The histopathological effect of methylmercury on the brain in orange spotted grouper (Epinephelus coioides) in Zangi Creek and laboratory

Abstract

Histopathological studies reflect the overall health of a community in an ecosystem and can be used as a biomarker of pollutants, distinguishing the presence and amount of the pollutant within the organism as well as assessing the risks the organism is facing. These studies can also provide a range for the pollutant concentration to indicate healthy and polluted environments. In order to assess the defects brought by industrial pollutants the histopathological defects of Methylmercury (MeHg) were studied. Methylmercury, is the organic derivative of mercury, possessing more dangerous and harmful effects than mercury. Methylmerucry is highly hydrophobic and due to its lipophilicity, it penetrates through the blood brain barrier and targets the brain more than the other tissues. Therefore the effect of MeHg on Epinephelus coioides brain in the laboratory and in Zangi Creek were studied. Fish were adapted to the tanks and exposed to 10, 20, 40 and 80 µg/L of methylmercury chloride in the Fisheries Center of Zangi Creek. The brains were collected at days 7, 14 and 30 of exposure and the depuration studies were performed for 7 and 14 days. The brains were dissected and sectioned for haemotoxylin and eosin staining and monitored under microscope. The defects pertained hyperanemia and hemorrhage, karyolysis and necrosis, nuclear dust, endothelium hypertrophy, hydrobic degeneration, vacuolation, cloudy swelling and edema. These defects were seen in different parts of the brain such as olfactory bulb, cereberum, optic lobe, cerebellum, diencephalon and medulla. The most defected part of the brain in response to MeHg seems to be the cerebellum. Depuration studies were performed for 7 and 14 days and mainly showed edema, cloudy swelling and hydropic degeneration in most parts (no rescue of phenotype seen). Fish were studied both in exposure to MeHg and preyed from field to compare the defects and assess the health and safety of the creek. The Histopathological Alteration Index (HAI) was assessed to show the amount of defects brought by the pollutant and revealed severe and irreversible defects in higher concentration of exposures. Zangi Creek's HAI fell between the control group and the lowest MeHg concentration and is still considered as a safe site below the threat range.