Abnormalities of Renal Induction
If two ureteric buds arise from the mesonephric (Wolfian) duct, or if a single bud divides into two branches before it invades into the metanephrogenic blastema, a duplication occurs. Complete duplication results when two entirely separate ureters drain separate parts of the kidney. Partial duplication occurs when the two ureters join before reaching the bladder.
See more about duplication.
When the ureteric bud does not meet the nephrogenic blastema,
normal nephrogenesis does not occur. If the bud originates from the
mesonephric duct either caudal to or cranial to the normal position,
dysplasia may occur. When the ureteric bud arises from an
abnormal position on the mesonephric duct, the drainage of the
resulting ureter may also be abnormal (reflux or ureteric ectopy).
This may explain why some kidneys with reflux or ectopic ureters have
diminished function and capacity.
Early in development, the ureter goes through a period during
which it has no lumen (six weeks). As the metanephros is induced to
develop functioning glomeruli and tubules the ureter opens to become
a conduit. Occasionally, this canalization fails to occur resulting
in complete obstruction of the developing metanephros. Functioning
nephrons fill with fluid and become cystic because there is no outlet
for the fetal urine. This collection of cysts of varying sizes in the
renal fossa, called a multicystic
kidney has no function at birth.
Abnormalities of Ascent and Rotation
If the kidney fails to ascend, it remains unrotated (pelvic
kidney, also called an ectopic kidney or a pancake kidney because
of its flattened shape). Such kidneys often maintain their fetal
blood supply from the iliac vessels or the distal aorta.
See a picture of a pelvic kidney.
If the two developing kidneys make contact in their ascent, fusion
may occur, resulting in a horseshoe
kidney. This most often occurs with the lower poles of each
kidney fusing. The bridge of parenchyma by which they are fused is
called the isthmus. When the isthmus of a horseshoe kidney reaches
the inferior mesenteric artery the kidneys can rise no further. Like
pelvic kidneys, horseshoe kidneys have multiple renal arteries and
veins. Horseshoe kidneys are at increased risk for development of
hydronephrosis, kidney stones and renal cell carcinoma.
Occasionally, fusion of the two developing kidneys may occur with one kidney moving to the opposite side, following ascent of the other kidney. This condition is called crossed fused ectopia. See a picture of crossed fused ectopia.
Abnormalities of Renal Vasculature
As the kidney ascends, new blood vessels arise from the iliac
artery, the distal aorta and, finally, the upper abdominal aorta.
Occasionally, caudal branches of the kidney persist as the kidney
ascends. Accessory renal arteries may arise from the aorta adjacent
to the main renal artery, distal to the ostium of the main renal
artery, or even from the iliac artery. When multiple arteries occur,
each artery supplies a distinct segment of the kidney; there is no
collateral perfusion. Occlusion of one artery will result in
infarction of its associated segment. Why is this important?
Multiple renal veins may drain to the vena cava. Unlike the
arteries, however, the renal veins interconnect within the kidney. If
one renal vein is occluded, the remaining renal vein(s) will continue
to drain the entire kidney.
During development, the ureteropelvic junction (the point at which
the ureter and renal pelvis meet) may be narrow or kinked or it may
have abnormal peristalsis so that urine flow is obstructed. This
results in a pooling of fetal urine within the pelvis. Hydronephrosis
[literally, water within the kidney] is the term used to describe
this enlargement. Hydronephrosis is usually caused by a
ureteropelvic junction obstruction although it can result from
abnormalities of the ureter or bladder. Why is hydronephrosis
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©David A. Hatch, M.D., 1996