Defining mechanisms regulating functional regionalization of the mammalian ovary

About This Grant

SUMMARY A major priority in reproductive biology is to understand how female mammals establish and maintain the ovarian reserve, a finite, non-renewable pool of ovarian follicles necessary for female fertility and endocrine health. Females with a smaller ovarian reserve experience spontaneous primary ovarian insufficiency (POI), which leads to subfertility and early menopause. As over 70% of spontaneous POI cases lack a known etiology, there is an urgent need to understand the mechanisms that regulate establishment of the ovarian reserve during fetal and perinatal life. During development, the ovary undergoes dramatic structural remodeling that culminates by birth in the formation of two distinct regions, the medulla in the center and the cortex at the surface, with oocytes and pre-granulosa cells (PGs) assembling into follicles within each region. Medullary and cortical follicles play different roles in reproduction. Medullary follicles undergo activation shortly after birth in a process termed “first-wave folliculogenesis” and are depleted by puberty. In contrast, cortical follicles remain quiescent until puberty. Only the cortical follicles constitute the ovarian reserve. These two populations are established from a single finite pool of oocytes, thus differences in the allocation of ovarian cells to cortical and medullary regions during development have lifelong impacts. Despite this major impact on female fertility, how structural remodeling of the ovary during fetal and perinatal development influences medullary versus cortical cell fate during development remains a key question in ovarian biology. Using 3D imaging, computational analyses, and single-cell-scale spatial transcriptomics, we have made inroads into the mechanisms that define medullary vs cortical fates. Our preliminary findings indicate early, localized expression of Thbs1 (Thrombospondin 1, TSP1) in the ovarian medulla, and enrichment of the mechano-sensing channel Piezo1 at the outer surface of the ovary during morphogenic folding. The proposed project will test the hypothesis that prenatal expression of TSP1 in medullary PGs, combined with increasing cortical tension sensed by PIEZO1, establishes distinct mechanical and transcriptional niches within the ovary. These distinct environments further specify medullary versus cortical cell fates, thereby predisposing follicles to either activate at birth or retain long-term quiescence, ultimately shaping the ovarian reserve. Using advanced 3D imaging, biomechanical assays, single-cell multiomics, and mouse genetic models, we will define the mechano-structural regulation of cortical specification and the role of PIEZO1 (Aim 1); determine the role of TSP1 in medulla patterning (Aim 2); and characterize the regulatory mechanisms that link ovary regionalization in fetal life to perinatal follicle activation or quiescence (Aim 3). This project will fill key gaps in our knowledge of the fetal and perinatal origins of ovarian health and disease. This will enhance our ability to predict reproductive disorders that affect fertility and hormone balance later in life, and may reveal early interventions that improve lifelong ovarian health, reducing the occurrence of POI and related health risks.