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The distribution of progesterone receptor in the 20-day-old fetal mouse: an autoradiographic study with [125I]progestin.

The distribution of progestin target sites in 20-day-old fetuses of estrogen-primed pregnant mice was investigated by thaw-mount autoradiography. Pregnant mice received a Silastic estradiol implant on day 17 and were ovariectomized on day 19 of pregnancy. Twenty-four hours after ovariectomy 10 prematurely delivered fetuses were each injected with 0.33 microgram/100 g BW [125I]progestin (SA, 2200 Ci/mM). To show specificity of progestin localization two additional fetuses were each injected sc with 20 micrograms R5020, a synthetic progestin, 1 h before the injection of [125I]progestin. The fetuses were frozen 2 h after injection of [125I]progestin, sectioned, and processed for thaw-mount autoradiography. Cells with nuclear uptake and retention of radioactivity were observed in numerous tissues, including certain regions of the oral mucosa and developing teeth, esophagus, larynx, skin, mammary gland, skeletal muscle, kidney, and reproductive glands and ducts. Injection of unlabeled R5020 1 h before [125I]progestin prevented nuclear concentration of radioactivity in all target tissues. The results indicate that progesterone receptors are expressed with a regional, cellular, and subcellular distribution in term fetal mouse tissues and suggest that progesterone is important to the growth and development of certain fetal tissues.