At the Crossroads of Fate-Somatic Cell Lineage Specification in the Fetal Gonad

Abstract

The reproductive endocrine systems are vastly different between males and females. This sexual dimorphism of the endocrine milieu originates from sex-specific differentiation of the somatic cells in the gonads during fetal life. Most gonadal somatic cells arise from the adrenogonadal primordium. After separation of the adrenal and gonadal primordia, the gonadal somatic cells initiate sex-specific differentiation during gonadal sex determination with the specification of the supporting cell lineages: Sertoli cells in the testis vs granulosa cells in the ovary. The supporting cell lineages then facilitate the differentiation of the steroidogenic cell lineages, Leydig cells in the testis and theca cells in the ovary. Proper differentiation of these cell types defines the somatic cell environment that is essential for germ cell development, hormone production, and establishment of the reproductive tracts. Impairment of lineage specification and function of gonadal somatic cells can lead to disorders of sexual development (DSDs) in humans. Human DSDs and processes for gonadal development have been successfully modeled using genetically modified mouse models. In this review, we focus on the fate decision processes from the initial stage of formation of the adrenogonadal primordium in the embryo to the maintenance of the somatic cell identities in the gonads when they become fully differentiated in adulthood.

Figure 1.

Lineage progression of the supporting and interstitial cells in the mouse testis. Somatic progenitor cells in the XY gonad (green) give rise to supporting/Sertoli cells (blue) or an interstitial progenitor population (pale orange). The supporting/Sertoli cell lineage is defined by the upregulation of Sry, followed by Sox9, that transforms them into a polarized epithelial cell type in the testis. As Sertoli cells mature postnatally (dark blue), they downregulate Amh expression and acquire androgen responsiveness. The interstitial progenitor cell population is defined by expression of Hes1 and other genes (Arx, CouptfII/Nr2f2, Mafb). The interstitial progenitor cells begin to express Gli1 in response to hedgehog signaling from Sertoli cells and differentiate into fetal Leydig cells (orange). A population of the interstitial progenitor cells remains as a nonsteroidogenic progenitor population that gives rise to adult Leydig cells (dark orange) postnatally. A second population of interstitial progenitor cells is specified in the mesonephros and enters the gonad during fetal development. These cells are poorly characterized, but they potentially express similar markers as the gonadal interstitial progenitors. The mesonephric interstitial progenitors differentiate into a population of fetal Leydig cells that express Nr5a1 and Cyp17a1, but do not express Gli1, and thus develop into Leydig cells independently of hedgehog signaling. The mesonephric interstitial progenitor cells do not contribute to the adult Leydig cell population.

Figure 2.

Pathways that govern Sertoli cell differentiation in the mouse. Sertoli cell differentiation steers the bipotential gonad from a female-biased fate (pink) to male fate (blue). (a) As the gonad begins to form, the MAPK pathway, SIX1/4, CBX2, LHX9, and CITED2 upregulate expression of GATA4/FOG, SF1, and WT1, which are necessary for the survival and expansion of the gonadal primordium. (b) At the beginning of testis determination, SF1, GATA4, and WT1 trigger Sry expression. Sry expression is also under epigenetic regulation by p300/CBP and JMJD1A. (c) SRY upregulates Sox9, which leads to upregulation of additional pro-testis pathways (FGF9 and PGD2) while simultaneously inhibiting the pro-ovarian WNT4/RSPO1/β-catenin pathway. WT1 enforces supporting cell fate by suppressing SF1 and promoting SOX9. (d) Under the control of SOX9, differentiating Sertoli cells begin to secrete DHH and AMH that control the appearance of Leydig cell lineage and dimorphic differentiation of the reproductive tract. SOX9 further promotes male fate by repressing the pro-ovarian forkhead box L2 (FOXL2).

Figure 3.

