Urinary System Development
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Direct answer
The urinary system develops from intermediate mesoderm in three overlapping stages — pronephros, mesonephros and metanephros, the permanent kidney. In the fifth week the ureteric bud sprouts from the mesonephric duct near the cloaca and invades the metanephric blastema; reciprocal induction converts the bud into the pelvis, calyces, collecting ducts and ureter, and the blastema into nephrons. The kidney then ascends from the sacral region to the lumbar fossa, rotating so the hilum faces medially.
What you must remember
- Three stages: pronephros (cervical, week four, non-functional), mesonephros (thoracic, functioning by week six, its duct persisting), metanephros (permanent, from week five, urine by about week ten).
- Ureteric bud fate: dilates into the renal pelvis and branches into major and minor calyces and about a million collecting tubules; it also forms the ureter, whose distal end is absorbed into the bladder to form part of the trigone.
- Metanephric blastema fate: metanephric vesicles become Bowman capsules, proximal and distal tubules and loops of Henle; nephron formation continues until about weeks 32 to 36.
- Ascent and rotation: kidneys develop in the pelvis and ascend during weeks six to nine, the hilum turning from ventral to medial, with blood supply shifting from iliac to aortic branches.
- Anomalies of ascent: horseshoe kidney — fused lower poles trapped below the inferior mesenteric artery, prone to obstruction and stones; ectopic pelvic kidney; crossed ectopia.
- Anomalies of branching: double ureter and double pelvis from premature bifurcation or two buds; an ectopic ureter opening below the sphincter causes incontinence.
- Potter sequence: bilateral agenesis or severe hypoplasia gives oligohydramnios, limb deformities, flat facies and lethal pulmonary hypoplasia; infantile polycystic disease is autosomal recessive.
- Bladder and urachus: the vesical part of the urogenital sinus forms most of the bladder, the trigone being mesodermal from absorbed mesonephric ducts; the urachus becomes the median umbilical ligament, leaking urine if patent.
Common confusion
Students reverse the inducer and the induced: the ureteric bud induces the blastema to form nephrons, and the blastema induces the bud to branch — either failing yields renal agenesis. Keep ducts separate: collecting ducts are bud derivatives, nephrons are blastemal. The trigone is mesodermal while the rest of the bladder is endodermal — a subtle point examiners relish.
Exam-focused takeaway
Theory questions ask for the three kidneys and their fates, derivatives of bud and blastema, kidney ascent, and the embryology of horseshoe kidney, polycystic disease, Potter sequence and patent urachus. In the viva, explain reciprocal induction and why a horseshoe kidney cannot rise past the inferior mesenteric artery.
Frequently asked questions
Which parts of the kidney come from the ureteric bud?
The ureter, renal pelvis, major and minor calyces and all the collecting tubules; the bud arises from the mesonephric duct near the cloaca.
Where do nephrons come from?
From the metanephric blastema induced at the tips of the collecting ducts; Bowman capsule, tubules and loops of Henle are all blastemal derivatives.
Why does a horseshoe kidney lie low?
Its fused lower poles catch on the inferior mesenteric artery during ascent, holding the kidney low with the ureters crossing ventral to the isthmus.
What is the Potter sequence?
Bilateral renal agenesis or severe dysplasia causes marked oligohydramnios, which deforms limbs and face and fatally arrests lung growth — pulmonary hypoplasia is the cause of death.
What is the fate of the urachus?
The allantois-derived channel from bladder apex to umbilicus obliterates into the median umbilical ligament; persistence gives a patent urachus, cyst or sinus at the umbilicus.