Ureterknospe

Ureterknospe, or ureteric bud, is an embryonic outgrowth of the distal mesonephric (Wolffian) duct that initiates the development of the kidney's collecting system. In humans and other mammals it emerges as a single budding event from the duct around the fourth week of gestation and invades the surrounding metanephric mesenchyme (metanephros).

The bud undergoes extensive branching morphogenesis, producing the collecting ducts, renal pelvis, and calyces, and ultimately gives rise to the ureter that connects the kidney to the bladder. The metanephric mesenchyme signals to the bud via glial cell line-derived neurotrophic factor (GDNF) binding to the RET receptor on the bud's epithelium; this GDNF-RET pathway drives bud outgrowth and branching. Additional signaling networks, including WNT (notably WNT11), FGF and BMP family members, regulate growth, branching pattern, and organogenesis. The process is reciprocal: the ureteric bud promotes nephron formation in the surrounding metanephric mesenchyme, while mesenchymal signals refine the bud's branching.

Disruptions of ureteric bud development can lead to congenital abnormalities of the kidney and urinary tract, such as renal agenesis, duplex collecting systems, vesicoureteral reflux, or hydronephrosis. Mutations and dysregulation of key genes, including RET, GDNF, PAX2, and others, have been linked to these conditions.