weeks, the vagina opens into the pelvic portion of the urogenital sinus and it becomes the vaginal vestibule. After fetal ovarian follicular growth (20-22 weeks), there is rapid ventral outgrowth of the perineum, the urethral and vaginal openings separate, and the urethra is brought to the surface. The clitoris becomes incorporated into the fused anterior ends of the genital folds (labia minora), which continue to grow posteriorly. Genital swellings lateral to the labia minora become the labia majora, anteriorly continuous as the mons pubis. The growth of the labia minora is greater than that of the labia majora; they are seen protruding out of the labia majora at 23-25 weeks of gestation. After 26 weeks, the labia majora have grown sufficiently to cover the labia minora [1, 2]. At midtrimester there is no difference in the distribution of androgen receptors between male and female fetuses in the external genitalia; estrogen receptors are present only in the genitalia of the female fetus. It is not known when estrogen receptors appear or what induces their appearance. Their lack may be what protects male fetuses from the effects of maternal estrogen.
Development of a bladder and urethra separate from the vagina requires the growth of a membrane from the cranial to the caudal region in the urogenital sinus. Anomalies of female embryogenesis during this process lead to a variety of clinically recognized disorders. Estrogen is responsible for the vascularity and thickness of vaginal tissue. Failure of the labia to fuse in the normal female fetus may be due in part to the lack of fetal androgen production and low 5-α reductase activity (and in part to maternal estrogen stimulation of ER-positive urethral folds), causing the labia minora to diverge laterally. There is extensive circumstantial evidence that 17β-estradiol and progesterone influence the postnatal physiology of extragenital and especially genital (vulval) skin in the human female.
Molecular Biology
The AR protein is a member of the steroid hormone-activated transcription regulator family, a subset of the superfamily of ligand-dependent transcription factors. Receptors such as estrogen receptor-α (ER-α) and ER-β, mineralocorticoid receptors, glucocorticoid receptors, and progesterone receptors affect transcription through steroid hormone-receptor interaction. The AR protein, like other steroid hormone receptors, consists of three functional domains. The carboxy-terminal (ligand-binding domain) is responsible for androgen binding, the central domain accounts for DNA binding (DNA-binding domain), and the amino-terminal plays a role in transcriptional activation. Together, the three functional domains determine the ability of androgen-AR interactions to regulate transcription. The AR protein is encoded by 8 exons in a single gene located on Xq 11-12. The most frequently seen mutations are nonsense or missense point mutations, with clusters of mutations seen on exons 5 and 7 which code for the ligand-binding domain of the AR. Mutations of the AR gene result in abnormal AR protein binding capabilities which leads to an inability of androgen hormones such as testosterone and DHT to properly regulate transcription. Defects in androgen-AR protein binding directly affect the development of urogenital mesenchyme into male or female external genitalia in 46, XY genotypical males [3].
The role of androgens in sexual differentiation was first elucidated in 1940 by Jost [2] who concluded that the testes were responsible for external genital differentiation through secretion of androgens. Recent studies have shown that androgen-related effects in a cell or tissue may be regulated through paracrine signaling by neighboring cells. In male genital differentiation in particular, the prostate demonstrates mesenchymal (stromal)-epithelial interactions that appear to be an integral part of the epithelium’s differentiation into characteristic branched ducts of the prostate. Steroid hormones such as 17β-estradiol and progesterone are capable of altering epithelial proliferation and organization through paracrine signaling of stromal cells. This paracrine signaling appears to be influenced by steroid-steroid hormone receptor interaction in the nearby cells.
There is also evidence that androgens have direct, non-AR-regulated effects on cells. Direct intracellular response to androgens can occur quickly, within seconds to minutes. This so-called ‘nonclassical’ androgen response pathway can involve several mechanisms such as activation of sex hormone-binding globulin receptors, activation of phosphatidylinositol 3-OH kinase (PI3-K) signaling, and modulation of voltage-and ligand-gated ion channels and transporters. The idea that the role of AR extends beyond the classic androgen-AR mechanism of transcription regulation is intriguing in partial androgen insensitivity syndrome (PAIS). It is possible the varied phenotypical presentations seen in PAIS are in part related to other cellular signaling roles separate from AR that androgens play in genital differentiation.
