Estrogen target sites in the cloacal region of female and male chick embryos.

After intravenous injection of 3H-estradiol in the 12-day old chick embryo, radioactivity is concentrated in nuclei of certain cells in the cloacal area. The nuclear labeling is observed in mesenchymal cells along the different portions of the cloaca, and in an unidentified tissue located laterally to the cloaca. The labeled mesenchymal cells display a definite pattern of distribution along the epithelial wall of the cloaca, identical both in male and in female embryos. In the adjacent bursa of Fabricius, cells do not concentrate labeled hormone in their nuclei. The presence of estrogen receptors in the cloacal area of embryos of either sex adds evidence, at the cellular level, to support the concept of a "neutral", or undifferentiated, sex with estradiol inhibiting this "neutral" male differentiation.