The ureter anlage and the formation of the evacuating urinary tract
The ureter anlage (stage 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:
- Renal pelvis
- Major and minor calices
- Collecting ducts
The cranial end of the ureter anlage subdivides dichotomically many times due to the inductive effect of the metanephric blastema.
The renal pelvis arises from the swollen end of the ureteric bud (stage 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 (stage 18).
In this interactive diagram you can study the development of the extra and intra-renal urine excretory system, especially of the ureter, of the renal pelvis, of the major and minor calices, as well as of the collecting ducts over the course of stages 14 to 20 (ca. 33 to 46 days).