Which endoneurial microvessel histologic measurements are least influenced by vasomotor tone?

The thickness and area of rat sciatic endoneurial microvessel components (endothelial cells, basement membranes and wall) were assessed to test whether collapse or vasoconstriction was typical of immersion (I) as compared to perfusion (P) or in situ (S) fixation and to determine whether area, but not thickness, would be unaffected by collapse or vasoconstriction. In S and in P fixation there was no apparent collapse or vasoconstriction; in fact, none of the measurements were significantly different between themselves except for index of circularity (IC), which was a little lower in S than in P fixation. By contrast to S or P, in I fixation, the lumen was much smaller and more irregular and the wall was thicker. Area of wall (and of its components) was not significantly different among I, P or S fixation, but thickness was significantly greater in I than in P or S fixation. For comparison to such properties of vessels as permeability, thickness may be the appropriate measurement, but for other purposes, such as determining the amount of basement membrane in diabetes mellitus, area may be a better measurement as it is less affected by collapse or vasoconstriction.