6

Practical Urology: EssEntial PrinciPlEs and PracticE

Critical Steps in Further Development

1. Absence of the entire genito-urinary ridge.

While differentiating into this “final stage kid-

2. Failure of mesonephric duct development.

3.

Failure of ureteral bud development from

ney,” during the 9th to 12th weeks of fetal life, the

the mesonephric duct.

metanephros undergoes three critical steps in

4.

Failure of the ureteral bud to reach the

further development. These include ascendance,

metanephros.

rotation,and revascularization.First,the kidneys

5. Failure of the vascular supply to invade the

ascend from the level of the fourth lumbar verte-

bral level to the first lumbar or 12th thoracic

metanephrogenic blastema.

level. This ascent of the kidney is due to both

Clinically, if agenesis is due to ureteral bud

cephalad migration as well as the differential

absence, a hemitrigone would be seen cysto-

growth of the caudal part of the fetus. Second, in

scopically. On the other hand, if the ureteral bud

addition to ascent, the kidneys also undergo an

develops normally but the metanephros fails, a

approximate 90° rotation from a plane with the

normal trigone would be visualized and a blind

pelvis anterior to one with the pelvis lying medial

ending ureter should be identifiable on retro-

and somewhat posterior. Third, as each kidney

grade urography. In patients with renal agenesis

ascends and rotates, it is successively supplied by

secondary to absence of the mesonephric duct

arteries which are located higher and higher on

proper, the genital duct apparatus would also be

the urogenital ridge. They can arise from the

absent on the ipsilateral side. Furthermore, if

iliac, middle sacral, lower aorta, and inferior

the entire genito-urinary ridge failed to develop,

mesenteric artery, and eventually from the final

not

only the kidney, ureter, and genital duct

position on the aorta just below the superior

structures, but the testis as well would be absent

mesenteric artery.

on the ipsilateral side.

Failure of any of these progressive steps to

Renal agenesis can be unilateral or bilateral.

occur in proper sequence and order will result

The unilateral absence of a kidney is not

in anomalous development. Such

anomalies

uncommon and occurs in approximately one of

include anomalies of volume and

structure,

every 500 births. Bilateral agenesis is fortu-

anomalies of number,anomalies of location,anom-

nately quite rare and, when present, is classi-

alies of rotation, anomalies of form, and anoma-

cally seen in association with the typical Potter’s

lies of the vessels.

facies. In such cases, other anomalies are almost

Anomalies of the Kidney

always present and generally incompatible with

life. It is noteworthy that in unilateral renal

agenesis the existing kidney is normally larger

While there are a great variety of anomalies asso-

than a kidney which has hypertrophied because

of destruction of its mate by injury or disease.

ciated with each of these categories,we will briefly

discuss the more common and serious anomalies

associated with renal maldevelopment.

Renal Aplasia

Before discussing renal agenesis, aplasia, and

hypoplasia, it is important to reiterate that the

In the patient with renal aplasia, nephrogenic tis-

presence of a normal ureteral bud is a prerequi-

sue is present but it has failed to develop. These

site to normal renal development. In other

very small kidneys generally do not function and

words, the quality of the ureteral bud reflects

the ureter is generally equally poorly developed.

not only the development of the ureter itself but

Embryologically, they are best explained by inad-

also the appearance and development of the

equate stimulation of the metanephros by a bud

bladder trigone and kidney as well.

of poor quality or inadequate vascularization of

the metanephrogenic blastema. Again, the oppo-

Renal Agenesis

site kidney is usually significantly hypertrophied.

It therefore stands to reason that in the vast

Renal Hypoplasia

majority of cases of renal agenesis, a ureteral

In renal hypoplasia, the kidney is grossly nor-

bud abnormality existed and could be due to

any of the following:

mal but small. Microscopically, the kidneys are