Fig. 1.

The grid used to estimate %EF is composed of an unbiased counting frame with inclusion (dashed) and exclusion (continuous) borders, and parallel lines 4 mm apart. The shape of the counting frame was chosen based upon the use of the same grid for slit diaphragm length density (not required for endothelial fenestration studies). There is 1 coarse line (where the intercepts with endothelial coverage are counted) per 7 fine lines. The short lines on the coarse lines are 4 mm apart and, similar to fine lines, are used to define fenestrated versusnon-fenestrated coverage. The endothelial coverage was arbitrarily called non-fenestrated if the distance between the two fenestrae on either side of the coarse line was more than 3 fine and coarse lines, otherwise it was called fenestrated.