Ovarian follicular granulosa cells surround and nurture oocytes, and produce sex steroid hormones. It is believed that during development the ovarian surface epithelial cells penetrate into the ovary and develop into granulosa cells when associating with oogonia to form follicles. Using bovine fetal ovaries (n = 80) we identified a novel cell type, termed GREL for Gonadal Ridge Epithelial-Like. Using 26 markers for GREL and other cells and extracellular matrix we conducted immunohistochemistry and electron microscopy and chronologically tracked all somatic cell types during development. Before 70 days of gestation the gonadal ridge/ovarian primordium is formed by proliferation of GREL cells at the surface epithelium of the mesonephros. Primordial germ cells (PGCs) migrate into the ovarian primordium. After 70 days, stroma from the underlying mesonephros begins to penetrate the primordium, partitioning the developing ovary into irregularly-shaped ovigerous cords composed of GREL cells and PGCs/oogonia. Importantly we identified that the cords are always separated from the stroma by a basal lamina. Around 130 days of gestation the stroma expands laterally below the outermost layers of GREL cells forming a sub-epithelial basal lamina and establishing an epithelial-stromal interface. It is at this stage that a mature surface epithelium develops from the GREL cells on the surface of the ovary primordium. Expansion of the stroma continues to partition the ovigerous cords into smaller groups of cells eventually forming follicles containing an oogonium/oocyte surrounded by GREL cells, which become granulosa cells, all enclosed by a basal lamina. Thus in contrast to the prevailing theory, the ovarian surface epithelial cells do not penetrate into the ovary to form the granulosa cells of follicles, instead ovarian surface epithelial cells and granulosa cells have a common precursor, the GREL cell.

Although not often discussed, the ovaries of women with polycystic ovary syndrome (PCOS) show all the hallmarks of increased TGF-β activity, with increased amounts of fibrous tissue and collagen in the ovarian capsule or tunica albuginea and ovarian stroma. Recent studies suggest that PCOS could have fetal origins. Genetic studies of PCOS have also found linkage with a microsatellite located in intron 55 of the extracellular matrix protein fibrillin 3. Fibrillins regulate TGF-β bioactivity in tissues by binding latent TGF-β binding proteins. We therefore examined expression of fibrillins 1-3, latent TGF-β binding proteins 1-4, and TGF-β 1-3 in bovine and human fetal ovaries at different stages of gestation and in adult ovaries. We also immunolocalized fibrillins 1 and 3. The results indicate that TGF-β pathways operate during ovarian fetal development, but most important, we show fibrillin 3 is present in the stromal compartments of fetal ovaries and is highly expressed at a critical stage early in developing human and bovine fetal ovaries when stroma is expanding and follicles are forming. These changes in expression of fibrillin 3 in the fetal ovary could lead to a predisposition to develop PCOS in later life.

Studies of selenium (Se) status indicate that Se is necessary for fertility but how precisely is not known. We aimed to show that Se was important in bovine female reproductive function. The elemental distribution in the bovine ovary (n = 45 sections) was identified by X-ray fluorescence (XRF) imaging. Se was consistently localized to the granulosa cell layer of large (>10 mm) healthy follicles. Inductively Coupled Plasma - Mass Spectrometry revealed tenfold higher Se in the bovine follicle wall compared to corpora lutea. Gene expression analysis of selenoprotein genes GPX1, GPX3, VIMP and SELM in bovine granulosa cells revealed that only GPX1 was significantly up-regulated in large healthy follicles compared to the small healthy or atretic follicles (P < 0.05). Western immunoblotting identified GPX1 protein in bovine granulosa cells of large healthy follicles, but not of small healthy follicles. To assess if GPX1 was important in human follicles, cumulus cells from women undergoing IVF/ICSI with single embryo transfer were collected. Oocytes and embryos were cultured and transferred independently in 30 patients undergoing elective single embryo transfer. Gene expression of GPX1 was significantly higher in human cumulus cells from cumulus-oocyte complexes yielding a pregnancy (P < 0.05). We present the first XRF imaging of mammalian ovaries showing that Se is consistently localized to the granulosa cells of large healthy follicles. We conclude that Se and selenoproteins are elevated in large healthy follicles and may play a critical role as an antioxidant during late follicular development.

Polycystic ovary syndrome (PCOS) affects around 10% of young women, with adverse consequences on fertility and cardiometabolic outcomes. PCOS appears to result from a genetic predisposition interacting with developmental events during fetal or perinatal life. We hypothesised that PCOS candidate genes might be expressed in the fetal ovary when the stroma develops; mechanistically linking the genetics, fetal origins and adult ovarian phenotype of PCOS. In bovine fetal ovaries (n = 37) of 18 PCOS candidate genes only SUMO1P1 was not expressed. Three patterns of expression were observed: early gestation (FBN3, GATA4, HMGA2, TOX3, DENND1A, LHCGR and FSHB), late gestation (INSR, FSHR, and LHCGR) and throughout gestation (THADA, ERBB4, RAD50, C8H9orf3, YAP1, RAB5B, SUOX and KRR1). A splice variant of FSHB exon 3 was also detected early in the bovine ovaries, but exon 2 was not detected. Three other genes, likely to be related to the PCOS aetiology (AMH, AR and TGFB1I1), were also expressed late in gestation. Significantly within each of the three gene groups, the mRNA levels of many genes were highly correlated with each other, despite, in some instances, being expressed in different cell types. TGFβ is a well-known stimulator of stromal cell replication and collagen synthesis and TGFβ treatment of cultured fetal ovarian stromal cells inhibited the expression of INSR, AR, C8H9orf3 and RAD50 and stimulated the expression of TGFB1I1. In human ovaries (n = 15, < 150 days gestation) many of the same genes as in bovine (FBN3, GATA4, HMGA2, FSHR, DENND1A and LHCGR but not TOX3 or FSHB) were expressed and correlated with each other. With so many relationships between PCOS candidate genes during development of the fetal ovary, including TGFβ and androgen signalling, we suggest that future studies should determine if perturbations of these genes in the fetal ovary can lead to PCOS in later life.