Sexual Differentiation — Embryology of the Reproductive Tract

• The SRY gene on the Y chromosome drives the indifferent gonad to become a testis. Without SRY, the ovary is the default pathway.
• The testis then secretes two independent signals from two different cells:
• Sertoli cells → anti-Müllerian hormone (AMH), which causes regression of the Müllerian ducts.
• Leydig cells → testosterone, which maintains the Wolffian ducts into the epididymis, vas deferens and seminal vesicles.
• Dihydrotestosterone (DHT), produced from testosterone by 5-alpha reductase, virilises the external genitalia and prostate. Testosterone acts internally, DHT externally — a distinction that explains 5-alpha reductase deficiency, where internal ducts are male but external genitalia are ambiguous at birth and virilise at puberty.
• Without testicular hormones, Müllerian ducts persist into the uterus, fallopian tubes and upper vagina, Wolffian ducts regress, and external genitalia are female. Female development does not require ovarian hormones.

Failure of gonadal determination (the SRY step)

• SRY mutation or deletion in a 46,XY embryo (Swyer syndrome, complete gonadal dysgenesis): bilateral streak gonads, so no AMH and no testosterone — a uterus and tubes persist and external genitalia are female.
• SRY translocation onto the X during paternal meiosis gives 46,XX testicular DSD — a phenotypic male with small firm testes and azoospermia.
• 45,X and mosaic 45,X/46,XY: accelerated follicular atresia (Turner) or asymmetric gonadal development (mixed gonadal dysgenesis, classically a streak gonad on one side and a testis on the other).

Defects in androgen synthesis or action (46,XY undervirilisation)

• Androgen receptor mutations (X-linked recessive) — complete or partial androgen insensitivity.
• 5-alpha reductase type 2 (SRD5A2) deficiency (autosomal recessive) — testosterone present, DHT absent.
• Enzyme blocks in testosterone synthesis — 17-alpha-hydroxylase and 17,20-lyase deficiency, lipoid CAH (StAR), 17-beta-hydroxysteroid dehydrogenase deficiency.

Defects in AMH signalling

• AMH or AMH receptor type 2 mutations → persistent Müllerian duct syndrome: a virilised male who also retains uterus and tubes, often with cryptorchidism.

Androgen excess in a 46,XX embryo

• Congenital adrenal hyperplasia, overwhelmingly 21-hydroxylase deficiency (roughly 95% of cases), then 11-beta-hydroxylase deficiency.
• Placental aromatase deficiency, maternal virilising tumour (luteoma, arrhenoblastoma), or exogenous androgens/danazol.

• The gonad is the switch, hormones are the output. SRY drives Sertoli cell differentiation; downstream SOX9 and steroidogenic factor-1 sustain testis cord formation. Anything that removes SRY function converts the pathway to the ovarian default, and anything that removes the gonad entirely (a streak) produces a female phenotype regardless of karyotype — because female development is hormone-independent.
• Two independent testicular signals explain why phenotypes dissociate. Sertoli AMH regresses the Müllerian ducts; Leydig testosterone maintains the Wolffian ducts. A defect in one leaves the other intact, which is why persistent Müllerian duct syndrome produces a normally virilised male who still has a uterus, and why complete androgen insensitivity produces a female-appearing patient with no uterus (AMH worked perfectly).
• Testosterone acts locally and by paracrine diffusion, so Wolffian derivatives (epididymis, vas, seminal vesicle) require high local concentration; a unilateral dysgenetic gonad therefore gives ipsilateral duct failure — the anatomic asymmetry of mixed gonadal dysgenesis.
• DHT amplifies the androgen signal at the genital tubercle, urogenital sinus and urogenital folds. It has higher receptor affinity and a slower dissociation than testosterone, which is why the external genitalia need it: without DHT the tubercle stays as a clitoris-like phallus, the folds fail to fuse (hypospadias, bifid scrotum), and the prostate never forms. At puberty the surge of testosterone can partially substitute at the receptor, producing the penis-at-twelve virilisation of 5-alpha reductase deficiency.
• In 46,XX androgen excess, the gonad is a normal ovary and the ducts are normal Müllerian structures — androgen reaches only the external genitalia. The result is clitoromegaly and labial fusion with a normal uterus and ovaries, which is why these patients are potentially fertile.
• The urogenital sinus timing rule: androgen exposure before about the 12th week causes labioscrotal fusion and a common urogenital sinus; later exposure causes clitoromegaly alone.

