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Abstract Ovarian follicles are the fundamental structures that support oocyte development, and communications between oocytes and follicle somatic cells are crucial for oogenesis. However, it is unknown that whether exposure to microgravity influences cellular communications and ovarian follicle development, which might be harmful for female fertility. By 3D culturing of ovarian follicles under simulated microgravity (SMG) conditions in a rotating cell culture system, we found that SMG treatment did not affect the survival or general growth of follicles but decreased the quality of cultured follicles released oocytes. Ultrastructure detections by high-resolution imaging showed that the development of cellular communicating structures, including granulosa cell transzonal projections and oocyte microvilli, were markedly disrupted. These abnormalities caused chaotic polarity of granulosa cells (GCs) and a decrease in oocyte-secreted factors, such as Growth Differentiation Factor 9 (GDF9), which led to decreased quality of oocytes in these follicles. Therefore, the quality of oocytes was dramatically improved by the supplementations of GDF9 and NADPH-oxidase inhibitor apocynin. Together, our results suggest that exposure to simulated microgravity impairs the ultrastructure of ovarian follicles. Such impairment may affect female fertility in space environment.

Abstract Background Ovarian follicles, which are the basic units of female reproduction, are composed of oocytes and surrounding somatic (pre) granulosa cells (GCs). A recent study revealed that signaling in somatic preGCs controlled the activation (initial recruitment) of follicles in the adult ovaries, but it is also known that there are two waves of follicle with age-related heterogeneity in their developmental dynamics in mammals. Although this heterogeneity was proposed to be crucial for female reproduction, our understanding of how it arises and its significance is still elusive. Results In the current study, by deleting the key secreted factor KIT ligand from preGCs and analyzing the follicle cell developmental dynamics, we revealed distinct patterns of activation and growth associated with the two waves of follicles in mouse ovary. Our results confirmed that activation of adult wave follicles is initiated by somatic preGCs and dependent on the KIT ligand. By contrast, activation of first wave follicles, which are awakened from germ cells before follicle formation, can occur in the absence of preGC-secreted KIT ligand in postnatal ovaries and appears to be oocyte-initiated. We also found that the asynchronous activity of phosphatidylinositol 3 kinases (PI3K) signaling and meiotic process in embryonic germ cells lead to the follicle heterogeneity in postnatal ovaries. In addition, we supplied evidence that the time sequence of embryonic germ cell development and its related first wave follicle growth are correlated to the time of puberty onset in females. Conclusion Taken together, our study provides evidence that asynchronous development of embryonic oocytes leads to the heterogeneity of postnatal ovarian follicle activation and development, and affects the timing of onset of puberty in females.

Ovarian follicles are the fundamental structure to support oocyte development, which provides mature oocytes for offspring. This process requires granulosa cells (GCs) to respond to the midcycle surge of hormones, leading to GC proliferation and differentiation by a series of genes’ transcriptional expression changes. Epigenetic mediator, Polycomb Repressive Complex 1 (PRC1) has been reported to function in fetal ovarian development. However, its functional relevance to folliculogenesis and ovulation remains unknown. In this study, we demonstrated that GC-selective depletion of PCGF2, a key component of PRC1, led to the loss of follicles, ovulation defects, and a lengthened estrus cycle, resulting in subfertility in female mice. The expression of PCGF2 is in the GCs of growing follicles and increases after human chorionic gonadotropin (hCG) stimulation. PCGF2 bound to the promoter of the key ovulation gene progesterone receptor (Pgr) and upregulated the expression of Pgr by targeting the epigenetic modification of H2AK119ub1 after hCG surge. Consistently, the expression of downstream genes of Pgr also sharply decreased, which resulted in the follicular rupture failed and oocyte entrapped in corpus luteum in GC-specific Pcgf2 knockout mice. Together, our study identified that PCGF2 is essential for folliculogenesis and ovulation via modulating hormone receptor expression.