الفهرس 
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Abstract
It is well documented that the bony fishes comprise the largest and the most diverse class of the vertebrates, with over 20,000 species worldwide. Compared to other vertebrates, fish have relatively small brains. Fish brain varies greatly between species, This variation gives the opportunity to correlate the role of ecological factors on brain structure and function. Structure of the brain of different fishes varies in between spawning and pre spawning season markedly affected by hormonal alteration that happen in fishes in these periods. The present study aimed to illustrate the structural variations of cerebellum during reproductive cycles. Also, the cerebellar structure and function and its correlations with the sex hormonal variation during pre-and spawning seasons Four kinds of freshwater fishes; Malapterurus electricus, Synodontis schall, Labeo niloticus and Lates niloticus were collected from River Nile at Mansoura Governorate. The fishes were sacrificed and brains were separated and dissected for separating the cerebellum. The cerebellum was subjected for light and transmission electron microscopy and immunohistochemistry (GFAP and FSH) while the whole brain was subjected to biochemical analysis (dopamine, serotonin, ATPase, aromatase, LDH and G6PD), isoenzyme electrophoresis of LDH and G6PD, Fatty acids content and finally Random Amplified Polymorphic DNA (RAPD)-PCR. During spawning season : The Purkinje cells possessed pyknotic nuclei with a moderate condensed heterochromatin materials and convoluted nuclear envelope. The cytoplasm possessed fragmented rough endoplasmic reticulum and electron –dense compacted mitochondria. Electron-dense heterochromatin was highly detected within the nucleus of the granular cells, Demyelination of the nerve axons were also remarked. In Lates niloticus, a stricking observations were remarked. This was characterized by ensheathing the damaged granular cells by a thin coat of nerve fibers. Lates niloticus is highly susceptible to hormonal alterations during spawning season and exhibited increased cytological alterations in neuronal cells. ATPase, aromatase, lactic dehydrogenase and glucose-6-phosphate dehydrogenase activities were altered in the brain of the studied fishes during pre-spawning and spawning season. The brain activities of ATPase, aromatase and glucose 6-phosphate dehydrogenase were markedly increased in the spawning season and reached their maximum activities in the brain of Lates niloticus in comparison with the pre-spawning season. However, the brain lactic dehydrogenase activity was markedly increased during the pre-spawning season in comparison with the spawning season. This study illustrated that, the brain of the studied fishes exhibited genetic variations and their cerebellar structures are highly sensitive to hormonal alterations during pre-and spawning periods assessed by structural and functional changes.