Urinary System Development

What is the embryologic origin of the urinary system?

The intermediate mesoderm forms elevations along the posterior abdominal wall → gives rise to urinary & genital systems, which are embryologically and anatomically interwoven.

What structures pass through the kidney hilum and what is the renal sinus?

Hilum transmits renal artery, vein, ureter, lymphatics, autonomic nerves. Renal sinus = space within hilum containing renal pelvis.

Name the divisions of the renal collecting system.

Renal pelvis → major calyces → minor calyces → collecting tubules/ducts → duct of Bellini → ureter.

What are the three overlapping kidney systems and their timeline?

• Pronephros (cranial, week 4) – nonfunctional, transient.

• Mesonephros (thoracic/lumbar, week 4–8) – interim kidney.

• Metanephros (pelvic, week 5–40) – permanent kidney.

Describe the pronephros.

From cervical intermediate mesoderm. Has nonfunctional tubules (no glomeruli). Pronephric duct runs caudally → opens into cloaca → becomes mesonephric duct (induces mesonephros).

Describe the mesonephros.

Appears late week 4, disappears by week 8. Has Bowman’s capsule + glomerulus, but no loop of Henle. Tubules drain into mesonephric duct → urogenital sinus. Caudal tubules persist as efferent ductules of testis.

What are the derivatives of the mesonephric duct?

• Ureteric bud (future ureter).

• Trigone of bladder.

• In males: ductus deferens, seminal vesicles.

• In females: degenerates.

What are the two sources of the permanent kidney (metanephros)?

• Ureteric bud (from mesonephric duct) → collecting system (ureter, pelvis, calyces, collecting ducts).

• Metanephric blastema (mesoderm) → nephrons (Bowman’s capsule, PCT, loop of Henle, DCT).

When does the metanephros begin to function?

Appears week 5, functions by week 9–10 → produces urine added to amniotic fluid.

Describe kidney ascent and rotation.

Kidneys ascend from pelvis to abdomen by week 9. Stop when contacting suprarenal glands. Supplied by successive arteries; caudal vessels degenerate → final supply from abdominal aorta. Rotate 90° medially (hilum faces medial). Accessory renal arteries common.

What causes unilateral vs. bilateral renal agenesis?

Failure of ureteric bud to develop/branch → no induction of nephrons.

• Unilateral: 1/1000, asymptomatic.

• Bilateral: 1/3000, fatal, causes oligohydramnios + Potter sequence.

Differentiate multicystic dysplastic kidney vs. polycystic kidney disease.

• Multicystic dysplastic: irregular cysts, “grape‑like,” unilateral OK, bilateral fatal.

• Polycystic: multiple cysts, renal failure.

  • AR (childhood): cysts from collecting ducts, Potter sequence, fatal.

  • AD (adult): mutations in PKD1/2, cysts from nephron parts, renal failure in adulthood.

What are renal ectopia and malrotation?

• Ectopia: kidney fails to ascend → pelvic kidney, pancake kidney, crossed ectopia.

• Malrotation: hilum faces anterior, posterior, or lateral. Often asymptomatic but ↑ risk of UTI, stones, hydronephrosis.

What is a horseshoe kidney?

Fusion of inferior poles → U‑shaped kidney in hypogastrium. Ascent blocked by inferior mesenteric artery. Associated with Turner syndrome (45,XO), ↑ risk of Wilms tumor, infection.

What causes duplications of the urinary tract?

Division of ureteric bud.

• Incomplete → bifid ureter.

• Complete → double kidney with bifid ureter.

• Two buds → supernumerary kidney.

What is the embryonic origin of bladder and urethra?

Cloaca divided by urorectal septum → urogenital sinus (anterior) + anorectal canal (posterior).

• Bladder (except trigone): from urogenital sinus (endoderm).

• Trigone: from mesonephric ducts.

• Urethra: pelvic + phallic parts (male vs. female derivatives).

What is the urachus?

Remnant of allantois. Connects bladder apex → umbilicus. Becomes median umbilical ligament. If patent → urachal fistula (urine leaks from umbilicus).

What is bladder exstrophy?

Ventral body wall defect → bladder exposed. Associated with epispadias in males. Results in incontinence, requires reconstructive surgery.

What are ectopic ureteral orifices?

Ureter opens outside bladder (neck, urethra, vagina). Caused by failure of incorporation into bladder wall. Leads to continuous incontinence.

Summarize embryonic precursors and derivatives.

• Metanephric blastema (intermediate mesoderm): nephrons.

• Ureteric bud (mesonephric duct): ureter, pelvis, calyces, collecting ducts.

• Mesonephric duct (pronephric duct): trigone.

• Urogenital sinus (cloaca): bladder (except trigone), urethra (male: prostatic, membranous, spongy; female: entire urethra).