GONADAL SEX-DIFFERENTIATION IN CHICKEN EMBRYOS - EXPRESSION OF ESTROGEN-RECEPTOR AND AROMATASE GENES

Estrogen is implicated in sexual differentiation of the avian gonad. E xpression of the estrogen receptor and aromatase genes was therefore e xamined at the time of gonadal sex differentiation in chicken embryos, using reverse transcription and the polymerase chain reaction (RT-PCR ). Estrogen receptor (cER) transcripts were detected in the gonads of both presumptive sexes at embryonic days 4.5, 5.5 and 6.5, and in fema le but not male urogenital tissues at day 3.5. Aromatase (cAROM) trans cripts were detected in female but not male gonads from day 6.5 of emb ryogenesis, and in adult gonads of both sexes. Both female and male em bryos thus express cER mRNA before morphological differentiation of th e gonads, which begins on day 5, whereas cAROM expression begins at or shortly after the onset of differentiation and is female-specific. Ex amination of other tissues showed that, in 5.5-day-old embryos, cER ex pression was limited to the gonads; no transcripts were detected in th e mesonephric kidney, liver, brain, hindlimb or heart of either sex. I n 9.5-day-old female embryos, cER and cAROM transcripts were present i n both the left (ovarian) and the right (regressing) gonads. Altogethe r, these observations imply that the gonads of both sexes develop the capacity to respond to estrogens early in embryogenesis, before morpho logical differentiation, whereas the capacity to synthesize estrogens is female-specific and occurs later, at the time of differentiation. T hese observations are consistent with estrogens having a key role in o varian development. (C) 1997 Elsevier Science Ltd. All rights reserved .