Chromatin deactivation in pregranulosa cells contributes to primordial follicle formation

The establishment and maintenance of the primordial follicle (PF) pool are critical for determining the ovarian reserve in females, ensuring efficient oocyte production, particularly in farm animals such as egg-laying chickens. This process involves the breakdown of germ cell cysts and the encapsulation of individual oocytes by somatic pregranulosa cells (pre-GCs) to form PFs, which then enter a lasting state of dormancy. The proper development of pre-GCs is essential for their interaction with and protection of oocytes, facilitating successful follicle formation. How pre-GCs organize to regulate PF formation and thesubsequent quiescence entry remain poorly understood. This study investigated gene expression and chromatin regulation in chicken pre-GCs during the PF establishment to identify epigenetic mechanisms that could enhance ovarian reserve and improve egg production. Cleavage Under Targets and Tagmentation sequencing assay revealed a global reduction in active chromatin as evidenced by the depletion of acetylation of lysine 27 on histone H3 deposition. This chromatin deactivation caused the downregulation of target genes involved in promoting cell apoptosis and blocking cell adhesion, thereby supporting cell survival and enhancing cell-cell communications necessary for follicle assembly. Further validation using an ex vivo chicken ovary organ culture system demonstrated that prolonged chromatin activation, achieved through histone deacetylase inhibitor-mediated chromatin acetylation, significantly reduced the number of PFs and resulted in the persistence of unruptured germ cell cysts. Forcibly deactivating the chromatin increased PF formation. Therefore, the chromatin deactivation regulates the establishment of PFs and their entry into quiescent states, offering potential strategies to improve ovarian reserve and egg production in different animals.