Gene expression of steroidogenic enzymes in chicken embryonic gonads

This paper reviews the molecular aspects of sex differentiation in birds and describes the sex-reversal effects of aromatase inhibitor on mRNA expression of steroidogenic enzymes. Although it is unknown whether the sex-determining gene resides in the W chromosome or whether a gene-dosage mechanism is involved in sex determination, it is clear that anti-Müllerian hormone (AMH) and androgen play a key role for testicular development, whereas estrogen is important for ovarian formation. Our in situ hybridization results demonstrated that mRNA expression of P450aromatase first takes place on day 6.5 of incubation exclusively in the female gonad. Treatment with aromatase inhibitor in ovo suppressed mRNA expression in genetic females. This suggests that ovarian development is promoted by aromatase expression via estrogen production. On the other hand, previous reports demonstrated that formation of the testis is accompanied by Sox9 and AMH mRNA expression, which start on days 6 and 8, respectively. This suggests that Sox9 plays an important role for the male determining pathway which leads to AMH expression and formation of the testis.