| With the development of modern industry and agriculture, the release of heavy metals into the environment has been accelerating year by year. Heavy metals contamination poses a threat to human health and life quality, which becomes a serious environmental and social problem. Mercury(Hg) a kind of heavy metal element of strong contamination and serious toxicity to human health., Mercury spilt in the environment will concentrate into food through bioaccumulation of food chains, especially the aquatic food chain, and hence seriously harms the nervous and immune system of animals and human.As Hg is a big hazard to people health, researches on Hg's physiological toxicity mechanism and how to harness this threat have been widely focused all over the world.toxicity Specifically, the harm of Hg on the neural system as well as the molecular mechanism of has been addressed. However, reports of the effect of Hg on hard-bone fish is rarely encountered. Fish is an important component of water ecological system, which seats in the higher trophic postion of aquatic food chain and is thus directly related to human health. In this paper, we studied the morphological changes of the nervous system in loach(Misgurnus Anguillicaudatus) under the treatments of mercury, so as to provide more morphological datum for futher research:1. Brains of normal healthy loaches were dissected and embedded in wax, and transverse or longitudinal sections were cut and Nissl stained. The anatomy of loach brain was observed and investigated with light microscopy so as to provide a base for the research of Hg's toxicity on the never system.2. Immunohistochemical ABC method was applied to study the distribution of Glutamatergic and S-100 immunoreactive (IR) structure in the cerebellum of normal and Hg-treated (0.2 mg/L ) groups of loach. The results showed that the number of glutamatergic neurons were not significantly changed at the 6th day after Hg treatment, and increased little after the 9th day. However, from around 15th day after Hg treatment, the Glutamatergic neurons began decreasing. Compared to the control group, the immunoreactivity of Glutamatergic neurons in the Hg-treated group were markedly weakened, and the number of Glutamatergic neurons was also significantly decreased after 21st day of Hg treatment. Compared with that of controls, the number of S100-IR cells in Hg-treated loaches increased significantly, and the increase was proportional to exposure time. All these may be related to Hg's poison to neurons in the cerebellum, which activate glial reactions to protect neurons.3. The disrtribution and number of S100 immunoreactive (S100-IR) structure in the retina of loaches under the Hg2+ treatment were investigated.. Immunohistochemical ABC method was applied to display the S100-IR structure. The results showed that S100-IR cells appeared first in the nerve fibre layer (NFL) and ganglion cell layer (GCL) and then in the other layers. Compared with that of controls, the number of S100-IR cells in Hg-treated loaches increased significantly, and the increase was proportional to exposure time. In conclusion, using S100 antibodies, the neurotoxic effects of Hg appear to promote astrocytic reactions in the retina. The increase of glial cells suggested that they could play an important role in resistence to the toxicity effect of inorganic mercury on neurons in the brain.From morphological changes in the brain and sense organ of loach under the Hg pollution, it shown that Hg harmed to nervous system of loach significantly and this effect was proportional to exposure time. These reports supply a basic morphological information of Hg's toxicity on the brain of loaches, which helps to futher research the physiological and molecular mechanisms underlied.
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