SITES OF ESTROGEN-RECEPTOR AND AROMATASE EXPRESSION IN THE CHICKEN-EMBRYO

Estrogens have been implicated in sexual differentiation of both the g onads and the genitalia of birds. In chicken embryos, the gonads are s teroidogenically active from an early age, and the aromatase gene, (cA ROM), necessary for estrogen synthesis, is expressed only in females a t the time of gonadal sex differentiation. However, no studies have di rectly demonstrated the distribution of estrogen receptor (cER) transc ripts or proteins in the embryonic avian reproductive system. Whole-mo unt in situ hybridization and immunohistochemistry were used here to i dentify sites of estrogen receptor expression in the embryonic chicken urogenital system. Estrogen receptor mRNA was observed in both male a nd female gonads prior to morphological differentiation, at Stage 26 ( 4.5 days of incubation), and continued until after sexual dimorphism a t Stage 32 (7.5 days). Transcripts of cER were also detected in the Mu llerian ducts and developing external genitalia of both sexes. Estroge n receptor protein was analysed in the embryonic gonads by immunohisto chemistry and found to be most abundant in the cortex of the left ovar y, although it was also present in the medulla of both female gonads. No significant cER protein expression was detected in the male gonad b y immunohistochemistry. In contrast, the aromatase gene was expressed in the gonads of female embryos from the onset of sexual dimorphism bu t was not detectable in male gonads at any stage examined. These findi ngs suggest that estrogen involvement in both gonadogenesis and genita l development in chickens is mediated by the estrogen receptor. (C) 19 97 Academic Press.