Figure 14.10 A to C. Ascent of the kidneys. Note the change in position between the mesonephric and metanephric systems. The mesonephric system degenerates almost entirely, and only a few remnants persist in close contact with the gonad. In both male and female embryos, the gonads descend from their original level to a much lower position. D. Scanning electron micrograph of a mouse embryo showing the kidneys in the pelvis. B, bladder; K, kidney A, adrenal gland; G, gonad; T, tail.
(Fig. 14.10). In the pelvis the metanephros receives its arterial supply from a pelvic branch of the aorta. During its ascent to the abdominal level, it is vascu-larized by arteries that originate from the aorta at continuously higher levels. The lower vessels usually degenerate, but some may remain.
Figure 14.11 A. Unilateral pelvic kidney showing the position of the adrenal gland on the affected side. B and C. Drawing and photomicrograph, respectively, of horseshoe kidneys showing the position of the inferior mesenteric artery. BW, bladder wall; U, ureters.
CLINICAL CORRELATES Abnormal Location of the Kidneys
During their ascent the kidneys pass through the arterial fork formed by the umbilical arteries, but occasionally one of them fails to do so. Remaining in the pelvis close to the common iliac artery, it is known as a pelvic kidney (Fig. 14.11 A). Sometimes the kidneys are pushed so close together during their passage through the arterial fork that the lower poles fuse, forming a horseshoe kidney (Fig. 14.11, B and C). The horseshoe kidney is usually at the level of the lower lumbar vertebrae, since its ascent is prevented by the root of the inferior mesenteric artery (Fig. 14.11 B). The ureters arise from the anterior surface of the kidney and pass ventral to the isthmus in a caudal direction. Horseshoe kidney is found in 1/600 people.
Accessory renal arteries are common; they derive from the persistence of embryonic vessels that formed during ascent of the kidneys. These arteries usually arise from the aorta and enter the superior or inferior poles of the kidneys.
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