Distribution and regulation of progesterone receptor in the urogenital tract of the chick embryo. An immunohistochemical study.

The early appearance, cellular distribution, and hormonal regulation of the progesterone receptor was studied in the urogenital tract of the chick embryo using antibodies to the receptor molecule. In embryos at day 5 of incubation the receptor is revealed in cell nuclei of the mesenchyme and the coelomic epithelium near the primordium of the urogenital sinus. In embryos at days 6 to 10 immunostained cells are found in the mesenchyme surrounding the urodeal and proctodeal epithelia. The first difference between male and female embryos appears at day 8, with a higher density of progesterone receptor-containing cells along the urogenital sinus epithelium in females. The female type of receptor-positive cell distribution can be induced in males by oestradiol treatment. Anti-oestrogens applied from day 0 of incubation do not prevent or delay the appearance of PR, but induce a male-type distribution in female embryos. In the gonads, immunostained cells appear unambiguously at day 6 in the medulla. At later stages, the receptor is revealed mostly in the medulla, although there are also positive cells in the cortex of the left ovary. The immunoreactivity is not significantly modified by oestradiol or anti-oestrogens. The mesonephros is devoid of immunoreactivity, whereas most cells of the metanephric mesenchyme are receptor-positive. In the Mullerian ducts progesterone receptor is not detected in control embryos of either sex until after day 10. Between days 6 and 7 Mullerian ducts become responsive to oestradiol, which induces progesterone receptor in luminal epithelial cells. In the spinal cord, receptor is detected in neurones of the ventral horn and the meninges, starting at day 6. The progesterone receptor is up-regulated by oestradiol in the mesenchymal cells. Mullerian ducts, and mesothelium, but not in the gonads or the spinal cord. In none of the tissues or organs where the receptor appears naturally during development could an anti-oestrogen treatment with Tamoxifen or RU39411 block or delay its constitutive appearance. The widespread and specific distribution of the progesterone receptor, its programmed appearance in various organs, and its precise hormonal regulation, are in favour of a morphogenetic role of progesterone in tissue differentiation, related or not to sexual differentiation of the urogenital tract.