Studies on the structure and histochemistry of the epidermis in the air-breathing teleost Mastacembelus armatus (Mastacembelidae, Pisces).

The epidermis of Mastacembelus armatus is equipped with secretorily active gland cells which are found in large numbers in both the middle and outermost epidermal layers. The middle layer is formed by sacciform glandular cells characterized by an enormous secretion occupying the bulk of the gland cell that pushes the nucleus along with the cytoplasm to cell periphery. This secretion consists of coarse granules that probably transform into the homogeneous material prior to its extrusion to the exterior via the apical pore. The histochemical reactions indicate the presence of total proteins aromatic proteins, tyrosine and tryptophan in addition to the 5-hydroxytryptamine(5-HT) histochemically detectable by both diazo-safranin, alkaline diazo-reactions and histofluorescence procedures (FIF and GIF techniques). In the outermost layer are present numerous flask-shaped mucous cells (globlet cells) which secrete copious amounts of strongly acidic mucous glycoproteins containing O-acetylated sialic acids detected by some recent sophisticated methods in addition to the routine enzyme extraction techniques based on the use of neuraminidase. A mucous coating at the surface epidermal layer could be not located and the profuse secretions of the goblet cell mucins can affect the modalities of the cutaneous respiration of this fish provided with high vascular adaptations of the dermis. The presence of biogenic amines and especially of 5-HT could be correlated with a possible irritating action to produce the predator repellant or intimately connected with the synthesis and release of the integumentary products of fish skin. Its function, remains to be solved with biochemical methods suitable to investigate thoroughly the chemical nature of the proteinaceous product in each species showed with sacciform cells.