Hes1 in the somatic cells of the murine ovary is necessary for oocyte survival and maturation.

The Notch pathway plays an important role in ovary development in invertebrates like Drosophila. However its role for the mammalian ovary is unclear. Mammalian Hes genes encode transcriptional factors that mediate many of the activities of the Notch pathway. Here, we have studied the function of Hes1 during embryonic development of the mouse ovary. We find that Hes1 protein is present in somatic cells and oocyte cytoplasm and decreases between E15.5 and P0. Conventional Hes1 knock-out (KO), Hes1 conditional KO in the ovarian somatic, and chemical inhibition of Notch signaling decrease the total number, size and maturation of oocytes and increase the number of pregranulosa cells at P0. These defects correlate with abnormal proliferation and enhanced apoptosis. Expression of the proapoptotic gene Inhbb is increased, while the levels of the antiapoptotic and oocyte maturation marker Kit are decreased in the Hes1 KO ovaries. Conversely, overactivation of the Notch pathway in ovarian somatic cells increases the number of mature oocytes and decreases the number of pregranulosa cells. Fertility is also reduced by either Hes1 deletion or Notch pathway overactivation. In conclusion, our data suggest that the Notch-Hes1 pathway regulates ovarian somatic cell development, which is necessary for oocyte survival and maturation.