Fenestrations in the internal elastic lamina at bifurcations of human cerebral arteries.

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Abstract
Measurements of fenestrations (or windows) in the internal elastic lamina at the bifurcation of human cerebral arteries, were obtained from photomicrographs (scanning electron microscope). Thirteen of 28 bifurcations revealed regions of enlarged fenestrations among the normal fenestrations in the vicinity of the apex. The mean diameter of the enlarged fenestrations (7.0 +/- 0.34 SEM micrometer) was significantly greater than the mean diameter (2.1 +/- 0.13 SEM micrometer) of the normal fenestrations. The number of fenestrations per sq mm was less (2606 +/- 284 SEM per sq mm) for the enlarged fenestrations than for the normal fenestrations (4518 +/- 397 SEM per sq mm). The proportion of the area of internal elastic lamina comprised of fenestrations increased to 15.0 +/- 1.1 SEM percent for the enlarged fenestrations from a mean of 1.8 +/- 0.16 SEM percent for the normal fenestrations. Fenestrations from bifurcations without enlarged fenestrations, demonstrated characteristics similar to the normal fenestrations. More than 80% of the specimens exhibited a gap in the internal elastic lamina in the apical region of the bifurcation. Based on a comparison of stress concentration factors, we propose that the presence of enlarged fenestrations represents a weakness in the internal elastic lamina at the bifurcation apex which may contribute to the initiation of microaneurysms.

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