Light and electron microscope studies on the kidney of Oreochromis niloticus fingerlings exposed to zinc, copper and mercury, singly or in combinations

Sublethal concentrations of zinc as zinc sulphate, copper as copper sulphate and mercury as mercuric chloride were used to treat the water media, singly or in combinations, where Oreochromis niloticus fingerlings were reared chronically. Daily mortality was noted until 45 days of exposure and computed every after 15 days as average cumulative mortality rate (ACMR). ACMR was highest in treatment group 8A (Zn to the 2.5th power/Cu to the 0.5th power/Hg to the 0.15th power) and lowest in treatment group 2B (Zn to the 2.5th power) on the 15th day sampling period, and highest among Group 5A (Zn to the 2.5th power/Cu to the 0.5th power) samples and lowest among Group 2B (Zn to the 2.5th power) on the 30th day. Interaction of these metals for ACMR showed synergism between zinc and mercury (6A), copper and mercury (7A) and among the 3 metals (8A) but antagonistic between zinc and copper (5A). The fingerlings were sampled after 15, 30, and 45 days of exposure for light and electron microscope studies of the tail Kidney. Histologically, the following changes were observed among the treated samples. Hyperemia, as evidenced by many blood cells in the renal interstitia and interstitial damage that resulted compacted tubules were observed. In both effects, antagonism was observed between zinc and copper in all sampling periods. Antagonistic interaction was also observed between zinc and mercury and among the 3 metals for hyperemia in all sampling days, and between copper and mercury until the 30th day only. After 45 days of exposure, synergism between copper and mercury in hyperemia was observed. Likewise, synergism between copper and mercury was observed with hyperemia among 15th-day samples and between zinc and mercury among 30th-day samples. The present study provides additional information on the nature of interaction of heavy metals that may be present in the environment of countries undergoing massive industrialization. Previous studies mainly focused on mortality as the index of the nature and extent of interaction of pollutants on the organisms. The emphasis of the present study, however, is on using the different histological anomalies as parameters