Pathology of fetal congenital nephrosis: immunohistochemical and ultrastructural studies.

The kidneys of four human fetuses aged 18 to 20 weeks of gestation with congenital nephrosis of the Finnish type (CNF) were studied by immunohistochemistry and electron microscopy to elucidate the pathogenesis of the disease. The immunohistochemical stainings for laminin, type IV collagen, fibronectin, brushborder antigens (BBA), Tamm-Horsfall protein (TH), and binding of wheat germ agglutinin (WGA) did not reveal changes as compared to the age-matched control kidneys. Proximal tubules of the CNF kidneys showed excessive accumulation of coarse granular alpha fetoprotein (AFP) and large absorption droplets with paracrystalline and membraneous structures were seen in electron microscopy. The dilated tubules were both of proximal and distal origin as judged from the BBA and TH stainings. It is suggested that the formation of the dilated tubules results from degeneration caused by obstruction and excessive protein loading of the tubular epithelium. Demonstration of the coarse granular AFP in the proximal tubular epithelium and lumina indicates heavy fetal proteinuria and serves as an additional diagnostic marker for prenatal CNF.