Histochemical studies of epithelial mucosubstances, respiratory and pentose shunt enzymes in the skin epidermis of the air-breathing fish Mastacembelus erythrotaenia (Mastacembelidae, Pisces).

The skin epidermis of the teleost fish Mastacembelus erythrotaenia has been studied by histochemical methods. 4 cell types were considered in the comparison; mucous goblet cells, sacciform cells, eosinophilic granular cells, and Malpighian cells. Succinate, isocitrate, glucose-6-phosphate, malate, and alpha-glycero-phosphate dehydrogenase did not indicate any changes in activity in the outer epidermal cell layers in comparison with the lower ones where the weakening or absence of the reactions could be inferred to a reversible metabolic state in the fields of the tricarboxylic acid cycle and pentose phosphate pathways. The strong activities of the above oxido-reductive enzymes found in the basal layer were correlated with the proliferative activity of stem cell differentiation and kinetic processes of cell division. The major component of the mucus produced by the mucous goblet cells consists of weakly acidic mucins showing in their carbohydrate moieties both the sialic acids and sulphated components with a lower degree of sulphation as evidenced by the appropriate histochemical methods. The sacciform cells produce a tryptophan rich protein and serotonin, the latter presumably acting in the control of mucus release since the N-acetylneuraminic acid is known to be an integral part of the serotonin receptor (Gottschalk 1972). Other functions related to the above amine may include its toxicity against the potential predators.