Estrogen inhibits the early development of mouse follicles through regulating the expression of Kit ligand

► We analyzed the effects of estrogen on follicles using in vitro and in vivo system. ► Estrogen inhibits the development of earlier follicles. ► We examined the expression of Kit ligand in ovaries treated by estrogen. ► Both mRNA and protein expression levels were decreased in the treatment group. ► Estrogen effects the development of follicles through Kit ligand. Estrogen inhibits cyst breakdown and primordial follicle assembling of germ cells, but little is known about the underlying mechanisms. We aimed to analyze the effects of estrogen on the early development of mouse follicles using an in vitro ovary culture system and in vivo injection. Newborn mouse ovarian tissues were cultured in vitro for 2 or 4 days with estrogen of 0 M, 10 −8 M and 10 −4 M, respectively, and neonatal mice were injected with 5 mg/kg/day estrogen. We found that the percentages of different-stage follicles significantly varied between the control and estrogen-treated groups. In vitro experiments showed that the unassembled follicles accounted for 70.5 ± 2.7% and the primordial follicles accounted for 29.5 ± 2.7% in the treatment group, but in the control group, ovaries had 61.7 ± 8.4% unassembled follicles. In vivo experiments showed that the percentages of unassembled follicles and primordial follicles were 37.1 ± 5.2% and 51.6 ± 2.4% in the control group, while they were 72.6 ± 5.2% and 25.1 ± 5.5% in the treatment group. Moreover, we analyzed the expression of Kit ligand in mouse ovaries treated by estrogen with real-time PCR and western blot technology, and found that compared with the control group, both mRNA and protein expression levels were decreased in the treatment group ( P < 0.05). These results indicate that estrogen inhibits the development of mouse ovarian follicles by regulating the expression of Kit ligand.