also normal but there is a reduced quantity of

Such ascendance does not change the location

tissue. Once again, they are usually associated

of the ureteral opening in the urogental sinus,

with a corresponding poorly developed ureter

i.e. the trigone.

and trigone. Inadequate ureteral bud develop-

Horseshoe kidneys are examples of abnormal

ment or vascularization is again thought to

fusion where the lower and occasionally upper

play a major role in their etiology. Renal hyp-

poles of the two renal blastemas are fused by an

oplasia is usually unilateral but can be bilateral

isthmus composed of either solid parenchymal

and may be confused with chronic pyelone-

tissue or a fibrous band which crosses in front of

phritis. An important point to remember in

the aorta and vena cava. In a rare instance, the

renal hypoplasia is that these kidneys usually

isthmus may be situated behind or between the

are unable to hypertrophy with contralateral

great vessels.

nephrectomy.

This abnormal fusion of the right and left

metanephros generally occurs between the fourth

Renal Ectopia

and eighth week of fetal development while the

mesonephros is the functional renal unit and

Renal ectopia can be simple or crossed. The

prevents normal ascent and rotation of the kid-

neys. The fusion and lack of rotation explain the

ectopic kidney can be normal or hypoplastic

anterior position of the ureters in such patients.

and is usually malrotated with the pelvis lying

The horseshoe kidney is typically located with

anterior or posterior from the norm. The usual

the isthmus overlying the L-3 or L-4 level. In

cause is arrest of normal upward migration by

about 5% of patients, the upper poles are fused.

abnormal persistence of vascular attachments

When both poles are fused, the finding is

which then prevent normal ascendance of the

described as a doughnut kidney. As might be

developing kidney from its pelvic fetal location

expected, the blood supply is very anomalous in

to its normal position high in the retroperito-

these cases.Twenty-five percent of these patients

neum.Thus,aberrant renal vascularity is almost

remain asymptomatic throughout life,and when

always present and can originate from any

symptoms do occur they are generally related to

nearby major artery such as the aorta, the ili-

obstruction and its aftermath of the ureters as

acs, the middle sacral, or inferior mesenteric

they cross the isthmus.

arteries. Simple ectopia is not uncommon,

occurs in approximately one of every 800

patients, and is three times more common on

Ureteral Development

the left side.

In crossed ectopia, there is an 85% incidence

of fusion with the contralateral kidney, which is

The next major area of discussion deals with the

generally in a normal position. A variety of con-

ureters and particularly their assumption of a

figurations or shapes may occur, but the fusion

normal position in the developing urogenital

is generally pole to pole. As in simple ectopia,

sinus (see Fig. 1.4). You will recall that the ure-

the blood supply is aberrant. The ureters, how-

teral bud originally sprouted from the dorsal

ever, generally enter the bladder in a normal

surface of the mesonephric duct just proximal

position.

to its junction with the cloaca. Because of the

manner of growth and absorption of the meso-

nephric duct into the developing urogenital

Renal Fusion

sinus, the ureter gradually assumes a more lat-

eral and eventual anterior position on the meso-

Renal fusion can take many shapes or forms.

nephric duct.

The lump or cake kidney is a solid, irregular, or

With continued expansion of the urogenital

lobulated organ which is usually quite low in

sinus, the mesonephric duct with its ureteral

position with an aberrant blood supply. Again,

bud is literally absorbed. They eventually sepa-

as in ectopia, the ureters generally enter the

rate from each other through a somewhat com-

bladder in a normal manner because, as you

plicated form of growth with the ureteral orifice

recall, the ureteral bud arises from the meso-

moving in a lateral and cephalad direction while

nephric duct to enter the metanephros and

the mesonephric duct, which has now separated

ascends into a normal or abnormal position.

from the ureteral bud, continues to move in a