the secretion is released into the lumen of the central vacuole is discussed. It is suggested that this proteinaceous material does not pass through the Golgi system, but flows directly from endoplasmic cisternae to the vacuole.