Structural, histochemical and functional aspects of the epidermis of fishes

Significant progress has been made in the last 30 years in our knowledge of cellular and physiological aspects of fish epidermis This review surveys the histology, histochemistry (including lectin- and immunohistochemistry) and function of the epidermis of various teleosts and cyclostomes in the light of the new data. The epidermis is a multipurpose tissue, although the secretory function is dominant. Mucous secretions are produced by different types of epithelial cells, in particular those located in the superficial layer. Specific unicellular glands in the skin of gnathostome fish are the goblet cells, the sacciform cells and the club cells. In addition, lampreys and hagfish possess specific secretory cell types. Other specialized types are the holocrine venom cells which are aggregated in venom glands, the mitochondria-rich cells (ionocytes) and the photocytes, the latter grouped with other cell types to form photophores that are embedded in the skin. The outer epidermal cells and the mucous goblet cells secrete the epithelial mucous layer, whose carbohydrate composition changes with stress and environmental conditions. The mucus contains neuronal and endothelial forms of nitric oxide synthase (NOS), the enzyme responsible for the production of nitric oxide (NO), a prominent vascular and neuronal messenger that regulates many epithelial functions. The cytokeratins in the skin tissues have different distribution patterns in the various cell types, correlated with specific epithelial differentiation or, in hagfish, the modulation of the viscoelastic properties of skin mucus Club cell and sacciform cell secretions are now considered as storehouses of biologically active substances. Their expression is paralleled with that present in glandular cells of the skin of other lower vertebrates. The fine structure and histochemical properties of club cells are clearly different from those of the venom cells in fish and amphibians. Mitochondria-rich chloride cells (ionocytes), although primarily concentrated in the branchial epithelium, also occur in various areas of the skin. The skin chloride cells, like those in the gill, secrete monovalent ions in sea water and take up ions in fresh water. Fish skin has a diffuse paraneuronal cell system responsible for the production of bioactive compounds that have possible regulating functions. Neuropeptides occur in the Merkel cells, the mechanoreceptors (neuromast hair cells) and the electroreceptors (sensory cells of the ampullary and tuberous organs) of the skin. Basal cells of the taste buds are the source of neuron-specific enolase, serotonin, bombesin and somatostatin. Photophores contain serotonin and adrenalin, which play a neuromodulatory role in the regulation of luminescence. The functions of the skin secretions are primarily protective, as a result of their antimicrobial properties. Further analysis of the components of epidermal cells and unicellular glands of aquatic animals will disclose many protective factors