17β-estradiol and estrogen receptor alpha protect mouse ovarian follicle development by repressing atresia

Mice administered an inhibin antiserum, equine chorionic gonadotropin (eCG) mixture called CARD HyperOva (OVA)/human chorionic gonadotropin (hCG), ovulate more oocytes than those administered eCG/hCG. In this study, the mechanism by which more oocytes are ovulated was investigated. Apoptotic cells were not observed in ovaries 24 and 48 h after OVA injection, and INHBA expression was absent in the growing follicles with apoptotic cells, whereas granulosa cells in follicles expressing INHBA also expressed estrogen receptor α (ESR1). ESR1 and CYP19A1 expression and 17β-estradiol (E₂) in sera significantly increased in OVA-injected mice. Esr1 and Cyp19a1 expression and E₂ concentration increased in ovaries cultured with activin A. The ovulation number increased in mice administered diethylstilbestrol. Taken together, these results suggest that ESR1 and E₂ are involved in the inhibition of follicular atresia, which increases ovulation and oocyte number. This study may provide valuable information on the molecular mechanisms underlying mouse follicle selection.