Surface areas of basolateral membranes in renal distal tubules estimated by vertical sections.

The surface area of the Na,K-ATPase-rich basolateral membranes in the medullary thick ascending limb of the distal tubule in the rabbit kidney was determined stereologically using the method of 'vertical sections', whereby unbiased surface estimates are obtained by imposing a cycloid test-lattice on micrographs of ultrathin sections cut roughly longitudinal to the tubular axis. The unbiased estimate of the surface area of basolateral membranes per tubule length in the thick ascending limb was 1.45 x 10(6) microns 2/mm. The results are compared with previous surface area measurements in this segment of the tubule and discussed with respect to the contributions from all sampling levels to the real biological variation. An optimized sampling scheme with a roughly fourfold reduction in workload is suggested.