Two day old female rats were injected sc with 1.0 μg of 3H estradiol per 100 g body weight and killed 2 hr later. Oviducts and uteri were removed, mounted on tissue holders, and frozen in liquified propane. Four μm frozen sections were cut in a cryostat and mounted according to the dry mount and thaw mount autoradiographic procedures. The sections were exposed at -15°C for different periods of time, then photographically processed and stained with methyl green pyronin. Radioactivity was found concentrated in nuclei of cells of the luminal epithelium in the uterus, but not in the oviduct, while cells of the connective tissue and muscularis showed nuclear concentration of radioactivity in both organs. Pretreatment with estradiol inhibited the nuclear concentration of radioactivity, whereas pretreatment with 5α dihydrotestosterone had no effect. It is concluded that both the uterus and oviduct of the neonatal rat have receptors for estrogen as early as 2 days after birth.
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