Embryologic Correlations

CASE 1: Multicystic Renal Dysplasia

Clinical History:

A 24-year-old primigravida is now in the 18th week of her pregnancy and has felt minimal fetal movement. An ultrasound shows a markedly decreased amniotic fluid volume, and the male fetus is, therefore, poorly visualized. The fetal kidneys appear enlarged and cystic, though one kidney appears larger than the other, and the renal pelves are not enlarged. The bladder appears quite small. The pregnancy continues until 34 weeks, when a preterm delivery results from preterm labor. The baby survives less than 5 minutes.

  1. What is the embryologic abnormality?

  2. The ureteric bud, an outgrowth of the mesonephric duct in the 5th week, penetrates the metanephric blastema in the location of what will become the retroperitoneum. The ureteric buds eventually give rise to the renal pelves, calyces, and collecting tubules. The nephrons with glomeruli, proximal convoluted tubules, loops of Henle, and distal convoluted tubules form from the metanephric blastema. When this process does not proceed properly, then a disorganized mass of renal tissue develops. There are glomeruli and renal tubules, but not in an organized cortex. Instead, there is a fibrous stroma in which prominent cysts are present. Even cartilage may be seen. The pediatric term "dysplasia" in this setting refers to disordered organ development. This condition is called multicystic renal dysplasia.

  3. What is a karyotype likely to show?

  4. 46, XY. This condition is typically not associated with chromosomal abnormalities.

  5. How do you explain the decreased amniotic fluid? What is this called?

  6. Amniotic fluid is mainly composed of fetal urine produced by the kidneys and passed through the urinary tract into the amniotic cavity that surrounds the fetus. Fetal urine production begins after the 10th week, but increases as the kidneys enlarge. Kidneys that are absent (agenesis) or malformed (dysgenesis) will not make urine. An obstruction in the urinary tract (such as posterior urethral valves from the persistence of an embryologic remnant of the prostatic urethra) will also lead to decreased amniotic fluid, known as oligohydramnios. The oligohydramnios leads to deformation of the fetus, often called the "Potter sequence" (after Edith Potter, who described these findings), with findings such as contractures of joints, club feet (talipes equinovarus, or varus deformity), infraorbital creases, and flattened nose. All of these findings in the sequence result from the constriction of the developing fetus with no room in the amniotic cavity.

  7. What other cystic disease of the kidneys can be congenital?

  8. Autosomal recessive polycystic kidney disease (ARPKD) is another congenital condition. In this condition, the kidneys are completely replaced by small cysts and the kidneys are massively and symmetrically enlarged. The recurrence risk is 25%. ARPKD is much less common than multicystic renal dysplasia. Autosomal dominant polycystic kidney disease (ADPKD), though inherited, manifests with renal problems in adulthood.

  9. What is the recurrence risk in this case?

  10. Multicystic renal dysplasia is most often a sporadic condition. In some cases, it can be part of a multiple malformation syndrome (such as Mickel-Gruber syndrome) with a specific recurrence risk. Multicystic renal dysplasia can be unilateral or involve just part of a kidney. The remaining renal tissue will enlarge to the size of two normal kidneys. Renal agenesis may be unilateral. This may be the reason for apparent absence of one kidney on imaging studies performed on adults.

  11. Name the most common urinary tract anomaly.

  12. The most common anomaly is a partial or complete duplication of one or both ureters. During embryologic development, the ureteric bud may split, and each split bud then lead to development of part of the kidney. This anomaly is seen in about 1 in 200 adults. Most cases are incidental, but there is a potential for urinary stasis and increased risk for urinary tract infection.