Expression of five steroidogenic genes including aromatase gene at early developmental stages of chicken male and female embryos.

In the course of avian embryo development, estrogen has been indicated to play a key role in gonadal differentiation by the inhibition of aromatase (P-450arom) that synthesizes estrogen from androgen. Biosynthesis of estrogen requires not only P-450arom but also other enzymes for a steroidogenic pathway. To elucidate gonadal differentiation, the steroidogenic pathway should be studied comprehensively in the early developmental stages including that of sex differentiation. Therefore, in the present study, the expressions of the steroidogenic genes, P-450scc, 3beta-HSD, P-450c17, 17beta-HSD and P-450arom, were measured at the developmental stages (days 2-9 of incubation) of chicken embryos by quantitative RT-PCR. Transcripts for all the genes studied, except for P-450arom were detected in all the developmental stages examined, indicating that mRNAs for the steroidogenic enzymes required to convert cholesterol to androgens are present in the avian embryo before gonadal differentiation. In contrast, P-450arom mRNA was detected in female embryos during days 5-9 of incubation but not in male embryos throughout incubation. The onset of P-450arom gene expression at day 5 coincides with the stage of gonadal differentiation, corroborating the role of estrogen in the process of gonadal differentiation in chicken.