Classification of Ambiguous Genitalia
Chromosomal Disorders
Sex Reversal, Turner’s Syndrome, True Hemaphroditism, Mosaics, Mixed Gonadal Dysgenesis, and Pure Gonadal Dysgenesis. 46, XY sex reversal is a DSD resulting in individuals of the 46, XY genotype with female phenotypic characteristics or ambiguous genitalia and associated complete gonadal dysgenesis in ~1/20,000 births. The commitment of undifferentiated gonads to testes initiates the development into the male phenotype in normal 46, XY males. The Y chromosome has long been thought to carry the testis-determining factor (TDF). In the absence of the Y chromosome, such as in 46, XX normal females or 45, X Turner’s syndrome, the individuals are phenotypically female. The SRY is located at Yp11.3 and it was hoped that the discovery of SRY would lead to a rapid expansion of knowledge regarding early sex determination. However, SRY as the only TDF was shown to be a gross oversimplification. Many other genes active in sex differentiation have been confirmed, including genes coding for transcription factors such as SOX9 [Sry-type high-mobility group (HMG) box 9], DMTR1, GATA4, DAX1, SF1, WT1, LHX9, and DSS and signaling molecules antimullerian hormone (AMH), WNT4, FGF9, and DHH. Genes coding for the male phenotype have been found on X chromosomes (e.g. DSS) and autosomes 9, 10, and 17 (e.g. DMTR1).
Genetic mutations at the distal end of the short arm of chromosome 9 have been demonstrated in cases of 46, XY sex reversal. Namely, doublesex and MAB-3 related transcription factors 1 (DMRT1) and 2 (DMRT2) are mapped to band 9p24.3. DMRT1 appears to play a role in early sex differentiation while the role of DMRT2 is in somitogenesis. Genetic mutations at the distal 9p locus are typically de novo, resulting from deletions at 9p more often than point mutations. The shortest deletion at 9p still results in the loss of both DMRT1 and DMRT2, allowing for the possibility of both playing a role in early sex determination. This may also account for multiple phenotypic anomalies seen in association with XY sex reversal in certain individuals with 9p mutations. Individuals with 9p mutations and XY sex reversal have been observed to display abnormal facies, hypotonia, and cardiac and gastrointestinal defects, as well as mental and motor developmental delay.
Overvirilized Females
Fetal Sources and Congenital Adrenal Hyperplasia
Genital masculinization in a chromosomally normal (XX) female infant, consisting of clitoromegaly and various degrees of vaginal abnormality, may result from congenital adrenal hyperplasia (CAH). This DSD has a prevalence of 1 in 15,000 births worldwide and is associated with a deficiency in 21-hydroxylase. Consequent virilization occurs because the external genitalia are competent to respond to DHT. The milder enzyme deficiency, i.e. nonclassical 21-hydroxylase deficiency (NC21OHD), was found to be the most common autosomal recessive disorder in humans. The disease frequency of NC21OHD varies between ethnic groups, with the highest ethnic-specific disease frequency in Ashkenazi Jews (1 in 27). NC21OHD is diagnosed based on serum elevations of 17-OHP and confirmed with molecular genetic analysis. Females with ‘classical’ 21 - OHD are exposed to excess androgens prenatally and are born with virilized external genitalia. Potentially lethal adrenal insufficiency is characteristic of two thirds to three quarters of patients with the classical salt-wasting form of 21-OHD. Non-salt-wasting 21-OHD may be diagnosed based on genital ambiguity in affected females, and/or later on the occurrence of androgen excess in both sexes. Another CAH variant, p450 oxidoreductase deficiency may also result in ambiguous genitalia [