Ultrastructural changes in the respiratory lamellae of the catfish, Heteropneustes fossilis after sublethal exposure to malathion

Transmission electron microscopy study of the gills of Heteropneustes fossilis, exposed to 4 mg/liter of malathion (1/3 of LC50) for 24, 48, 72, and 96 h showed significant changes in its ultrastructures. Exposure to the pesticide after 24 h caused a slightly disarrayed condition in the double layered epithelial structure. Lymphatic spaces became more apparent, and a few chloride cells appeared which protruded toward the peripheral margin of the secondary lamellae. Chloride cells were exposed to the exterior by an apical pit. Pinocytosis was observed with marginal folds (MF) originating from the pillar and epithelial cells. Some vascular constrictions were also seen in the capillaries with erythrocytes. After 48 h exposure, the outer epithelial cells were stretched into a thin boundary wall and lymphatic spaces were engorged with plasma exudate. Chloride cells transversed the whole epithelium of the lamella and came into direct contact with lymphoid space and exterior to epithelial lining. Basement membrane of the capillaries became thicker. After 72 h a distorted lamellar epithelium ruptured in a few places allowing many spheroid bodies and some chloride cells come out. Marginal folds of pillar cells migrated into vascular spaces. Basement membrane of capillaries became thicker and blood channels were constricted causing vascular stasis. No erythrocytes were visible. Blood channels were filled with leukocytes and amoebocytes. After 96 h exposure to malathion narrowing of lymphatic spaces, proliferation of epithelial cells and development of pinocytotic vesicles from marginal folds of pillar cell flanges were observed. Only marginal blood channels maintained normal configuration. Vascular stasis due to thickening of the basal lamina were still evident in centrally located blood channels filled with leukocytes. Vascular stasis would likely cause a decrease in respiratory efficiency. This study has revealed that the gills of H. fossilis were affected by a sublethal dose of malathion. The ultrastructural damages to the gills were observed as early as at 24 h exposure, but the most severe damage occurred at 72 h exposure. However, signs of gill structure regeneration were seen in malathion-exposed fish after 96 h.