Obstetrics and Gynaecology chapters
Embryology of the female genital and urinary tract
How the indifferent gonad becomes an ovary or testis, how the Müllerian and Wolffian ducts form the internal organs, how the vagina and external genitalia form, and how the kidney, ureter and bladder develop, with the timings and remnants PGET asks about.
- The gonad is indifferent until about 7 weeks; the SRY gene makes it a testis, whose Sertoli cells make AMH (Müllerian ducts regress) and Leydig cells make testosterone (Wolffian ducts persist); DHT virilises the external genitalia.
- Without AMH the Müllerian (paramesonephric) ducts form the tubes, uterus, cervix and upper vagina; the lower vagina comes from the sinovaginal bulbs of the urogenital sinus; canalisation runs upwards from the hymen.
- Germ cells peak at 6-7 million at 20 weeks, fall to 1-2 million at birth and 300,000-400,000 at menarche; the ureteric bud grows from the mesonephric duct in week 5 to form the collecting system.
On this page
Overview
Genital development has four phases (Gynaecology by Ten Teachers):
| Phase | Timing |
|---|---|
| Indifferent gonad | 4-6 weeks |
| Gonadal differentiation | About 7 weeks |
| Ductal differentiation | 9-11 weeks |
| External genitalia | 10-12 weeks |
The genital and urinary systems grow together from the intermediate mesoderm and share the mesonephric duct and the urogenital sinus. That is why genital anomalies often come with renal anomalies.
The indifferent gonad
- The genital (gonadal) ridge appears on the medial side of the mesonephros (embryonic kidney) on the back wall of the coelom in the fourth to fifth week.
- Primordial germ cells arise outside the embryo proper, in the wall of the yolk sac, and migrate along the gut mesentery into the ridge. Without them, the gonad does not form properly.
- Until about 7 weeks the gonad is bipotential (could become ovary or testis).
- The chromosomes decide the path: the SRY gene (sex-determining region of the Y chromosome) turns it into a testis. Ovarian development is also an active gene-driven process, not simply "no SRY".
Male pathway (for contrast)
| Cell | Hormone | Effect |
|---|---|---|
| Sertoli cell | Anti-Müllerian hormone (AMH) | Müllerian ducts regress |
| Leydig cell | Testosterone | Wolffian ducts form epididymis, vas deferens, seminal vesicles, ejaculatory duct |
| Genital skin | Testosterone → DHT by 5-alpha-reductase | Genital tubercle → penis; labioscrotal folds fuse → scrotum; urethral folds close → penile urethra |
- In the fetus, placental hCG drives the fetal Leydig cells to make testosterone.
- Both hormones act locally on each side, so a one-sided testis gives one-sided duct effects.
Duct fate depends on two separate hormones: AMH removes the Müllerian ducts, testosterone keeps the Wolffian ducts. External genitalia need DHT. In complete androgen insensitivity (46,XY), AMH works, so there is no uterus, but the genitalia look female; in 5-alpha-reductase deficiency the internal male ducts form but the external genitalia are ambiguous and virilise at puberty.
Female pathway
Ovary
- Granulosa cells (from the proliferating coelomic epithelium) surround each germ cell to make a primordial follicle: an oocyte inside one layer of granulosa cells. Theca cells form outside, separated by a basal lamina.
- Oocytes enter the first meiotic division and arrest in prophase I until atresia or the cycle of ovulation.
- Ovaries descend from the back wall to the pelvis, keeping their blood supply from the aorta.
| Time | Oocyte number (both ovaries) |
|---|---|
| 20 weeks of fetal life | 6-7 million (the peak) |
| Birth | 1-2 million |
| Menarche | 300,000-400,000 |
| Menopause | Almost none |
The maximum number of oocytes is before birth (20 weeks), not at puberty. No new oocytes are made after fetal life; loss is by atresia.
Genital ducts
- With no AMH, the Müllerian (paramesonephric) ducts develop. They form lateral to the mesonephric ducts, grow down, cross in front of them and meet in the midline.
- Cranial unfused parts → fallopian tubes (the cranial openings become the fimbrial ends).
- Caudal fused parts → uterus, cervix and upper vagina. A midline septum forms and is then absorbed.
- With no testosterone, the Wolffian (mesonephric) ducts regress, leaving remnants.
Vagina
- The fused Müllerian ducts touch the back wall of the urogenital sinus, raising the Müllerian tubercle.
- The sinus wall grows two solid sinovaginal bulbs, which fuse with the tubercle to make a solid vaginal plate.
- The plate canalises from below upwards (starting at the hymen and moving towards the cervix), finishing by about the sixth month; by about 5 months uterine septal resorption is complete.
- The hymen is the thin membrane left where the vaginal lumen meets the urogenital sinus.
| Part | Origin |
|---|---|
| Upper vagina (most of it) | Müllerian ducts |
| Lower vagina | Sinovaginal bulbs (urogenital sinus) |
| Hymen | Junction of vaginal plate and urogenital sinus |
A girl with Müllerian agenesis (MRKH) still has a short blind lower vagina (from the urogenital sinus) and normal ovaries (from the genital ridge, not the Müllerian duct), so breasts and pubic hair are normal.
External genitalia
- Without androgens the external genitalia do not virilise.
