DISTRIBUTION PATTERNS OF THE PARANEURONAL ENDOCRINE-CELLS IN THE SKIN, GILLS AND THE AIRWAYS OF FISHES DETERMINED BY IMMUNOHISTOCHEMICAL AND HISTOLOGICAL METHODS

The neuro-endocrine cells of fish skin and respiratory surfaces, and t heir bioactive secretion as far as is known, are reviewed, and compare d with similar elements in tetrapods, particularly amphibians. In the skin of teleost fish, immunohistochemistry has shown that Merkel cells react for serotonin, neuron-specific enolase and enkephalins. The pha rmacology is not established in dipnoans or lampreys. In some teleosts , neuromasts react for substance P and leu-enkephalins; substance P is also reported from some ampullary organs (electroreceptors). Taste bu ds of teleosts may react for enkephalin and substance P. Basal cells o f taste buds react for serotonin and neuron-specific enolase. Some uni cellular skin glands of teleosts express bioactive compounds, includin g serotonin and some peptides; this ectopic expression is paralleled i n amphibian skin glands. The dipnoan Protopterus has innervated pulmon ary neuro-endocrine cells in the pneumatic duct region with dense-core d vesicles. In Polypterus and Amia the lungs have serotonin-positive n euro-endocrine cells that are apparently not innervated. In fish gills , a closed type of neuro-endocrine cell reacts for serotonin, an open type for enkephalins and some calcium-binding proteins (calbindin, cal modulin and S-100 protein). The functions of neuro-endocrine cells in fishes await investigation, but it is assumed they are regulatory.