Effects of perinatal steroid hormonal changes on the cytoarchitecture of brain

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To elucidate the mechanism of the formation of sexual dimorphism, we examined the number of apoptotic cells in the medial preoptic nucleus in male and female rats of various developing stages. Many apoptotic cells in the postnatal day 1 and 4 were found in the preoptic periventricular nucleus and its number was significantly larger in male than in female. The number of apoptotic cells in the medial preoptic nucleus was increased in the postnatal day 4 and 7 and significantly larger in female than in male. It was found that the distribution of apoptotic cells depends on the sexuality, the location of the area and postnatal period, indicating that the apoptosis contributes the formation of sexual dimorphism in the medial preoptic area. The medial preoptic area contains highest number of estrogen receptors. Therefore we expanded our experiments to investigate the subcellular localization of estrogen receptor in cultured cells, in which estrogen receptor linked to GFP was over expressed. With or without ligand, the subcellular localization of estrogen receptor was localized in the nucleus, not in the cytoplasm.It was concluded that the sexual dimorphism in the brain was due, at least in part, to the apotosis of the neurons in the specific areas and these apoptosis was strongly linked to the gene expression of steroid hormone receptor.