Steroidogenesis in Experimentally Induced Atretic Follicles of the Hamster: A Shift from Estradiol to Progesterone Synthesis

The effects of phenobarbital-delayed ovulation on in vitro steroidogenesis and aromatase activity of LH-stimulated preovulatory follicles was ascertained in the cyclic hamster. After 3 days of ovulatory delay, antral follicles exhibited early signs of atresia (degenerating granulosa cells and pyknotic nuclei within oocytes). On days 2 and 3 of delay, LH-stimulated follicles produced more progesterone (P 15–20 ng P/follicle·90 min) in vitro than control proestrous follicles (2.5 ng P/follicle·90 min). Concomitant with the increase in LH-stimulated P production was a decline in the in vitro production of androstenedione (A) and estradiol (E₂) on day 2 (A, 5 ng/follicle·90 min; E₂,4 ng) and day 3 (1 ng A, <0.2 ng E₂)-proestrous controls: 8 ng A or E₂/ follicle ·90 min. The steroidogenic profile of 1-day delayed follicles was similar to proestrous controls. A similar E₂-P shift was observed in vitro when the steroidogenic profile of preovulatory follicles on the morning of proestrus (2.5 ng P, 8 ng A or E₂) was compared with post-LH surge follicles (25 ng P, <1 ng A or E₂) and with new corpora lutea (36 ng P, <0.2 ng A, <1 pg E₂). A slight increase in the aromatizing activity of follicles was observed on days 1 and 2 of delay. However, atretic antral follicles (day 3 of delay) exhibited an aromatizing capacity similar to that of proestrous controls. Collectively, these results indicate that the loss of E₂-synthesizing capacity in atretic antral follicles (induced by ovulatory delay) is due to the lack of A precursor. In addition, the results indicate that phenobarbital-delayed ovulation induces a shift from E₂ to P synthesis in 3-day delayed follicles (early atretic). This E₂-P shift is similar to that in Graafian follicles on the afternoon of proestrus but occurs at a much slower rate.