mesenchyme to an epithelium for nephron formation is also mediated by the ureteric buds, in part through modification of the extracellular matrix. Thus fi-bronectin, collagen I, and collagen III are replaced with laminin and type IV collagen, characteristic of an epithelial basal lamina (Fig. 14.7B). In addition, the cell adhesion molecules syndecan and E-cadherin, which are essential for condensation of the mesenchyme into an epithelium, are synthesized. Regulatory genes for conversion of the mesenchyme into an epithelium appear to involve PAX2 and WNT4 (Fig. 14.7B).
Wilms' tumor is a cancer of the kidneys that usually affects children by 5 years of age but may also occur in the fetus. Wilms' tumor is due to mutations in the WT1 gene on 11p13, and it may be associated with other abnormalities and syndromes. For example, WAGR syndrome is characterized by aniridia, hemi-hypertrophy, and Wilms' tumor. Similarly, Denys-Drash syndrome consists of renal failure, pseudohermaphrodism, and Wilms' tumor.
Renal dysplasias and agenesis are a spectrum of severe malformations that represent the primary diseases requiring dialysis and transplantation in the first years of life. Multicystic dysplastic kidney is one example of this group of abnormalities in which numerous ducts are surrounded by undiffer-entiated cells. Nephrons fail to develop and the ureteric bud fails to branch, so that the collecting ducts never form. In some cases these defects cause involution of the kidneys and renal agenesis. Renal agenesis may also occur if the ureteric bud fails to contact and/or induce the metanephric mesoderm. Bilateral renal agenesis, which occurs in 1 /10,000 births, results in renal failure. The baby presents with Potter sequence, characterized by anuria, oligohydram-nios (decreased volume of amniotic fluid), and hypoplastic lungs secondary to the oligohydramnios. In 85% of cases other severe defects, including absence or abnormalities of the vagina and uterus, vas deferens, and seminal vesicles, accompany this condition. Common associated defects in other systems include cardiac anomalies, tracheal and duodenal atresias, cleft lip and palate, and brain abnormalities.
In congenital polycystic kidney (Fig. 14.8) numerous cysts form. It may be inherited as an autosomal recessive or autosomal dominant disorder or may be caused by other factors. Autosomal recessive polycystic kidney disease, which occurs in 1/5,000 births, is a progressive disorder in which cysts form from collecting ducts. The kidneys become very large, and renal failure occurs in infancy or childhood. In autosomal dominant polycystic kidney disease, cysts form from all segments of the nephron and usually do not cause renal failure until adulthood. The autosomal dominant disease is more common (1/500 to 1/1,000 births) but less progressive than the autosomal recessive disease.
Figure 14.8 A. Surface view of a fetal kidney with multiple cysts (arrowheads) characteristic of polycystic kidney disease. B. Section of the kidney in A, showing multiple cysts.
Duplication of the ureter results from early splitting of the ureteric bud (Fig. 14.9). Splitting may be partial or complete, and metanephric tissue may be divided into two parts, each with its own renal pelvis and ureter. More frequently, however, the two parts have a number of lobes in common as a result of intermingling of collecting tubules. In rare cases one ureter opens into the bladder, and the other is ectopic, entering the vagina, urethra, or vestibule (Fig. 14.9C). This abnormality results from development of two ureteric buds. One of the buds usually has a normal position, while the abnormal bud moves down together with the mesonephric duct. Thus it has a low, abnormal entrance in the bladder, urethra, vagina, or epididymal region.
Figure 14.9 A and B. A complete and a partial double ureter. C. Possible sites of ectopic ureteral openings in the vagina, urethra, and vestibule. D and E. Photomicrographs of complete and partial duplications of the ureters (U). Arrows, duplicated hilum; B, bladder; K, kidneys; ML, median umbilical ligament.
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