metanephros

Metanephros is the final stage of kidney development in amniotes and forms the definitive adult kidney. In humans, kidney development begins with the pronephros and mesonephros, but the metanephros arises around the fifth week of gestation and becomes functionally mature around the ninth week.

Developmentally, the metanephros arises through interaction between two embryonic structures: the ureteric bud, which sprouts from the caudal end of the mesonephric duct, and the surrounding metanephric mesenchyme. The ureteric bud undergoes branching to generate the collecting system, including the renal pelvis, calyces, and collecting ducts. The metanephric mesenchyme differentiates into the nephron components, forming the proximal and distal tubules, the loop of Henle, and the Bowman’s capsule. This reciprocal induction between the ureteric bud and the mesenchyme is regulated by signaling pathways and transcription factors such as GDNF and RET, WT1, and PAX2.

Anatomically, the metanephros develops in the adult retroperitoneal location and consists of a renal cortex and a medulla. Nephrons, the functional units, are formed from the metanephric mesenchyme and connect to the collecting ducts derived from the ureteric bud. The mature kidney filters blood via glomeruli, reabsorbs and secretes substances along the tubules, and concentrates urine. Arterial supply to the kidney comes from the renal arteries, with venous drainage to the renal veins, and urine drains via the ureter.

Clinical relevance includes congenital anomalies such as renal agenesis, hypoplasia, or dysplasia, which arise from disrupted metanephric development. The metanephros remains the functional kidney throughout life in humans and other amniotes.