Module
20
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Urinary sytem

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. (animation)

Various anomalies are connected with this ascent and will be described in detail in the chapter on pathology.

Fig. 16 - Migration of the kidneys Stage 15 (ca. 36 days)
media/module20/t4p1_migr_sans_fleche.gif

  1. Gonad
  2. Mesonephros
  3. Allantois
  4. Tuberculum genitale
  5. Cloacal membrane
  6. Cloaca
  7. Posterior intestine
  8. Metanephros
  9. Mesonephric duct
    (Wolffian duct)
  10. Ureter anlage

Legend
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 - Migration of the kidneys Stage 18 (ca. 44 days)
media/module20/t4p2_migr_sans_fleche.gif

1
Gonad
3
Bladder being formed
4
Genital tubercle
5
Cloacal membrane
6
Rectum
7
Posterior intestine
8
Metanephros
9
Mesonephric duct

Fig. 18 - Migration of the kidneys Stage 23 (ca. 56 days)
media/module20/t4p3_migr_sans_fleche.gif

1
Gonad
3
Bladder being formed
4
Genital tubercle
6a
Rectum
7
Posterior intestine
8
Metanephros
9
Mesonephric duct
10a
Ureter
11
Urogenital orifice
12
Anal orifice

Legend
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.