The microanatomy of the skin of an air-breathing fish, Notopterus notopterus is described and related to the distribution of various histochemical constituents involved in keratinization.
In addition to distinct unicellular mucous glands, secreting sulphated acid mucopolysaccharides, the epidermis of TV. notopterus is mainly composed of polygonal cells containing sulphated acid mucopolysaccharides, concentrated in the distal peripheral areas of the cells. The mucopolysaccharide contents of these polygonal cells gradually increase as they mature and move towards the outer surface. They may ultimately form a part of the protective keratinized layer.
The mature keratinized cells give positive reactions with Papanicolaou's stain, contain cystine bound SS groups, cysteine bound SH groups, calcium and moderate amounts of sulphated acid mucopolysaccharides. These cells having flat pyknotic nuclei undergoing karyolysis, are fused together at their lateral margins to form an intact sheet which is frequently seen sloughing off from the underlying epidermal cells.
The process of cornification in N. notopterus is comparable to that of the amphibians and the aquatic reptile–the turtle.
There exists a well developed lymphatic system in between the cells of the stratum germinativum having lymphatic spaces often penetrating deep into the middle layer. This is an adaptation which contributes largely to the protective function of the fish skin.
The layers between the scales and the basement membrane contain sulphated acid mucopolysaccharides. This together with keratinization has been correlated with the peculiar mode of life of the fish, often facing the problems of desiccation.
The epidermis is anchored on the dermis by fine strands of collagen fibres connecting the basement membrane and the scales, and by numerous fine papilla-like projections of the basement membrane projecting in between the cells of the stratum germinativum.
Numerous fine elastic fibres in the connective tissue pockets in which scales are lodged, provide for free movement of the scales when the body of the fish is bent during swimming.
Pit organs singly or in groups are found distributed in the epidermis.