Development of the Genitourinary System — Kidneys, Bladder, Genital Ducts and Their Anomalies

During embryonic folding in the fourth week, the intermediate mesoderm forms a longitudinal swelling on the posterior body wall — the urogenital ridge. Its lateral part, the nephrogenic cord, forms the urinary system; its medial part, the gonadal (genital) ridge, forms the gonads. This shared origin explains why kidney and genital-tract anomalies so often occur together. For the full germ-layer map, see germ layers.

Three sets of kidneys appear in a cranial-to-caudal sequence within the nephrogenic cord. Each overlaps with the next, and only the last persists as the adult kidney.

At the start of week 5, the metanephric blastema (metanephric mesoderm) in the sacral region secretes GDNF, which acts on the RET receptor of the mesonephric duct and induces an outgrowth — the ureteric bud. The two tissues then induce each other: the bud branches into the blastema, and every branch tip induces a cap of blastema to form a nephron.

Molecular control is examinable at a basic level: Pax2 is essential for specifying renal tissue, Lim1 patterns the pronephric and mesonephric ducts, and BMP4 (from the blastema) and Gremlin (from the bud) balance each other so that only one ureteric bud forms on each side. Disruption of these signals produces congenital anomalies of the kidney and urinary tract (CAKUT) — renal agenesis, dysplasia, multicystic dysplastic kidney and others.

The metanephric kidneys start low in the sacral region, close together. Between weeks 6 and 9, growth of the lower body draws them apart and they ascend to the lumbar region, rotating so the hilum faces medially. As they rise they pick up blood supply from successively higher branches of the aorta, and the lower branches degenerate. A kidney that stops ascending keeps a low, often multiple, arterial supply.

In week 4 the urorectal (urogenital) septum divides the cloaca into the rectum behind and the urogenital sinus in front. The urogenital sinus forms the bladder and urethra. The terminal mesonephric ducts are absorbed into the posterior bladder wall: the ureters come to open separately and higher, while the ducts' openings move down. The triangle between them becomes the trigone, whose mesodermal lining is later replaced by endoderm.

Until about the sixth week both sexes have two pairs of genital ducts: the mesonephric (Wolffian) and the paramesonephric (Müllerian). Sex determination depends on the SRY gene on the Y chromosome, which drives the gonad to become a testis. Two testicular hormones then decide the duct fate:

• Anti-Müllerian hormone (AMH, Müllerian-inhibiting substance) from Sertoli cells makes the Müllerian ducts regress.
• Testosterone from Leydig cells (from weeks 9–10) stabilises and virilises the Wolffian ducts. Without testosterone the Wolffian ducts regress.
• Dihydrotestosterone (DHT), made from testosterone by 5-alpha-reductase, masculinises the external genitalia.

The Müllerian ducts run lateral to the Wolffian ducts cranially, cross ventral to them, and meet in the midline caudally. Their unfused cranial ends form the uterine tubes. The caudal ends fuse (from about 6–8 weeks) into the uterovaginal primordium, which forms the uterus and cervix. A midline septum initially divides the fused cavity; it is normally resorbed.

The tip of the uterovaginal primordium meets the posterior wall of the urogenital sinus at the sinus (Müllerian) tubercle, which induces paired sinovaginal bulbs. These form a solid vaginal plate that later canalises to give the lower two-thirds of the vagina; the hymen separates the vaginal canal from the urogenital sinus. Most of the vaginal epithelium is therefore endodermal, while its muscular wall is mesenchymal.

Female reproductive-tract anomalies are estimated in 0.1–3% of live births. Because the Müllerian ducts and mesonephros share intermediate mesoderm, every Müllerian anomaly should prompt a search for renal anomalies. MRI is considered the gold-standard imaging test. StatPearls follows the classic seven-class scheme:

A horseshoe kidney is the most common renal fusion anomaly, seen in about 1 in 500 people, with a male-to-female ratio of about 2:1. The lower poles of the two metanephric blastemas fuse across the midline (in over 90% of cases) between the 4th and 6th weeks, before the renal capsule forms. The fused kidney is malrotated, with collecting systems facing anteriorly, and its ascent is classically said to be arrested by the inferior mesenteric artery at about L3.

• Ureters pass anterior to the isthmus, which predisposes to ureteropelvic junction obstruction (up to 33%), hydronephrosis, stones and infection.
• Blood supply is variable and multiple, arising wherever ascent stopped.
• Associations: Edwards syndrome (trisomy 18) in about 67%, Turner syndrome in about 20%, Down syndrome in about 1%.
• Higher risk of renal tumours, including Wilms tumour in children.

The external genitalia are indifferent until about week 9: a genital tubercle, paired urogenital (urethral) folds and labioscrotal swellings surround the cloacal membrane. DHT drives the male pattern; its absence (with maternal oestrogens) gives the female pattern, completed by about week 20.

Incomplete fusion of the urethral folds gives hypospadias — a urethral opening on the ventral surface of the penis (glanular, coronal, penile or perineal). 5-alpha-reductase type 2 deficiency (46,XY) causes ambiguous genitalia at birth because DHT cannot be made, even though testosterone-dependent Wolffian structures are normal.