Histopathological changes in the olfactory organ of rainbow trout (Oncorhynchus mykiss) induced by early chronic exposure to a sublethal copper concentration

Sequential histopathological changes related to a long-term sublethal copper exposure were investigated in the olfactory organ of rainbow trout, Oncorhynchus mykiss (formerly Salmo gairdneri Richardson, 1836). Fish were continuously exposed to an average copper level of 22 μg/L from the 14th day after fertilization (embryo group) or from hatching (alevin group) for a period of 41 and 37 weeks, respectively. A control group was reared in well water. Specimens of the exposed and the control groups were sacrificed every month for histological examination by light microscopy. Morphological alterations were identical in both embryo and alevin groups and appeared at the same posthatching periods. The first change, noted from the 8th week posthatch, was an increase in goblet cells. Epithelial lesions, which looked like patches of necrotic tissue randomly distributed in the olfactory sheet, were reported from the 28th week posthatch. From the 32nd week, cellular shrinkage related to degeneration and necrosis of individual cells led to a pattern of loose epithelium in many olfactory areas. These sequential histopathological changes suggest an initial increased mucus production which, however, does not appear effective enough to ensure long-term protection of the olfactory rosette. Stopping the copper exposure led to some morphological restoration. Ten weeks after copper removal, a few epithelial lesions and shrinkage were still noted. The delay between morphological and functional recoveries supports the concept of an anatomical redundancy of the epithelium-to-bulb connections.