Metanefro

Metanefro (metanephros in English) is the final and permanent stage of vertebrate kidney development. It arises during embryogenesis from the interaction of two embryonic structures: the ureteric bud, an outgrowth of the mesonephric duct, and the metanephric mesenchyme (metanephric blastema). Through reciprocal induction, these tissues organize into the definitive kidney that will function after birth. The term combines Greek roots meta- 'beyond' and nephros 'kidney'.

Formation and architecture: The metanephros forms in the posterior urogenital ridge. The ureteric bud invades the metanephric mesenchyme, undergoing branching to generate the collecting system—renal pelvis, calyces, and collecting ducts—while the metanephric mesenchyme differentiates into nephron components: glomerulus with Bowman's capsule, proximal tubule, loop of Henle, and distal tubule. In humans, metanephrogenesis begins around weeks 5–6 of gestation; nephron formation continues after birth, with maturation of nephrons progressing over years. Adult kidneys typically contain about one million nephrons per organ, though numbers vary.

Significance: The metanephros is the functional organ of the kidney in the adult. Disruptions in metanephric development can cause congenital anomalies such as renal agenesis, hypoplasia, or dysplasia and can affect urinary tract formation. Development is regulated by signaling pathways and transcription factors, including GDNF-RET signaling and genes such as PAX2 and WT1, which coordinate ureteric bud branching and nephron formation. Evolutionarily, the metanephros is the characteristic definitive kidney in mammals and other vertebrates.