Expansion
SRY drives testis formation; without it the default pathway is female
Expansion
- SRY gene on the Y chromosome drives the indifferent gonad to become a testis
- Without SRY, the default is ovarian development
- Sertoli cells produce anti-Mullerian hormone, causing regression of the Mullerian ducts
- Leydig cells produce testosterone, which maintains the Wolffian ducts (epididymis, vas, seminal vesicles)
- Dihydrotestosterone, via 5-alpha reductase, masculinises the external genitalia and prostate
Disorders map onto the steps:
- Complete androgen insensitivity: 46XY with a defective androgen receptor. Testes and anti-Mullerian hormone are present, so there is no uterus, but external genitalia are female. Presents as primary amenorrhoea with absent pubic hair and a blind vagina
- 5-alpha reductase deficiency: internal male structures with ambiguous or female external genitalia at birth, virilising at puberty when testosterone surges
- Congenital adrenal hyperplasia: 46XX virilised by excess adrenal androgens
- Swyer syndrome: 46XY with SRY mutation, giving a female phenotype with streak gonads