In preprints: rethinking the foundations of the ovarian reserve Free

During oogenesis, as oocytes transition from clusters of germ cells into individually encapsulated units, pre-granulosa (PG) cells surround each oocyte. This initiates the assembly of primordial follicles, the most immature stage of the follicular hierarchy (Wallace and Kelsey, 2010). Interactions between PGs and oocytes are crucial for follicle assembly, survival, and long-term dormancy. In the widely accepted ‘two-wave model’ based on mouse studies, two distinct PG lineages specify follicle fate (Niu and Spradling, 2020). The first wave governs the formation of short-lived follicles in the medullary region of the ovary that are activated early after birth. The second wave guides the formation of dormant, long-lived primordial follicles in the cortical region of the ovary, which constitute the ovarian reserve. This model proposes that granulosa cell lineage dictates follicle fate, determining whether a follicle remains quiescent or becomes activated. Although this model is appropriate for mice, primate developmental timelines, physiology and reproductive needs differ significantly (Cunha et al., 2019). The question of whether oogenesis aligns with this paradigm in humans and other primates remains unresolved.

To address this conceptual gap, Wamaitha et al. combined single-cell RNA-sequencing, spatial transcriptomics and immunofluorescence microscopy of fetal ovaries obtained at different gestational stages (5 weeks post-conception to 6 months postnatal) from rhesus macaques, a species with a developmental timeline and ovarian architecture comparable to humans (Wagenen and Simpson, 1965). Across these stages, they identified three distinct PG cell populations: the expected PG1 and PG2 subtypes corresponding to the two-wave model in mice, and a novel PG1.5 population with intermediate transcriptional features. The authors found that all three granulosa subtypes contributed to quiescent and activated follicles, suggesting a developmental continuum rather than a strict binary lineage as in mice. Instead of lineage, the location of a follicle within the ovary was shown to determine its fate: quiescent follicles were restricted to the ovarian cortex, while activated follicles localized to the medulla region. Consistently, both dormant and activated follicles contain PG1.5 and PG2 cell types, confirming that granulosa identity alone does not dictate follicle fate in primates. This finding shifts the paradigm to the ovarian cortex as a regulator of follicular dormancy and identifies the medullary region as a trigger for follicle activation, a conceptual advance with significant implications for primate development.

Alongside their redefinition of follicle fate, Wamaitha et al. uncovered a transient population of hormonally active medullary follicles that are activated by gestational week 19. They expressed CYP19A1 (aromatase) in estrogen-producing granulosa cells and CYP17A1 in androgen-producing theca-like cells – features of the canonical adult ‘two-cell’ steroidogenic system of follicular hormone production (Hillier et al., 1994). However, these fetal-activated follicles were morphologically distinct: they were cyst-like, and their oocytes ultimately underwent apoptosis as indicated by high expression of cleaved caspase 3 at 6 months postnatal. These findings suggest that the purpose of these follicles is not reproduction, but rather endocrine signaling, perhaps contributing to mini puberty, a hormonal surge that occurs shortly after birth in primates and supports brain and gonadal development (Renault et al., 2020).

The idea that some follicles are designated to signal temporarily before undergoing programmed cell death is a provocative concept that broadens the purpose of folliculogenesis beyond reproduction to encompass animal development.

If follicle fate is determined by spatial location rather than cell lineage, the disruption of ovarian tissue architecture during fetal development may have long-lasting effects. For example, POI (Touraine et al., 2024) may result from a mass shift of follicles into the medullary ovarian area, prompting their premature activation due to the early depletion of the follicle reserve. Likewise, PCOS, commonly associated with excess androgens and disrupted folliculogenesis (Joham et al., 2022), may stem from the abnormal persistence or dysregulation of theca-like cells that produce androgens. Overall, this study necessitates reevaluating the perspective that adult reproductive disorders typically arise during adolescence or adulthood, urging consideration of early fetal development as a crucial period in establishing ovarian structure and disease etiology. This research also lays a foundation for investigating and potentially addressing female reproductive disorders by redirecting focus from granulosa cell lineage to the ovarian microenvironment. As the engineering of female reproductive tissues develops into a groundbreaking method for preserving germ cells and revitalizing ovarian function (Almeida et al., 2023; Brownell et al., 2022; Dadashzadeh et al., 2021), the new ovarian biology uncovered in this study could significantly guide the mimetic potential of reconstructed tissues.

The preprint by Wamaitha et al. outlines a developmental framework for understanding the formation of the non-renewable ovarian reserve in a species pertinent to human biology. It raises crucial questions that, when addressed, could elevate the field of female reproductive biology. What processes create the cortical niche that maintains follicular dormancy? Is it possible to reprogram the fate of follicles by modifying ovarian structure? How do medullary follicles synchronize hormone production with programmed cell death? Lastly, what molecular signals ensure a proper balance between ovarian reserve and temporary endocrine function? This research will undoubtedly inspire new investigations in models that relate to human biology, aiming to reveal the molecular and mechanical signals influencing cell fate in the fetal ovary.

We thank members of the Mogessie Lab for discussions.