- Between weeks 5 and 7, the cloacal folds around the cloacal membrane fuse in front to form the genital tubercle.
- The urorectal septum splits the cloacal membrane into a front urogenital membrane and a back anal membrane; the perineum forms between them.
- Female genitalia are recognisably female by about 12 weeks.
| Indifferent structure | Female | Male |
|---|---|---|
| Genital tubercle | Clitoris | Glans and body of penis |
| Urethral (urogenital) folds | Labia minora | Floor of penile urethra |
| Labioscrotal swellings | Labia majora | Scrotum |
| Urogenital sinus | Vestibule, lower vagina, urethra, paraurethral (Skene's) and Bartholin's glands | Prostatic and penile urethra, prostate, bulbourethral glands |
Homologues and remnants
| Embryonic structure | Female | Male |
|---|---|---|
| Indifferent gonad | Ovary | Testis |
| Mesonephric (Wolffian) duct | Epoophoron, paroophoron, Gartner's duct; trigone of bladder | Epididymis, vas, seminal vesicle and ejaculatory duct; trigone |
| Paramesonephric (Müllerian) duct | Tubes, uterus, cervix, upper vagina | Appendix of testis, prostatic utricle |
| Gubernaculum | Ovarian ligament and round ligament of uterus | Gubernaculum testis |
- Epoophoron: blind tubules in the mesosalpinx between ovary and tube.
- Paroophoron: a few tubules nearer the uterus.
- Gartner's duct: the persisting lower mesonephric duct along the side of the uterus and lateral vaginal wall; it can form a Gartner's duct cyst in the anterolateral vaginal wall.
"Mesonephric = Male; paraMesonephric = Mummy (uterus)."
Urinary tract
Kidney and ureter
- Three kidneys form in sequence from the intermediate mesoderm:
- Pronephros (about week 3): never works and disappears.
- Mesonephros: works briefly; its duct (Wolffian duct) reaches the cloaca and persists.
- Metanephros: the permanent kidney.
- In week 5 the ureteric bud grows from the mesonephric duct. It branches to form the ureter, renal pelvis, calyces and collecting ducts, and it induces the metanephric tissue to form the nephrons (glomeruli, tubules, loops of Henle).
- All nephrons are formed by about 32-36 weeks, but they keep maturing after birth (so preterm babies handle salt, water and acid poorly).
- The kidneys ascend and rotate from the pelvis.
| Anomaly | Cause | Clue |
|---|---|---|
| Renal agenesis | Ureteric bud fails | Bilateral: no liquor, lung hypoplasia, Potter facies, incompatible with life |
| Pelvic kidney | Failure of ascent | Mass in pelvis |
| Horseshoe kidney | Lower poles fuse | Ascent stopped by the inferior mesenteric artery |
| Duplex kidney | Bud splits early | Two ureters; upper moiety ureter may open ectopically, lower moiety refluxes |
| Pelviureteric junction obstruction | Narrow junction | Hydronephrosis with normal ureter |
| Posterior urethral valves (boys) | Obstructing fold in posterior urethra | "Keyhole" bladder, bilateral hydronephrosis, oligohydramnios |
Fetal urine makes most of the liquor in later pregnancy, so bilateral renal agenesis gives severe oligohydramnios, pulmonary hypoplasia and Potter facies; it is lethal.
Bladder and urethra
- The urorectal septum divides the cloaca into the urogenital sinus (front) and the anorectal canal (back).
- The upper sinus forms the bladder; in females the lower sinus forms the whole urethra and the vestibule.
- The trigone forms from the absorbed lower ends of the mesonephric ducts.
- The allantois narrows to the urachus, which becomes the median umbilical ligament. A patent urachus leaks urine at the umbilicus.
Quick revision
- Indifferent until about 7 weeks; SRY → testis.
- AMH (Sertoli) kills Müllerian; testosterone (Leydig) keeps Wolffian; DHT virilises outside.
- Müllerian: tubes (unfused), uterus, cervix, upper vagina (fused).
- Lower vagina: sinovaginal bulbs; canalisation from below up.
- Oocytes: 6-7 million at 20 weeks → 1-2 million at birth → 300,000-400,000 at menarche.
- Female Wolffian remnants: epoophoron, paroophoron, Gartner's duct.
- Ureteric bud from mesonephric duct (week 5); nephrons from metanephric tissue.
- Renal anomalies in up to 30% of Müllerian anomalies.
Written by The House Job from standard references (Gynaecology by Ten Teachers; Obstetrics by Ten Teachers; Snell's Clinical Anatomy).
FAQ
Why is there no uterus in complete androgen insensitivity?
The testes form normally and their Sertoli cells make AMH, which removes the Müllerian ducts; only the androgen receptor is faulty, so the outer genitalia develop as female.
Where does the lower vagina come from?
From the sinovaginal bulbs of the urogenital sinus, which form the vaginal plate with the Müllerian tubercle and then hollow out from the hymen upwards.
What is a Gartner's duct cyst?
A cyst in the anterolateral vaginal wall from a persisting lower part of the mesonephric (Wolffian) duct; it is usually harmless.
Why are the ovaries normal in MRKH syndrome?
The ovaries come from the genital ridge and germ cells, not from the Müllerian ducts, so they make hormones and ovulate normally.