Dormant primordial follicles constitute the finite ovarian reserve, yet the mechanisms that decide which follicles awaken under physiological conditions remain incompletely defined. Zhang et al. (2020) showed that exogenous epidermal growth factor (EGF) activates dormant follicles in mouse and human ovarian tissue by elevating the oocyte-intrinsic CDC42–PI3K-AKT-FOXO3a cascade, and that EGF applied topically to the ovarian surface activates follicles in living mice; however, the physiological extra-follicle mechanisms operating in intact ovaries — which endogenous factors act, which ovarian cell types produce them, in what spatial and temporal pattern, and how they couple to the oocyte-intrinsic execution machinery — were left open.

Background

In mammalian ovaries, the entire female germ-cell reserve is laid down before or shortly after birth as primordial follicles: a dormant primary oocyte arrested in meiosis, enclosed by a single flattened layer of pregranulosa cells. From this pool, individual follicles are recruited into growth one by one over the reproductive lifespan — a transition called primordial follicle activation (also 'initial recruitment'), which is the rate-limiting step that sets how long the reserve lasts. The oocyte-intrinsic side of this decision is comparatively well understood: activation is executed by the phosphoinositide 3-kinase (PI3K) signaling cascade inside the oocyte, in which the kinases PI3K and AKT (protein kinase B) phosphorylate the transcription factor FOXO3a, driving it out of the oocyte nucleus (its nuclear retention is the hallmark of dormancy); the lipid phosphatase PTEN and the mechanistic target of rapamycin complex 1 (mTORC1) pathway in the surrounding pregranulosa cells gate this cascade (Adhikari and Liu, 2009; Zhang and Liu, 2015). A genetic landmark established that somatic cells, not the oocyte alone, initiate activation in adulthood (Zhang et al., 2014). The best-studied somatic-to-oocyte signal is KIT ligand (KITL, also called stem cell factor, SCF) made by pregranulosa cells, which binds the KIT receptor on the oocyte and feeds into PI3K-AKT-FOXO3a. The same Beijing laboratory later showed that CDC42 — a small Rho-family GTPase that cycles between an inactive GDP-bound and active GTP-bound form — is a rapid response target in dormant oocytes whose activation suffices to trigger follicle activation through PI3K (Yan et al., 2018). Clinically, drug-induced 'in vitro activation' (IVA) of residual dormant follicles in ovarian cortical tissue, pioneered by Kawamura et al. (2013) with disruption of the Hippo pathway (a conserved signaling pathway that restricts tissue growth) and AKT stimulation, has produced live births in patients with premature ovarian insufficiency (POI, the loss of ovarian function before age 40), but it requires surgical removal, culture, and re-transplantation of ovarian tissue. Zhang et al. (2020) searched among endogenous ovarian growth factors for an efficient upstream activator and found that brief exposure to epidermal growth factor (EGF) strongly activates primordial follicles in cultured mouse ovaries via CDC42–PI3K (measured as elevated CDC42-GTP, phosphorylated AKT and FOXO3a, and cytoplasmic FOXO3a translocation), that EGF improves follicle development in human cortical pieces ex vivo and after transplantation into host mice (xenografting), and — using a liquid-to-solid phase-transition biogel painted onto the ovarian surface — that topical in vivo EGF delivery activates follicles in normal mice and in a diphtheria-toxin-induced model of POI. The paper explicitly framed its limits: although EGF is a well-studied endogenous ovarian factor, 'the physiological extra-follicle mechanisms that governing the activating primordial follicles are still remaining elusive', and 'what factors and which cell types involved in the surrounding microenvironment to control the fate of primordial follicles are also unknown'. In other words, exogenous pharmacology had outrun physiological understanding: what actually wakes follicles in an intact, unperturbed ovary — the endogenous ligands or other environmental cues, their cellular sources, their timing and positioning, and how they engage the oocyte's own CDC42–PI3K machinery — remained, and remains, the unresolved question. Work since then has identified partial components — a pregranulosa-cell KIT ligand requirement for adult-wave activation, ovulation-coupled extracellular-matrix remodeling by the protease cathepsin L as a spatial trigger, mechanically maintained dormancy, and mapping of stromal paracrine niches in the human cortex — but no integrated physiological mechanism, and the identity and necessity of endogenous activating ligands remain contested.

Problem Statement

Define the physiological extra-follicle mechanisms — in the source's usage of the term, everything upstream of the oocyte, spanning both the follicle's own pregranulosa and other somatic cells and extrafollicular stromal, immune, or vascular sources — that govern the activation of dormant primordial follicles in intact ovaries. As posed by the source work, the question is which endogenous factors operate under physiological conditions in vivo to trigger activation — receptor-ligand activators, extracellular-matrix or mechanical cues, metabolic or other paracrine inputs, or a defined combination of them — and which ovarian cell types produce or relay these activating signals (for example pregranulosa cell subtypes, specific stromal fibroblast populations, immune or vascular cells). Also open, and part of what an integrated physiological mechanism would have to explain, are how such extra-follicle inputs are transduced to the oocyte-intrinsic execution machinery — the PI3K-AKT-FOXO3a cascade, which exogenous EGF was shown in the source work to stimulate through the GTPase CDC42, one identified node rather than an obligatory route — and the spatial and temporal pattern in which activating signals are deployed across the ovarian cortex, across the estrous cycle, and across the developmental waves of activation (the perinatal 'first wave' and the cyclic adult recruitment waves, whose initiating cells the genetic evidence cited below indicates may differ). A complete mechanism must jointly account for all four components — the identity of the endogenous activating factor or cue, the ovarian cell type(s) that produce or relay it, how it couples to the oocyte-intrinsic PI3K-AKT-FOXO3a cascade, and the spatial and temporal pattern in which it is deployed — and evidence for a single component alone, such as one endogenous ligand together with its source cell type, is a partial contribution rather than a resolution of the question. The question concerns the intact ovary under physiological conditions: the pharmacological responsiveness of cultured or topically treated ovaries to exogenous ligands, on which the source work established its proof of principle, does not by itself identify the endogenous signals or the cell types that produce them. The problem is posed for mammalian ovaries, on the mouse and human ovarian tissue evidence of the source work, with the human ovarian cortex as its principal translational target.

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