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

Ascent of the kidneys

The metanephros is formed in the sacral region at the level of the first sacral vertebra (S1) and the bifurcation of the aorta. In the adult, the kidney lies at the upper lumbar level. The "migration" (ascent) of the kidney occurs between the 6th and 9th week and the kidneys finally come to lie at the level of the 12th thoracal vertebra (Th12) under the suprarenal glands.
The mechanism that leads to this ascent of the kidneys is not an active migration but rather much more the result of the differing growths of the sacral and lumbar regions, which lead to an unfolding of the lower pole of the embryonic body. During its ascent, the kidney is supplied by a number of transitory vessels that all originate from the aorta. The definitive renal arteries stem from the lumbar region of the aorta, while the transitory vessels normally disappear.

Fig. 16
Ascent of the kidney at the end of the 5th week. The kidneys still find themselves in the sacro-lumbar region. The mesonephros, which in this stage is still to be found under the undifferentiated gonads, is in the process of atrophying.

Fig. 17
Ascent of the kidneys at the end of the 7th week. At this point the ureter - independent of the mesonephric duct (Wolffian duct) - inserts into the bladder. The mesonephros has disappeared.

Fig. 18
Ascent of the kidneys at the end of the 8th week. The kidneys find themselves at the upper lumbar level in their definitive position and are multilobular.