Paracrine regulation of steroidogenic cell development. (a) In the fetal mouse testis, Sertoli cells produce paracrine factors PDGFα and DHH that control the appearance and expansion of fetal Leydig cells in the interstitium after E12. Conversely, AMH from Sertoli cells negatively regulates the steroidogenic cell population. Fetal Leydig cells secrete activin A, which promotes proliferation of Sertoli cells via SMAD4 activation. Germ cells do not contribute to paracrine regulation of steroidogenic cells in the testis. (b) Unlike Leydig cells, which differentiate in the testis right after sex determination, the ovarian steroidogenic theca cells differentiate near birth and full steroidogenesis begins after birth. Also, oocytes participate in theca cell recruitment in the ovary through secreting GDF9, which induces expression of the hedgehog ligands DHH and IHH in the granulosa cells. DHH and IHH transform stromal progenitors into the theca cells that surround the follicles around birth in the mouse. Furthermore, granulosa cells promote theca cell recruitment by also producing PDGFα.

Figure 4.

Lineage progression of the supporting and stromal cells in the mouse ovary. Somatic progenitor cells in the XX gonad (green) commit to either supporting/granulosa cells (pink/lilac) or a stromal progenitor population (pale orange). Granulosa cells (GC) differentiate in two asynchronous waves. The first population of the granulosa cells (pink) is specified by upregulation of the cell cycle inhibitor Cdkn1b/p27 and subsequent induction of Foxl2, and it eventually gives rise to granulosa cells that surround the follicle in the medullary region of the ovary (dark pink). The second population of granulosa cells (lilac) expresses the RSPO1 receptor Lgr5 and actively proliferates to expand the cell pool prior to cell cycle arrest and differentiation through induction of Cdkn1b/p27 and Foxl2. The second wave of granulosa cell differentiation gives rise to granulosa cells of both medullary (dark pink) and cortical follicles (purple). The stromal progenitor cells in the XX gonads (light orange) remain as undifferentiated Lhx9-positive cells in the fetal ovary, but they potentially express markers for interstitial cell fate (Arx, CouptfII/Nr2f2, and Mafb). Near birth the hedgehog signaling from granulosa cells transforms the progenitor cells into the steroidogenic Gli1-positive theca cells (dark orange). Stromal progenitor cells from the mesonephros also contribute to the theca cell population. These cells potentially express genes similar to the gonadal stromal progenitors. Some of the stromal cell progenitors might remain in the postnatal ovary as a nonsteroidogenic stromal cell population, which expresses CouptfII and Arx, similarly to the nonsteroidogenic interstitial cells in the testis.

Figure 5.

Pathways that govern granulosa cell differentiation in the mouse. (a) As the gonad begins to form, SIX1/4, CBX2, LHX9, and CITED2 upregulate expression of GATA4/FOG2, SF1, and WT1, which are necessary for the survival and expansion of the adrenogonadal primordium. (b) In the XX somatic progenitor cells, where Sry is absent, WT1 and GATA4 promote maintenance of WNT4/RSPO1/β-catenin signaling. β-Catenin forms a feedforward loop by stimulating Wnt4 expression. (c) WNT4/RSPO1/β-catenin signaling inhibits the protestis signaling by SOX9 and FGF9 and induces female fate by promoting secretion of FST. (d) As pregranulosa cells begin to differentiate, they upregulate p27 and exit the cell cycle. In granulosa cells FOXL2 expression is initiated, which further strengthens female fate by antagonizing SOX9 and promoting FST expression.

Figure 6.

Lineage separation of the adrenogonadal primordium to adrenal and gonadal steroidogenic cells. Cells in the adrenogonadal primordium (green) express SF1, WT1. and GATA4. As separation of the adrenal and gonadal populations begins in the mouse, the gonadal progenitors (purple) retain expression of these factors, whereas adrenal progenitors (blue) downregulate WT1 and GATA4 and upregulate GATA6 expression. Few undifferentiated gonadal progenitor-like cells (green) remain in the adrenal gland. When LH levels are increased in the mouse by gonadectomy or administration of ectopic LH/human chorionic gonadotropin, these progenitor cells differentiate into gonad-like steroidogenic cells in the adrenal (purple). Some adrenal-like cells (light blue) reside around the gonad mesonephric border. Upon loss of WNT4 signaling, this population expands in the testes and ovaries.