Estrogen target cells in gonads of the chicken embryo during sexual differentiation

Estrogen target cells were searched for in the differentiating gonads of the chicken embryo in order (1) to establish at the cellular level that steroid hormones can play a physiological role in gonadal sexual differentiation, and (2) to localize their sites of action. An autoradiographic technique carried out with frozen sections was employed to demonstrate the presence of binding sites for radiolabelled hormone in the nuclei of the target cells. Target cells for [3H]estradiol are found similarly in gonads of both male and female embryos from 5½ (stage 27 of H and H) to 7 days of incubation. Estrogen target cells are observed in the germinal epithelium of the left but not the right gonad, and in the medulla of both the right and left gonads. In the medulla, numerous cells inside the cords are a target for estradiol. The germ cells, difficult to identify unmistakably in the experimental conditions, do not seem to be a target for estrogen hormones. A 100-fold excess of unlabelled estradiol abolishes the nuclear labelling, which is only slightly reduced after a similar excess of unlabelled dihydrotestosterone. It is concluded that the nuclear binding sites have a limited capacity for steroid hormones and are specific for estrogen hormones. The lack of clear and consistent nuclear labelling after [3H]dihydrotestosterone injection confirms the specificity of the [3H]estradiol nuclear labelling. At day 10 of incubation, only the undifferentiated remnant of the germinal epithelium in the left testis still displays labelled cells after [3H]estradiol injection. These observations confirm the determinative role currently ascribed to the estrogen hormones in the cortical differentiation, but they also emphasize that this role extends to the medulla of both gonads. In light of this presence of estrogen receptor sites in the medullary cords as well as in the germinal epithelium, one can assign the estrogen hormones more specific and diversified roles than currently believed. These roles also appear very precocious in the process of gonadal differentiation. Finally, the absence of target cells for estrogen hormones in the germinal epithelium of the right gonad accounts for the lack of cortical differentiation on the right side.