The newborn with ambiguous genitalia

• 46,XX CAH — the most common cause in the US nursery: clitoromegaly, fused labioscrotal folds, a single perineal opening, and no palpable gonads (ovaries do not descend). In salt-wasting disease the infant returns at 1–2 weeks with vomiting, poor feeding, dehydration, shock, hyponatraemia, hyperkalaemia and hypoglycaemia from combined aldosterone and cortisol deficiency.
• Hypospadias with bilateral non-palpable testes or a bifid scrotum should never be treated as isolated hypospadias — it is undervirilisation until proved otherwise.
• Asymmetry (one descended gonad, one absent; hemi-scrotum) suggests mixed gonadal dysgenesis.
• 11-beta-hydroxylase deficiency adds hypertension and hypokalaemia from 11-deoxycorticosterone excess — the opposite electrolyte picture to 21-hydroxylase deficiency.

The adolescent with primary amenorrhoea

• Complete androgen insensitivity — the classic stem is a tall phenotypic girl with normal breast development, scant or absent axillary and pubic hair, a blind-ending vaginal pouch, absent uterus, and testes found in the inguinal canal or as an unexpected "hernia" contents in infancy. Breasts develop because unopposed testosterone is aromatised to oestrogen.
• Swyer syndrome — normal-to-tall girl, no breast development, a uterus is present, streak gonads, markedly elevated FSH and LH.
• 5-alpha reductase deficiency — raised female, then at puberty deepening voice, muscle mass, phallic growth and testicular descent, with no gynaecomastia and a sparse beard/prostate.
• MRKH (Müllerian agenesis) — the key look-alike to CAIS: 46,XX, normal pubic and axillary hair, normal ovaries and cyclic ovulatory symptoms, absent uterus, frequently associated renal agenesis and skeletal anomalies.

Immediate steps in the ambiguous newborn (this is a social and medical urgency, not a cosmetic one)

• Do not assign sex before evaluation. Per the Chicago Consensus, evaluation belongs to a multidisciplinary team.
• Physical examination first: are gonads palpable? A palpable gonad below the inguinal ring essentially means testicular tissue and therefore Y-chromosome material — this single finding reorients the whole differential away from CAH.
• Karyotype with rapid FISH or PCR for SRY — the initial and pivotal test.
• Serum 17-hydroxyprogesterone, drawn after the first 24–48 hours to avoid falsely high newborn values; it is markedly elevated in 21-hydroxylase deficiency. All US states screen newborns for CAH with 17-OHP.
• Serial electrolytes and glucose through the first two weeks — salt-wasting is not present at birth and declares itself later.
• Pelvic/abdominal ultrasound for a uterus and for adrenal enlargement; presence of a uterus means AMH was absent or ineffective.

Second-tier and confirmatory testing

• AMH/inhibin B as a marker of functioning Sertoli tissue (low/absent in gonadal dysgenesis, normal-to-high in androgen insensitivity).
• Testosterone, DHT and the testosterone:DHT ratio, measured after hCG stimulation — an elevated ratio confirms 5-alpha reductase deficiency; testosterone that fails to rise at all indicates a synthesis block or dysgenesis.
• LH and testosterone both high with a female phenotype point to androgen insensitivity; markedly elevated FSH/LH with low testosterone point to dysgenesis.
• Molecular testing (AR, SRD5A2, CYP21A2, AMH/AMHR2) is the gold standard and increasingly a targeted DSD gene panel.
• Genitography, MRI, or diagnostic laparoscopy with gonadal biopsy define internal anatomy and assess for dysgenetic tissue.

Immediate stabilisation — adrenal crisis is the only true emergency here

• Isotonic saline with dextrose for volume and hypoglycaemia, plus IV glucocorticoid (hydrocortisone) at stress dosing given empirically before confirmatory labs return; treat hyperkalaemia as needed. The Endocrine Society CAH guideline stresses that treatment must not wait for the 17-OHP result.

Chronic medical therapy

• Glucocorticoid replacement: hydrocortisone is first-line in infants and growing children because its short half-life minimises growth suppression. The Endocrine Society advises against long-acting potent glucocorticoids (dexamethasone) in children still growing.
• Mineralocorticoid replacement: fludrocortisone, with sodium chloride supplementation in infancy, for salt-wasting disease.
• Stress dosing and an emergency injectable hydrocortisone kit with a medical alert identifier for every patient — the highest-yield chronic-care point.
• Prenatal dexamethasone to prevent virilisation of an affected 46,XX fetus is considered experimental and is not recommended outside a formal research protocol (Endocrine Society).

Sex-steroid replacement

• Oestrogen (with progestin if a uterus is present) after gonadectomy in CAIS or Swyer syndrome, to induce puberty, achieve peak bone mass and prevent osteoporosis.
• Testosterone for male-raised patients with 5-alpha reductase deficiency or partial androgen insensitivity; supraphysiologic dosing is sometimes required in partial insensitivity.

