Development of the deﬁnitive adult kidney (metanephros) begins when the two ureteric buds invade the paired masses of metanephric mesenchyme (see Plate 2-2). Through a process known as branching morphogenesis, which depends on reciprocal signals between each ureteric bud and its associated mass of metanephric mesen- chyme, the ureteric buds give rise to the ureters, renal pelves, calices, and collecting ducts, whereas the meta- nephric mesenchyme gives rise to nephrons.
Throughout this process, the two kidneys undergo separate but simultaneous development. As they undergo structural maturation, they also ascend in position (see Plate 2-5) from the sacral end of the fetus to the lumbar retroperitoneum.
Renal fusion can occur secondary to abnormalities in renal ascent, as in crossed renal ectopia (see Plate 2-6), or vice versa. In the former case, the superior pole of the crossed kidney ends up situated near the inferior pole of the normally positioned kidney, leading to fusion. In the latter case, a primary fusion event occurs, which then results in ectopia.
Horseshoe kidney, the most common type of renal fusion, occurs when an isthmus consisting of either ﬁbrous tissue or functioning renal parenchyma connects the two kidneys in the midline. The overall incidence of this abnormality is estimated to about 1 : 600, with males affected twice as often as females. The horseshoe kidney is especially common in patients with chromosomal disorders, such as trisomy 18 and Turner syndrome.
The horseshoe kidney is thought to result from fusion of the two metanephroi during the sixth week of development, while they are still near one another at the sacral end of the fetus. It is believed that abnormal lateral ﬂexion of the embryo may dislocate one kidney more medially, approximating it near the contralateral kidney and causing a fusion event. Ascent of the fused horseshoe kidney is prematurely terminated when the isthmus reaches the level of the inferior mesenteric artery, beyond which it is unable to cross.
The horseshoe kidney is typically situated in the lower lumbar region, below the normal position of the mature kidneys. The isthmus almost always connects the lower poles of the two fused kidneys, although in rare cases it may join the upper poles instead. The isthmus is usually situated anterior to the aorta and the inferior vena cava but may rarely be situated between these vessels or posterior to them both. Both renal pelves are usually oriented ventrally or ventromedially, secondary to a failure of rotation. The ureters insert normally into the bladder but are prone to reﬂux. In about 10% of patients, ureteral duplication is seen (see Plate 2-23). The renal vasculature is variable. The upper poles of each kidney are usually perfused by one or more ipsilateral branches of the aorta, whereas the lower poles and isthmus may receive their own branches from the aorta, iliac, or sacral arteries.
A horseshoe kidney rarely causes symptoms and is typically an incidental ﬁnding. A minority of patients, however, develop ureteropelvic junction obstructions, nephrolithiasis, or urinary tract infections. These com plications may result from the abnormally high ureteropelvic junction or kinking of the ureters as they cross over the fused isthmus. In addition, some patients may experience traumatic injury to the isthmus due to its midline position anterior to the spine. A smaller subset of patients may present during childhood with Wilms tumor, as horseshoe kidney increases the risk.
A small subset of patients with horseshoe kidney have concomitant abnormalities in other organ systems. Associated genital abnormalities include hypospadias and undescended testes in males, or vaginal septation and bicornuate uterus in females. Other associated abnormalities include neural tube defects and cardiac ventriculoseptal defects.
The “lump” or “cake” kidney is a renal fusion variant in which there is complete merging of the two kidneys, such that two separate masses are no longer distinguishable. This anomaly reﬂects a very early, complete fusion of the metanephroi. The symptoms, risks, and treatment o tions are largely the same as for horseshoe kidney.