Rudolph's Pediatrics, 22nd Ed.

CHAPTER 538. Normal Human Sex Differentiation

Melvin M. Grumbach

Phenotypic sex is determined by a sequential process that involves the establishment of chromosomal and genetic sex in the zygote at conception, gonadogenesis and the determination of gonadal sex (sex determination), and sexual differentiation of the genital tract and external genitalia as programmed by gonadal sex.1 Divergent differentiation from the sex chromosome constitution is possible at each level of sexual organization. Table 538-1 considers the hierarchal ontogeny of sex from chromosomal sex to gender identity.

Recent advances allow more accurate prenatal diagnosis of sex. This is of particular importance when embryo gender determination is desired for gender determinations of X-linked disorders. Despite preimplantation sex determination, in about 1% of cases of mistakes occur in preimplantation sex assignment. Ultrasound or amniocentesis allows sex determination in the second trimester, and chorionic villus sampling allows determination and molecular analysis of X-linked genetic disorders during the first trimester, but it is associated with a risk of fetal loss. Newer techniques that measure maternal plasma cell-free fetal nucleic acids (DNA and RNA), such as sex determining region (SRY) gene amplification, allow reliable determination of sex early in the first trimester.2

Table 538-1. Ontogeny of Sex

Chromosomal sex

Genetic sex (+ or − SRY gene or other critical sex-determining genes)

Gonadal sex

Sex of the genital duct derivatives

Sex of the external genitalia-phenotypic sex

Hormonal sex

“Brain sex”

Assigned sex

Gender identity

SRY, sex determining region Y.

HUMAN SEX DETERMINATION

In placental mammals, the male is the heterogametic sex, XY, and the female is homogametic, XX. The Y chromosome functions as a dominant male determinant. The XX and XY are the chromosomal basis of sex determination.1,3-5 The sex chromosomes and the autosomes harbor genes that regulate sex determination and, as a consequence, gonadogenesis. Although the presence of only a single X chromosome (as in 45,X gonadal dysgenesis) can lead to the beginning of ovarian organogenesis, with rare exceptions 2 X chromosomes are required for survival of oocytes in utero and the development of a normal ovary at birth. Two X chromosomes (in the absence of a Y chromosome or translocation of its testis-determining gene to an X-chromosome) lead to ovarian differentiation.

FIGURE 538-1. Diagram of human sex determination and differentiation. Intrinsic or extrinsic factors adversely affecting any stage of these processes can lead to disorders of sex development.

GONADAL DIFFERENTIATION

The gonads of both sexes develop from anlagen located on the medioventral region of the urogenital ridge at about the end of the fifth week of gestation.10 The transcriptional regulators, WT1 and SF1, lead to differentiation of the urogenital ridge into the bipotential primordial gonad. The Y-linked testis-determining factor gene, named SRY (sex-determining region on Y) is a powerful determinant of testicular organogenesis.12 Activation of SOX9 (SRY-Box 9) by SRY activates the SOX9 gene that leads to the differentiation of Sertoli cells, marking testicular differentiation. Deletions or point mutations of the SRY gene occur in about 20% of XY females with complete gonadal dysgenesis. Even though SRY is the master testis-determining switch, control of gonadogenesis is a much more complicated process. A number of other autosomal and X-linked genes are involved in this transcription and signaling cascade.

FIGURE 538-2. Major genes involved in male sex differentiation. AMH, anti-müllerian hormone; GATA4, transcription factor; SF1, steroidogenic factor 1; WT1, Wilms tumor suppressor gene.

A lack of the SRY gene was thought to lead to a default process of ovarian rather than testicular differentiation. However, now the delayed differentiation of the bipotential gonad into an ovary, characterized by transformation of oogonia into oocytes, at about the 11th or 12th weeks of gestation, is recognized as being related to XX-specific genes. Proteins expressed by the WNT4 and RSPO1 (R-spondin 1) genes13,14 activate and stabilize cytoplasmic β-catenin and the canonical WNT4/β-catenin pathway.15-17 RSPO1 acts as a transcriptional coactivator of β-catenin and is required for WNT4 expression in the primordial XX gonad, leading to the differentiation of ovarian granulosa cells (the equivalent cell to the Sertoli cell in the testis) B.17 SRY inhibits the β-catenin pathway.18

DIFFERENTIATION OF DUCTS AND EXTERNAL GENITALIA

The earliest gonadal sex differentiation is followed by the development of the paired genital ducts (wolffian ducts and müllerian ducts) and subsequently the urogenital sinus and external genitalia. Although the embryo possesses a male and female set of duct primordia, typically only the pair homologous with genetic sex develops completely; the opposite set retrogresses, persisting as vestigial structures. In boys, the wolffian ducts form the vas deferens, epididymis, and seminal vesicles. In girls, the müllerian ducts differentiate into the uterus, fallopian tubes, and the upper portion of the vagina. The urogenital sinus and the anlage of the external genitalia are neutral primordia that give rise to homologous structures in boys and girls. These homologous structures include the clitoris and penis, the labia majora and scrotum, the labia minora and corpus spongiosum that encloses the penile urethra, and the paraurethral glands and prostate.

In the absence of testes or their hormones, the fetus has an inherent tendency to develop along female lines irrespective of chromosomal sex. The fetal testicular hormones, antimüllerian hormone (AMH) (müllerian duct inhibitory factor or müllerian inhibitory substance) secreted by Sertoli cells,20,21 and testosterone secreted by fetal Leydig cells, are essential for differentiation of male sex structures and for retrogression of the female duct (Fig. 538-1). Antimüllerian hormone causes ipsilateral regression of the paired müllerian (female) ducts. Testosterone stimulates growth and stabilization of the wolffian (male) ducts and male development of the urogenital sinus and external genitalia (Fig. 538-2).22 Testosterone promotes male differentiation by direct actions that promote differentiation of the wolffian duct to the epididymis, vas deferens, and seminal vesicle. It is also the prohormone for dihydrotestosterone (DHT), formed by means of enzymatic reduction by 5α-reductase type 2 in the target tissues, to induce masculinization of the urogenital sinus, with formation of the prostate and male-type urethra, and masculinization of the primordia of the external genitalia to cause differentiation of the penis, penile urethra, and scrotum. Whereas a functioning fetal ovary is not a prerequisite for development of a female genital system, exposure of the female fetus to androgenic hormones can arrest female differentiation of the urogenital sinus and external genitalia and induce masculinization of the lower genital tract.