Histopathological analysis of acute toxicity of zinc chloride on the respiratory organs of air-breathing catfish Heteropneustes (Saccobranchus) fossilis (Bloch)

Abstract

Histopathological analysis of acute zinc chloride toxicity (75 ppm: 96 h LC50 value) to accessory respiratory organs (ARO) and gills of Heteropneustes fossilis shows that the exposure causes severe toxicopathological damage to both these respiratory organs, although only the gills come into direct contact with the ambient xenobiotic. The ARO becomes affected via the circulatory system. The mucous cells of these two tissues show the earliest toxic manifestation in the form of periodic fluctuations in their density, as well as staining properties. The marked toxicopathological alterations in the ARO include periodic necrosis and sloughing off of the respiratory epithelium, leading to haemorrhage and lamellar deformity, followed by their regeneration and hyperplasia. However, the lamellar elements re-differentiate on each occasion from the focal inflammatory tissues. Deposition of a large quantity of glycogen in the muscles is also indicative of disturbed aerial respiration. Damage to gills include periodic lifting of the respiratory epithelium, followed by its sloughing, vacuolization and extensive fusion of secondary lamellae, resulting in reduced respiratory area. Uncontrolled hyperplasia of epithelial cells following regeneration also increases gas diffusion distance. Certain similarities in the histopathological manifestations in these two tissues are possibly due to their common embryonic origin. Hence, the study of the histopathology of ARO of exposed Heteropneustes fossilis can also be used as a potential bioindicator to monitor altered water quality.