Three-dimensional organization and segmental ultrastructure of rat proximal tubules.

Aims: To provide a 3-D description and basic morphometric data along the course of subcapsular proximal tubules.
Methods: Proximal convoluted (PCT) and straight (PST) tubules were analyzed in series of up to one thousand 4-micron sections of perfusion-fixed rat renal cortex. Nine proximal 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 contacted the renal surface from 5 to 10 times. In contrast, mid-cortical tubules (total PT length 11.6 +/- 0.1 mm) and in particular deep cortical tubules (total PT length 12.1 +/- 0.9 mm) extended laterally and intermingled considerably with neighboring tubules. Epithelial height gradually decreased from around 11 microns in the PCT to 7.5 microns in the end of the PST. Brush-border height was around 4 microns in the first PT segment (S1), 2.3 microns in the second segment (S2) and 4.5 microns in the end of the proximal tubule. Tubular wall volume, excluding microvilli, decreased from around 1,450 micron3/microns tubule length in early PCT to 825 micron3/microns in terminal PST.
Conclusions: The results provide, at precisely defined nephron levels, quantitative ultrastructural data relevant to transport physiology.