Development of the proximal tubule of the fetal rat kidney: morphometry and ultrastructural findings.

The development of the kidney in fetal rats was investigated morphometrically and ultrastructurally. Both the length and volume of the proximal tubule increased with age. The total volume of functional glomeruli increased markedly from fetal day 20 onward. The ratio of total glomerular volume to total proximal tubular volume decreased with age and was less than 1 from fetal day 20 onward. The cells of the proximal tubule had numerous microvilli on their free surfaces and vacuoles in the apical region of the cytoplasm after fetal day 20. These findings indicate that the relative growth of the proximal tubule has exceeded that of the glomeruli by fetal day 20, and that increased filtration induces both the growth and the differentiation of the proximal tubule during development of the fetal rat kidney.