Application of scanning electron microscopy to kidney development and nephron maturation.

The present study uses scanning electron microscopy to review mammalian kidney development using the dog model. Canine mesonephros and metanephros are examined, focusing on the structure of the nephron from each and the maturation of the different cell types found in the metanephric nephron. The mesonephros possesses numerous nephrons each with a glomerulus, proximal, distal, and collecting tubule which empty into the mesonephric (Wolffian) duct. The mesonephros does not have cortical and medullary regions. The metanephric nephron not only possesses a glomerulus and the same tubular segments as described for the mesonephric nephron (proximal, distal, and collecting tubules), but also has tubular segments that are found in the medulla, namely the loop of Henle and thick ascending limb. Mammalian metanephric glomerulogenesis and tubulogenesis follows an orderly centrifugal pattern (from inner to outer cortex). The glomerular visceral epithelium initially has a columnar shape which is modified into a cuboidal sheet possessing numerous slender foot processes. The endothelium begins as a double layer with minimal fenestrae. With maturation, a single cell layer is formed possessing many large fenestrae. The maturation of the proximal tubule is documented by changes in the apical microvilli, and the foldings of the lateral and basal cell surfaces. Initially, the collecting tubule is lined by a primordial cell which differentiates into both the principal and intercalated cells.