MORPHOMETRICAL AND ULTRASTRUCTURAL STUDY ON THE PROTECTIVE EFFECTS OF MELATONIN IN METHYL MERCURY PERIPHERAL NEUROTOXICITY
Ehab A. A. El Shaarawy and Sherif Mohammed Sabry
Department of Anatomy, Faculty of Medicine,Cairo University
Mercury is a heavy metal contaminant. In nature, elementary mercury (Hg°) can be biotransformed and converted to methyl mercury (MeHg) which is the most toxic form of mercury in the environment that may lead to dangerous impact on the health of humans and animals due to the exposure in low doses for a short period of time. The present work aimed to study the tissue changes that may occur in the sciatic nerve of adult male albino rat on exposure to small doses of MeHg and to ensure the protective role of melatonin in peripheral neurotoxicity in these circumstances. Forty adult male albino rats were used in the current study. They were divided into four groups, 10 rats each. The first group served as control, the second group the animals were given MeHg daily for seven successive days, the third group received the same previous dose then was left to survive for 15 days for recovery and the fourth group received both 1 mg/Kg MeHg and 10 mg/Kg melatonin daily for seven days. At the end of the experiment, the animals were sacrificed by cervical dislocation. The sciatic nerve of each animal was dissected, excised and prepared for light and electron microscopic examination. In the current study it was found that most of the myelinated nerve fibers in group two (after MeHg intake), exhibited degeneration and their axoplasms appeared distorted and lost their normal pattern. The myelinated nerve fibers showed variable degrees of demyelination of the axons ranged from mild to severe (from grade I to VI). The endoneurium was seen abundant and distorted. In severe affection it appeared irregular, severely distorted and filled with tissue debris and multiple vacuoles. In group three it was found that the degenerated myelin sheaths were absent in some parts, and fragmented with loss of their normal lamination in other parts. Few regenerating myelinated fibers were seen with apparently normal axoplasms while other areas exhibited variable degrees of degeneration. Group four (given melatonin) showed regenerating myelinated nerve fibers in most of cases to the degree of normal appearance. Few cases showed mild degeneration either of grade I or II. On the other hand, the non-myelinated nerve fibers showed no affection in all experimental groups. At the end of this work it was concluded that the oral intake of methyl mercury in low doses for seven successive days had a toxic effect on sciatic nerve in adult male albino rat. The intake of melatonin in these cases protected the sciatic nerve and helped regeneration of the degenerated nerve fibers. It is also recommended to avoid methyl mercury exposure and use the melatonin as antioxidant to protect from the peripheral neurotoxic effect of this substance.