Genetic Control of Embryonic Gonadal Differentiation

Gonadal differentiation is a complex, multistep process that requires the sequential action and interaction of multiple gene products.

Early in the 1st trimester, the undifferentiated, bipotential fetal gonad begins as a thickening of the urogenital ridge, near the developing kidney and adrenal cortex. At 6 wk of gestation, the gonad contains germ cells, stromal cells that will become Leydig cells in testes, or theca, interstitial or hilar cells in the ovary; and supporting cells that will develop into Sertoli cells in testes or granulosa cells in ovaries. In the absence of a testis-determining factor, thought to be the SRY (Sex-determining Region on the Y chromosome), the gonad develops into an ovary. SRY may suppress a putative factor 2 that functions as repressor of male development.

A chromosome complement of 46,XX is necessary for the development of normal ovaries. Both the long and short arms of the X chromosome contain genes for normal ovarian development. The DSS (Dosage Sensitive/Sex reversal) locus associated with the DAX1 (DSS Adrenal hypoplasia on the X chromosome) gene, which is defective in patients with X-linked congenital adrenal hypoplasia and hypogonadotropic hypogonadism, is a member of the nuclear receptor superfamily and acts as a repressor of male gene expression. DAX1 acts by binding to a related nuclear receptor SF-1 (Steroidogenic Factor-1). In vitro, the signaling gene WNT4 stimulates expression of DAX1, resulting in the suppression of androgen synthesis in XX females. The WNTs are ligands that activate receptor-mediated signal transduction pathways and are involved in modulating gene expression as well as cell behavior, adhesion, and polarity. A key to its role in humans was elucidated by loss-of-function mutation of the WNT4 gene that was found in an 18 yr old 46,XX woman. She had absence of Müllerian-derived structures (uterus and fallopian tubes), unilateral renal agenesis, and clinical signs of androgen excess.

Mutations of the Wilms tumor 1 (WT1) gene, including alternative splicing, may also impact sex differentiation. WT1 mutations are associated with the Denys-Drash syndrome (early-onset renal failure with abnormal external genitalia and Wilms tumor). Haploinsufficiency of a 3-amino-acid (KTS) form of WT1 has been implicated in the gonadal dysgenesis of patients with Fraser syndrome (late-onset progressive glomerulopathy and 46,XY gonadal dysgenesis). Mutations in the FOXL2 and SF-1 gene are associated with ovarian failure. Other autosomal genes also play a role in normal ovarian organogenesis and testicular development. Several conditions of gonadal dysgenesis are associated with gross abnormalities of both autosomes and sex chromosomes. A deletion affecting the short arm of the X chromosome produces the typical somatic anomalies of Turner syndrome.

Development of the testis requires the short arm of the Y chromosome; a testis-determining factor at this site has been identified, and the gene for it has been cloned and designated SRY. During male meiosis, the Y chromosome must segregate from the X chromosome so that both X and Y chromosomes do not occur in the same spermatozoa. The major portion of the Y chromosome is composed of Y-specific sequences that do not pair with the X chromosome. However, a minor portion of the Y chromosome shares sequences with the X chromosome and pairing does occur in this region. The genes and sequences in this area recombine between the sex chromosomes, behaving like autosomal genes. Therefore, the term pseudoautosomal region is used to describe this portion of the chromosome, and the term indicates genetic behavior of these genes. The SRY gene is localized to the 35-kb portion proximal to the pseudoautosomal region of the Y chromosome. It contains a high-mobility group nonhistone protein (HMG box), suggesting that SRY may be a transcriptional regulator of other genes involved in sex differentiation. The gonadal ridge forms at around 33 days of gestation. SRY is detected at 41 days, peaks at 44 days when testis cords are 1st visible, and persists into adulthood.

Other genes that are found on autosomes are important in this process. SOX9, a SRY-related gene containing a region homologous with the high-mobility group box 9 (HMG box 9) of SRY, is located on chromosome 17. Mutations of this gene result in XY sex reversal and camptomelic dysplasia. Steroidogenic factor 1 (SF-1) on chromosome 9q33, is important in adrenal and gonadal development, as well as the development of gonadotropin-releasing hormone (GnRH)-secreting neurons in the hypothalamus. The Wilms tumor gene (WTI), especially the -KSTiso form on chromosome 11p13, is needed for early gonadal, adrenal, and renal development. Fibroblast growth factor-9 (FGF-9), GATA-4, XH-2, and SOY9 are also important.

When genetic recombination events on sex chromosomes extend beyond the pseudoautosomal region, X- and Y-specific DNA may be transferred between the chromosomes. Such aberrant recombinations result in X chromosomes carrying SRY, resulting in XX males, or Y chromosomes that have lost SRY, resulting in XY females. SRY acts as a transcriptional regulator to increase cellular proliferation, attract interstitial cells from adjacent mesonephros into the genital ridge, and stimulate testicular Sertoli cell differentiation. Sertoli cells act as an organizer of steroidogenic and germ cell lines and produce antimüllerian hormone (AMH) that causes the female duct system to regress. These cells express low levels of SRY. For additional genes involved in sex development, see Table 576-1.

Function of the Testes

Levels of placental chorionic gonadotropin peak at 8-12 wk of gestation and stimulate the fetal Leydig cells to secrete testosterone, the main hormonal product of the testis. Testosterone is then converted by the enzyme 5α-reductase to its more potent metabolite, dihydrotestoterone. This early period is critical for normal and complete virilization of the XY fetus. Defects in this process lead to different forms of atypical male development (Chapter 582.2). After virilization occurs, fetal levels of testosterone decrease but are maintained at lower levels in the latter half of pregnancy by luteinizing hormone (LH) secreted by the fetal pituitary; this LH-mediated testosterone secretion is required for continued penile growth, and to some degree also for testicular descent.

As part of the normal transition from intrauterine to extrauterine life, perhaps as a result of the sudden withdrawal of maternal and placental hormones, the newborn experiences a transient postnatal surge of gonadotropins and sex steroids. This is the so-called minipuberty.