The identification of a gene expression signature of primordial follicle activation in mouse pregranulosa cells

Female fertility is dictated by the number of oocyte-containing primordial follicles within the ovary. These follicles are established early in development and their selective activation represents the definitive first step towards oocyte maturation and ovulation. The intrinsic molecular mechanisms that regulate ovarian follicle activation are largely uncharacterised. In this study, we report a single-cell RNA sequencing (scRNAseq) dataset to examine mouse embryonic and neonatal ovaries across the developmental window of primordial follicle formation and activation. We identified five distinct clusters of granulosa cells and bioinformatic analysis revealed a population of pregranulosa cells that appeared to be undergoing follicle activation. This cluster was uniquely differentiated by the expression of Tnni3, Slc18a2, Fam13a and Klf2, all of which are novel to follicle activation. Finally, we examined the transcriptome of a mouse model where all primordial follicles enter folliculogenesis simultaneously (Cdkn1b-/-) and showed that the signature genes of activating pregranulosa cells was observed precociously in Cdkn1b-/- ovaries, further demonstrating that p27kip1 acts as an important repressor of follicle activation. Combined, this data provides insight into how primordial follicle activation is regulated in mammals and could be utilised to identify pathways to target to improve fertility preservation and infertility conditions in the future. ### Competing Interest Statement The authors have declared no competing interest.