Surgical and definitive management

• Prophylactic gonadectomy for dysgenetic gonads containing Y-chromosome material (Swyer, mixed gonadal dysgenesis) — done at diagnosis because gonadoblastoma risk is high.
• In complete androgen insensitivity the testes are usually retained until after spontaneous pubertal feminisation, with the timing shared-decision.
• Vaginal dilator therapy is first-line for vaginal hypoplasia/agenesis; surgical vaginoplasty is reserved for dilation failure (ACOG).
• Contraindicated/avoid: sex assignment in the delivery room, and early irreversible cosmetic genitoplasty performed without multidisciplinary input and informed parental/patient participation (Chicago Consensus).

Emergencies

• Adrenal crisis — the leading cause of death in classic CAH. Mechanism: absent cortisol and aldosterone during intercurrent illness. Signals: vomiting, lethargy, hypotension, hyponatraemia with hyperkalaemia, hypoglycaemia. Any febrile illness without stress dosing can precipitate it.
• Gonadoblastoma and dysgerminoma in dysgenetic gonads harbouring Y material — signals include a pelvic mass or rising tumour markers; malignant transformation drives the recommendation for prophylactic gonadectomy.

Complications of the underlying condition

• Infertility — universal in gonadal dysgenesis and CAIS; variable in 5-alpha reductase deficiency and partial androgen insensitivity; often preserved in well-controlled 46,XX CAH.
• Testicular adrenal rest tumours in males with poorly controlled CAH: ACTH-driven growth of adrenal rest tissue within the testis, causing firm testicular masses and obstructive azoospermia.
• Obstructed Müllerian remnants (imperforate hymen, transverse septum, non-communicating horn) → cyclic pelvic pain with primary amenorrhoea, haematocolpos/haematometra, retrograde menstruation and endometriosis.
• Renal and skeletal anomalies accompanying Müllerian agenesis, since the mesonephric duct induces the ureteric bud — unilateral renal agenesis is the classic association.
• Osteoporosis from unreplaced sex steroids after gonadectomy.
• Inguinal hernia containing a testis or, in persistent Müllerian duct syndrome, a uterus.
• Psychosocial distress, gender dysphoria and disclosure-related harm — a recognised outcome that the Chicago Consensus addresses directly with mandated psychological support.

Complications of treatment

• Glucocorticoid over-replacement: iatrogenic Cushing syndrome, growth suppression and reduced adult height, obesity, insulin resistance.
• Glucocorticoid under-replacement: androgen excess with precocious pseudopuberty, accelerated bone age and paradoxically short adult height, hirsutism and menstrual irregularity.
• Post-gonadectomy surgical menopause if oestrogen is not started, and surgical complications of genitoplasty including stenosis, fistula and impaired sexual sensation.

• "Is there a uterus?" separates the two great primary-amenorrhoea mimics. Complete androgen insensitivity (46,XY) has no uterus because AMH worked, and scant pubic/axillary hair because the receptor is dead. MRKH (46,XX) has no uterus but normal pubic hair and normal ovaries. The distractor examiners plant is calling one the other — check the hair and the karyotype.
• Testosterone is internal, DHT is external. In 5-alpha reductase deficiency the Wolffian ducts are fully male (epididymis, vas, seminal vesicles) while the external genitalia are ambiguous — and virilise at puberty. Confirm with an elevated testosterone:DHT ratio after hCG stimulation.
• No palpable gonads in a virilised newborn = 46,XX CAH until proved otherwise. Best next step: karyotype plus 17-hydroxyprogesterone, and start serial electrolytes/glucose — salt-wasting appears in the second week, not at birth.
• Female is the default. A streak gonad, regardless of karyotype, gives female internal and external genitalia — this is why Swyer syndrome (46,XY, SRY mutation) has a uterus and needs oestrogen plus progestin, and why the same patient needs prophylactic gonadectomy for gonadoblastoma risk.
• A boy with a uterus found at hernia repair or orchidopexy is persistent Müllerian duct syndrome — AMH or AMH receptor mutation; look for transverse testicular ectopia.
• Hypertension plus virilisation in a 46,XX infant is 11-beta-hydroxylase deficiency (deoxycorticosterone excess), not 21-hydroxylase deficiency, which instead salt-wastes.
• A 46,XX infant virilised together with a virilised mother points to placental aromatase deficiency or a maternal androgen-secreting source, not a fetal adrenal enzyme block.
• Every CAH patient needs stress-dose steroid teaching and an emergency hydrocortisone kit (Endocrine Society) — a favourite "single best next step" in the outpatient stem.