Premutations of the fragile X mental retardation 1 (FMR1) gene are associated with increased risk of primary ovarian insufficiency. Here we examined the localization of the Fmr1 gene protein product, fragile X mental retardation protein (FMRP), in rat ovaries at different stages, including fetus, neonate, and old age. In ovaries dissected from 19 days postcoitum embryos, the germ cells were divided into 2 types: one with decondensed chromatin in the nucleus was FMRP positive in the cytoplasm, but the other with strongly condensed chromatin in the nucleus was FMRP negative in the cytoplasm. The FMRP was predominantly localized to the cytoplasm of oocytes in growing ovarian follicles. Levels of FMRP in oocytes from elderly (9 or 14 months of age) ovaries were lower than in those from younger ovaries. These results suggest that FMRP is associated with the activation of oogenesis and oocyte function. Especially, FMRP is likely to be implicated in germline development during oogenesis.
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