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.
It must also be mentioned here that during their development the kidneys turn 90° towards the vertebral cords, so that the hili are medially oriented at the end, while they originally face ventrally. (
Various anomalies are connected with this ascent and will be described in detail in the chapter on pathology.