Histochemical analysis of glycoproteins in the unicellular glands in the epidermis of an Indian freshwater fish Mastacembelus pancalus (Hamilton)

Summary The unicellular glands in the epidermis of the Indian freshwater fish Mastacembelus pancalus consist of three types of mucous cells and sacciform cells. The histochemical properties of their secretory glycoproteins have been analysed by means of a battery of histochemical methods. These included methods for the identification and simultaneous visualization of oxidizable vicinal diols, O-acyl sugars, O-sulphate esters and sialic acid residues with or without side-chain O-acyl variants. Four general classes of glycoproteins (GPs) were identified. These included (i) GPs with O-sulphate esters and oxidizable vicinal diols, (ii) GPs with oxidizable vicinal diols and sialic acid residues with or without O-acyl substitution at C7, (iii) GPs mainly with O-sulphate esters, low moieties of GPs with oxidizable vicinal diols, O-acyl sugars and sialic acid residues with side-chain O-acyl variant predominantly at C8 (or which are di- or tri-substituted) or C9 and in traces of sialic acid residues without O-acyl substitution or with O-acyl substitution at C7, and (iv) GPs with traces of oxidizable vicinal diols, O-acyl sugars and sialic acid residues with O-acyl substitution at C8 (or which are di- or tri-substituted) or C9. The physiological significances of these GP classes and their release on the surface of the epidermis are discussed with special reference to their role in lubrication, protection and inhibition of the invasion and proliferation of pathogenic microorganisms in the epidermis, as adapted to the peculiar mode of life of the fish.