Collagen Organization in the Branching Region of Human Brain Arteries

Abstract

Background and Purpose-Unruptured saccular aneurysms are relatively common, occurring in 4% to 9% of autopsies.Their development at the apex region of brain artery bifurcations is attributed to a combination of structural factors and the effect of blood pressure. Collagen is a primary tension-bearing fabric of the vessel wall, and our purpose was to examine its 3-dimensional alignment at arterial branches. Methods-Sixteen segments of arteries from the circle of Willis, including bifurcations, were pressure distended, fixed, and sectioned in 1 of 3 orthogonal planes. We measured the 3-dimensional organization of collagen at the flow divider by using the polarized light microscope. An electron microscopy study performed in tandem provided measurements on the collagen fibril diameters and packing density. Results-Orientation data of the collagen fabric were obtained from sections from 3 different cutting planes. The tunica media of all bifurcations had an alignment that was primarily circumferential, and the medial gap (medial defect) was distinguishable at the apex of all bifurcations. The subendothelial layer was thin at the apex but thicker and more disorganized distally. Adventitial collagen showed little organization except for a high degree of alignment along the apex. Results from the electron microscopy study showed densely packed collagen fibrils of uniform diameter at the apex, compared with slightly smaller and less densely packed fibrils nearby. Conclusions-In the region of the medial gap, a narrow band of highly aligned tendonlike collagen running in the direction of the ridge of the flow divider was a consistent finding. This structure would provide strength and stability to the vessel and is inconsistent with the concept of an inherent defect in the structure of bifurcations. (Stroke. 1998;29:1595-1601.)

Widening of the basilar bifurcation angle: association with presence of intracranial aneurysm, age, and female sex

“…Previous studies suggested that the bifurcation apex may be protected from hemodynamic stresses by a narrow band of densely packed collagen fibers that covers the bifurcation medial pad and provides strength and stiffness to the region. 2,5,21 It was shown using surgically created arterial bifurcations in animals that the acceleration region adjacent to the medial pad was characterized by disrupted internal elastic lamina and loss of endothelium, which is consistent with early-stage IAs. 17 Whereas our simulations showed that the acceleration area grew from 0.38 mm to almost 7 mm with a larger angle, the medial pad was reported to take values between 0.42 mm 2 and 1.40 mm 2 .…”

Computational method for excluding fibers under compression in modeling soft fibrous solids

“…Specifically, in human arterial walls the collagen fibers are not perfectly aligned but are dispersed around a mean direction. Such a fiber dispersion has been observed in, for example, human arterial walls [1][2][3][4][5], the myocardium [6,7], corneas [8,9] and articular cartilage [10]. In particular, recent extensive experimental results [4] have shown that the collagen fiber dispersion in each of the layers of (healthy) human thoracic and abdominal aortas and iliac arteries is non-symmetric, in contrast to the rotationally symmetric fiber dispersion assumed in previous studies; see, for example, [11].…”

The genetics of intracranial aneurysms

“…Through this, the inner layers of the arterial wall would bulge and then form an aneurysm. But it was demonstrated that these gaps are equally common in patients with and without aneurysms (Stehbens 1989) and usually packed by dense collagen fibrils (Fujimoto 1996;Finlay et al 1998). Furthermore, the defect in the muscle layer is located in the wall of the aneurysmal sac and not at the neck of the aneurysm (Stehbens 1989).…”