Fine structure of the club cells in the skin of ostariophysan fish.

: The club cells of the epidermis of a number of species of cyprinoid and siluroid fish have been examined by electron microscopy. As reported by previous authors, most cellular organelles are confined to a perinuclear zone. The outer cytoplasm, hitherto described as fibrillar or vesicular, is characterized by the presence of filaments of approximately 10 nm diameter, which have a spiral configuration. These helices are arranged in random directions. The coil diameter is about 50 nm, but the pitch varies widely, between species and even sometimes in different examples in a single specimen. The outer cytoplasm also contains amorphous electron-dense material, some vesicular profiles, especially at the periphery of the cell, and some ribosomes. Ribosomes are more numerous in the small club cells deep in the epidermis which are presumed to be relatively juvenile. The coiling filaments are associated with desmosome plaques, like the tonofilaments of the epithelial cells. A further resemblance to tonofilaments was noted in two species of topical catfish, where chilling the tissue induced felting of the tonofilaments and also of the helical filaments of the club cells, which aggregated to form a capsule of electron-dense material surrounding the perinuclear cytoplasm. The functions of club cells are briefly not yet experimentally established, and that the recognition of specific pheromones in ostariophysan fish is a secondary phenomenon. There is evidence suggesting that is some circumstances a club cell may discharge part of the outer cytoplasm, and then be re-enclosed in the epidermis. In damaged epidermis, club cells occasionally become confluent above the basal layers of epithelial cells.