Urinary tract development

Three kidney systems

• Fetal urine is the main source of amniotic fluid in the second half of pregnancy.

Metanephros: reciprocal induction

• The ureteric bud sprouts from the caudal mesonephric duct and pushes into the metanephric blastema (intermediate mesoderm). Each induces the other; WT1 is needed by the blastema.
• Ureteric bud gives: ureter, renal pelvis, major and minor calyces, collecting ducts.
• Metanephric blastema gives: the nephron, from Bowman's capsule to the distal convoluted tubule. The two meet at the connecting tubule.
• Nephrogenesis runs from about week 8 and is complete by 34–36 weeks. No new nephrons form after birth, so a baby born very preterm has fewer nephrons for life.
• Fetal lobulation may persist: smooth notches between pyramids with normal cortex, not scars (scars lie over calyces).

Ascent and rotation

• The kidney does not climb; growth of the lumbar and sacral body carries the pelvis away, so the kidney comes to lie in the loin by week 9. It is supplied by successively higher arteries; one that persists becomes an accessory renal artery, an end artery entering a pole directly (it may compress the PUJ).
• The hilum, at first anterior, rotates 90 degrees medially. Incomplete rotation leaves the hilum facing forward at a normal level.
• Pelvic kidney: fails to ascend, often supplied by the common iliac artery; in pregnancy it can mimic an adnexal mass.
• Horseshoe kidney: fused lower poles are caught under the inferior mesenteric artery; associated with Turner syndrome.

Bud failures and duplication

• Bilateral renal agenesis: no bud induction, no fetal urine, oligohydramnios, pulmonary hypoplasia, Potter facies and limb contractures; lethal. Unilateral agenesis is often silent.
• Multicystic dysplastic kidney: faulty bud–blastema induction; non-communicating cysts like grapes, no functioning nephrons; usually unilateral with normal fluid.
• Duplex kidney: an early split bud gives a bifid ureter; two buds give a complete duplex system.
• Weigert–Meyer rule: the upper-pole ureter opens lower and more medially, may be ectopic and obstructs (ureterocele); the lower-pole ureter opens higher and laterally and refluxes.
• Ectopic ureter in a girl: the bud stays attached to the mesonephric duct and follows the Gartner duct to the vagina or vestibule, below the sphincter. She voids normally but is constantly damp. In boys it opens above the external sphincter, so they stay dry.
• The Müllerian duct depends on the mesonephric duct, so unicornuate uterus or MRKH with ipsilateral renal agenesis is common; image the kidneys in any uterine anomaly.

Cloaca and urorectal septum

• The cloaca is the endoderm-lined end of the hindgut, closed by the cloacal membrane. This and the oropharyngeal membrane are the only sites where ectoderm meets endoderm with no mesoderm between.
• The urorectal septum grows down between allantois and hindgut and divides the cloaca into the urogenital sinus in front and the anorectal canal behind (weeks 4–7).
• Where the septum meets the cloacal membrane it forms the perineal body, damaged in perineal tears. The membrane splits into the urogenital membrane in front and the anal membrane behind, which ruptures in week 8.
• Above the pectinate line the anal canal is endoderm (hindgut); below it is ectoderm (proctodeum).

Urogenital sinus and allantois

• The trigone is formed by the incorporated mesonephric ducts (mesodermal origin), the exception in an endodermal bladder.
• The allantois becomes the urachus, then the median umbilical ligament. Persistence: whole tract open, patent urachus (urine at the umbilicus); umbilical end open, urachal sinus; middle, cyst; bladder end, diverticulum. Urachal remnant at the bladder dome can give adenocarcinoma with mucus in the urine.

Anorectal and body wall anomalies

• Rectovestibular fistula (commonest anorectal malformation in girls): faulty division of the cloaca by the septum. Look for VACTERL anomalies.
• Persistent cloaca: one perineal opening for urethra, vagina and rectum; occurs in girls.
• Covered (imperforate) anus with a membrane: the anal membrane failed to rupture; a low anomaly.
• Bladder exstrophy: mesoderm fails to migrate over the cloacal membrane, so the thin wall ruptures and the posterior bladder wall lies exposed, with pubic diastasis and a bifid clitoris.

Remember Bud gives the drains (ureter to collecting duct); blastema gives the filter and tubules (capsule to DCT).

Exam trap In a duplex kidney the upper pole obstructs and the lower pole refluxes; the ureters cross so the upper pole drains lowest.

Exam trap Count perineal openings: a single opening is a persistent cloaca; separate urethra and vagina with the rectum opening into the vestibule is a rectovestibular fistula.