The layered fabric of cerebral artery fenestrations.

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Abstract
Intravascular bridges, resulting from developmental anomalies of brain arteries, are now better known as arterial fenestrations. Their tendency to develop aneurysms, similar to arterial bifurcations, makes their anatomy and microstructure important for study. Six segments of artery, each including a fenestration (five from the vertebrobasilar junction and one from the middle cerebral artery), were pressure distended, fixed, and sectioned. We made three-dimensional orientation measurements of smooth muscle and collagen, stained to enhance their birefringence, using the polarized light microscope. The general contour of the fenestrations is streamlined with a thickened layered subendothelium at the trailing or distal edge, structurally similar to the region of convergence of major brain arteries. Defects of the medial layer were found at both proximal and distal edges of all the fenestrations. Results included regional mean orientations of individual layers, with circular SDs. The medial layer was found to be coherently aligned perpendicular to the direction of blood flow, with a mean circular SD of 12 degrees. The adventitia was less coherent (mean circular SD, 16 degrees) with the same average orientation, and the multilayered subendothelium had layers of obliquely oriented fibers with a wide range of coherence for individual fiber groups. Layers of the side regions were analogous to those in segments of brain artery and differed significantly from the proximal and distal edges of the fenestration structure. The plasticity of form of the fenestrations at both the proximal and distal edges is in response to hemodynamic forces and is analogous to branching regions of brain arteries. Medial defects, a common feature in both brain arteries and fenestrations, may predispose the arterial fenestration to aneurysm formation.