Secretion in the epidermis of polypteriform fish.

Calamoichthys calabaricus possesses four types of epidermal secretory cell. Mucus is produced by the superficial epithelial cells and by goblet cells. Electron microscopical studies show that the other two types of unicellular gland present are structurally similar to the club cells and to the sacciform cells of teleost fish. The outer cytoplasm of the club cells contains filaments, coiled in spirals, which can be compared to the helical filaments previously described in club cells of eels, although they are less regularly arranged. Spiral filaments similar to those of Calamoichthys can be detected in some ostariophysan species studied by electron microscopy. The homologies of so-called club cells in various fishes are discussed. The lampreys and gadoids are now known not to have club cells; in eels the club cells contain a secretory vacuole in addition to cytoplasm with helical filaments. The club cells of ostariophysans have been distinguished, by some authors, as "alarm substance cells", but their cytological characteristics are so close to those of the club cells of Calamoichthys that, morphologically, they must be considered homologous. The sacciform cells are a separate type of unicellular gland; their fine structure in Calamoichthys is closely similar to that reported from certain teleost fish. In spite of the cell diversity, polypterid skin provides a model for a type of epidermis which is primitive in the actinopterygii.