Sex Determination and Gonadal Differentiation
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Direct answer
Gonadal sex determination is the process by which the bipotential gonad becomes a testis or an ovary, depending on whether the SRY gene on the Y chromosome is present. SRY activates SOX9 to form Sertoli cells, which make AMH, while fetal Leydig cells make testosterone that sustains the male ducts. Without these signals the gonad becomes an ovary and the default female tract develops.
What you must remember
- SRY: on the short arm of the Y chromosome, it acts as the testis-determining switch, and its absence leads to ovarian development.
- SOX9: downstream of SRY, it drives Sertoli cell differentiation and the testicular cord architecture.
- AMH: produced by Sertoli cells, it causes regression of the Müllerian ducts.
- Testosterone: made by fetal Leydig cells, it maintains the Wolffian ducts, and 5 alpha-reductase converts it to DHT to masculinise the external genitalia.
- Default pathway: without SRY, the Müllerian ducts persist as the uterus, tubes and upper vagina, while the Wolffian ducts regress.
- Swyer syndrome: 46,XY pure gonadal dysgenesis from an SRY defect gives a female phenotype with streak gonads, and gonadectomy is advised because of gonadoblastoma risk.
- 21-hydroxylase deficiency: congenital adrenal hyperplasia exposes a 46,XX fetus to excess androgen, causing virilisation with normal ovaries.
- Complete androgen insensitivity: a 46,XY individual with an androgen receptor defect has a female phenotype and testes, with no Müllerian structures because AMH acts normally.
From bipotential gonad to external genitalia
Sex determination runs as two linked chains, one for the gonad and one for the hormones it makes. The steps follow the chain from the bipotential gonad to the external genitalia.
- Bipotential gonad: both sexes start with a gonadal ridge that can become a testis or an ovary.
- SRY switch: in XY embryos, SRY is expressed in the supporting cells and pushes the gonad towards a testis.
- Sertoli and Leydig cells: SOX9 drives Sertoli cell differentiation, which makes AMH, and fetal Leydig cells make testosterone.
- Duct fate: AMH causes Müllerian duct regression, while testosterone maintains the Wolffian ducts.
- External genitalia: DHT, made from testosterone by 5 alpha-reductase, masculinises the external genitalia and prostate.
Each defect in this chain produces a predictable phenotype, which is how exam vignettes locate the broken step. A 46,XY child with a female phenotype and no uterus, for example, points to testes that made AMH but a blocked androgen receptor, as in complete androgen insensitivity.
Where the exam tries to catch you
Vignettes in this area test whether you can separate the karyotype from the phenotype.
- Trap: Assuming SRY directly shapes the external genitalia. Correct: SRY acts on the gonad, and the external genitalia depend on testosterone and DHT produced downstream.
- Trap: Equating a 46,XY karyotype with a male phenotype. Correct: Complete androgen insensitivity and Swyer syndrome give a female phenotype despite the XY karyotype.
- Trap: Expecting Müllerian structures in complete androgen insensitivity. Correct: Normal AMH from the testes regresses the Müllerian ducts, so the uterus and upper vagina are absent.
Frequently asked questions
Which gene on the Y chromosome determines testis formation?
SRY, located on the short arm of the Y chromosome, is the testis-determining gene. It acts in the bipotential gonad to drive SOX9 expression and push development towards a testis.
What does AMH do in male embryos?
Anti-Müllerian hormone, produced by Sertoli cells, causes regression of the Müllerian ducts. Its action explains why males do not develop a uterus or fallopian tubes.
Why does 21-hydroxylase deficiency virilise a 46,XX female?
In 21-hydroxylase deficiency, cortisol synthesis falls and androgen precursors accumulate, exposing the female fetus to excess androgen. The infant has a 46,XX karyotype with normal ovaries but ambiguous external genitalia.
What is Swyer syndrome?
Swyer syndrome is 46,XY pure gonadal dysgenesis, usually caused by an SRY defect. The patient has a female phenotype with streak gonads, and gonadectomy is advised because of gonadoblastoma risk.