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SummaryThe ultrastructural and histochemical characteristics of mucous cells and their functional significance in the gills of Schizothorax curvifrons were studied with the objectives that this study could serve as a basis for further studies in relation to histopathology, aquatic toxicology and stress conditions in aquaculture and natural or polluted environments. The electron microscopical analysis of the gills following fixation in Karnovsky's fluid and post fixation in osmium tetraoxide revealed that the mucous cells were covered by epithelial cells and their major part was occupied by firmly arranged large mucous globules of different electrodensities, displacing their nuclei towards the basal position. The cytoplasmic extensions and islets among the mucous globules and the cellular interdigitations between mucous cells and neighbouring cells were present and the release of mucin by exocytosis was also observed. Utilizing histochemical techniques that included methods of periodic acid Schiff (PAS) reaction and alcian blue method (AB pH 2.5) for localization of neutral and acid mucopolysaccharide secreting mucous cells showed that the gill epithelium was lined by mucous cells which were found to be PAS and AB positive, confirming the presence of both neutral mucous cells secreting hexose containing mucosubstances and acidic mucous cells secreting weakly acidic sulphated mucosubstances and sialomucins. The possible roles of the mucous cells and the products secreted by them have been discussed in this paper.

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The structural characteristics of chloride and pavement cells of the gills of snow trout, Schizothorax curvifrons, were investigated with the aid of the great depth and resolving power of a transmission electron microscope. Ultrastructural analysis showed that the chloride cells were distributed in the filament and interlamellar epithelia. They were circular to elongated in form having a central euchromatic nucleus, expanded tubular system and an abundance of mitochondria with parallel patterns of mitochondrial crests. On the other hand, the squamous to polygonal shaped pavement cells were endowed with irregular nucleus and electrodense cytoplasmic matrix embracing rough endoplasmic reticulum and lesser number of mitochondria, abundant filament bundles and dark vesicles of varying electrodensity. Some of the dark vesicles were bound to the apical plasma membrane of the primary lamellae bearing microridges covered with glycocalyx, however, the apical plasma membrane of the pavement cells of the secondary lamellae did not reveal any signs of the presence of micro ridges.

Summary Transmission electron microscopic study has been conducted on the epithelium of an unaltered intestinal tract of the snow trout, Schizothorax curvifrons. The ultrastructural analysis reveals that the epithelium comprises of enterocytes, goblet cells and rodlet cells. The enterocytes with basal nuclei were joined at their apical surfaces by desmosomes and microvilli forming a brush border were found on their apical plasma membrane, with a steady decrease in the length and density of the microvilli from the intestinal bulb towards the rectum. Spherical blebs were observed on the dense tips of the microvilli of the intestinal enterocytes. Distinct invaginations of the plasmalemma were found below the terminal net which is formed by the deep projection of the microfibrils of the microvilli in the apical cytosol. Endoplasmic reticulum, low density lipid droplets, heteromorphic mitochondria, and lysosomes were found scattered in the supranuclear cytosol of the enterocytes. The mucous secreting goblet cells interspersed between the enterocytes were occupied by tightly packed mucous globules of varying electrodensity. The rodlet cells were encountered only in the intestinal epithelium. The possible roles of the enterocytes and associated structures, goblet cells and rodlet cells have been discussed in this paper.