IN VIVO MICROSCOPIC STUDIES
The permanent mammalian kidney is derived from nephrogenic cord mesoderm. The nephron arises from the metanephric mesoderm, and the collecting ducts, calyces and renal pelvis arise from the ureteric bud that branches off the mesenephric duct.
The microscopy of the developing kidney has been described in detail (Larsen, 1975;Rhodin, 1967). Branches of the ureteric bud grow out into the metanephric mesenchyme, and the nephron develops in the metanephric tissue. The nephron progresses through four developmental stages. As the kidney develops, the earliest stages of nephron development are close to the kidney surface, and the older stages are next to the developing medulla. Rhodin (1967) described the four stages of kidney development in the mouse kidney and Larsen (1975) described the four stages in the rat kidney. In the newborn rat kidney, all four stages were distinguishable. Ellipsoid-shaped stage one vesicles with elongated lumens and prominent basement membranes were located near the kidney surface. Stage two S-shaped bodies were located near the kidney surface. The S-shaped bodies contained the proximal tubule anlages and glomerular anlages. Capillaries and developing collecting tubules were located near the S-shaped bodies. In stage three nephrons, capillary loops were observed within the developing glomeruli. Stage three developing podocytes had a few foot processes. The stage three developing proximal tubule cells had no brush borders; the short loops of Henle were lined by simple cuboidal cells. In stage four nephrons, the glomeruli were larger and contained capillary loops. The parietal layer of Bowman's capsule was lined by simple squamous cells. The proximal tubule cells had brush borders, and the loops of Henle were longer and were lined by simple squamous cells. Dodge (unpublished) studied the microscopy of the developing hamster kidney. Fetal 10-day, 11-day, 12day, 13-day, 14-day, 15-day, and neonatal kidneys were viewed at the light microscopic level. The four stages of nephron development in the kidney are illustrated in Figures 1 and2. In the 13-day fetal kidney, branches of the ureteric bud are observed, and condensation of mesenchyme has begun in the cortical area (Fig. 1). All four stages of nephron development can be seen in the 14-day fetal kidney (Fig. 2). Dorup and Maunsbach (1982) described the development of distal tubules and cortical collecting tubules in the human kidney. In stage four nephrons, the proximal tubules were structurally differentiated. There were 4-5 cell long tubules that followed the proximal tubules. The cells in these tubules still had a few brush borders and lateral interdigitations. In the developing