A stereological and electron microscopic study of the development of the nephron in prenatal and postnatal rats.

OBJECTIVE: To describe the morphometric and ultra-structural features of the kidney in fetal (20-day-old), newborn (7-day-old) and adult (180-day-old) rats.

MATERIALS AND METHODS: Kidneys from all animals were excised, fixed in 3% glutaraldehyde in 0.1 M phosphate buffer, post-fixed in 1% phosphate-buffered osmium tetroxide, and examined by stereological and light and electron microscopic methods.

RESULTS: Fetal kidneys displayed kidney corpuscles, glomeruli, and proximal and collective tubules at multiple developmental stages. Glomeruli in the outer surfaces of the kidney were less mature than those in the inner surface. Kidney corpuscles were made up of parietal cells and podocytes without feet. Kidneys from newborn rats were almost completely developed, while kidneys in adult rats were fully developed. Under stereological examination, the percent volume of cortex in fetal kidneys (86.19%) was higher than in newborn (53.77%) or adult rats (76.78%). Compared to both newborn and fetal rats, adult rats displayed the highest total volumes of distal and proximal tubules, but lower mean glomerular or Bowman's capsule volumes. On the other hand, the total number of glomeruli was increased in adult rats (32,600) as compared to newborn (17,896) and fetal (11,650) rats.

CONCLUSION: These data suggest that development of metanephric kidneys is not yet complete by gestational day 20, but is almost complete at postnatal week one. Furthermore, the developmental stage of the kidney, as determined by stereology, correlates well with the age of the rat.