20.3 The upper urinary tract: development of the metanephros

• Ureter anlage and metanephric blastema: reciprocal induction
• Morphologic differentiation of the urinary tract
  • The ureter anlage and the formation of the evacuating urinary tract
  • The metanephric blastema and the formation of the nephron
• Functional development
• Ascent of the kidneys

Morphologic differentiation of the urinary tract

The ureter anlage and the formation of the evacuating urinary tract

The ureter anlage 14 is an epithelial diverticulum which arises from the caudal portion of the mesonephric duct (Wolffian duct) at the level of the first sacral vertebra. Its enlarged end grows in the dorso-cranial direction and projects into the metanephric blastema.
It is the origin of the intra- and extrarenal excretory passages:

• Ureter
• Renal pelvis
• Major and minor calices
• Collecting ducts

Fig. 12
The renal excretory passages develop from the ureter anlage. This development is described in greater detail in an interactive diagram of region C (see below).

The renal pelvis arises from the swollen end of the ureteric bud 16 that subdivides 4 times over the course of the 4th to 6th weeks. 16 branches arise from this that partially then coalesce again in order to finally form 2 to 4 major calices.
At around the 7th week the minor calices start to develop. They discharge into the major calices. Further dichotomic branching follow - up to the 15th generation (roughly 32 weeks). Thereby the caliber of the tubules is reduced more and more so that finally 1-3 millions collecting ducts are formed in the periphery of the metanephric blastema 18.