3-DIMENSIONAL ORGANIZATION AND SEGMENTAL ULTRASTRUCTURE OF RAT PROXIMAL TUBULES

Aims: To provide a 3-D description and basic morphometric data along t he course of subcapsular proximal tubules. Methods: Proximal convolute d (PCT) and straight (PST) tubules were analyzed in series of up to on e thousand 4-mu m sections of perfusion-fixed rat renal cortex. Nine p roximal tubules were traced through the sections by computer-assisted 3-D reconstruction. Selected sections were reembedded and resectioned for ultrastructural morphometry. Results: Subcapsular PCTs (n = 3) had a total length of 12.0 +/- 0.2 mm (SEM), formed tight clusters and co ntacted the renal surface from 5 to 10 times. In contrast, mid-cortica l tubules (total PT length 11.6 +/- 0.1 mm) and in particular deep cor tical tubules (total PT length 12.1 +/- 0.9 mm) extended laterally and intermingled considerably with neighboring tubules. Epithelial height gradually decreased from around 11 mu m in the PCT to 7.5 mu m in the end of the PST. Brush-border height was around 4 mu m in the first PT segment (S1), 2.3 mu m in the second segment (S2) and 4.5 mu m in the end of the proximal tubule. Tubular wall volume, excluding microvilli , decreased from around 1,450 mu m(3)/mu m tubule length in early PCT to 825 mu m(3)/mu m in terminal PST. Conclusions: The results provide, at precisely defined nephron levels, quantitative ultrastructural dat a relevant to transport physiology.