An understanding of normal cerebral artery architecture is essential to the understanding of aneurysmal formation and progression. Cerebral arteries are composed of three primary layers: the inner tunica intima, tunica media (muscularis), and outer tunica externa (adventitia) (Fig. 1). The collagenous intima is covered by a layer of endothelial cells. The media is comprised of elastic fibers and smooth muscle cells that secrete many of the growth factors and cytokines essential for vascular remodeling. Separating the intima and media is the internal elastic lamina (IEL), which is essentially a longitudinal arrangement of fibers composed of an elastin core. This core is composed of tropoelastin molecules cross-linked by lysyl oxidase. The adventitia is composed of fibroblasts and collagen. Contrary to prevailing misconceptions, intracranial arteries have vasa vasorum (10). Unlike most systemic arteries, however, intracranial arteries lack an external elastic lamina, which might make them vulnerable to hemodynamic stress and aneurysmal formation (11). In normal cerebral arteries, these layers are generally intact. However, it is common for gaps to exist in the medial layer of intracranial arteries. Described by Forbus in 1930, these "medial defects of Forbus" are found most commonly at the apex of a bifurcation, but they also frequently exist at the lateral angles of arterial bifurcations (12). These medial defects were once believed to be congenital defects and were considered the locus minoris resistentiae (place of least resistance) of the arterial bifurcation. As we will discuss, however, our understanding of these defects has changed.

The extracellular matrix of cerebral arteries is composed of a collagen and elastin scaffold embedded in a collection of glycoproteins and proteoglycans (13). The tensile strength of

Figure 1 The upper panel shows the histology of a typical cerebral artery. The area within the box is shown at greater magnification in the bottom panel. All 3 layers of the vessel are seen, including the intima, media, and adventitia. The IEL is also clearly seen in this magnification. Abbreviation: IEL, internal elastic lamina.

a vessel is governed primarily by its elastin and collagen content (14). Elastin fibers are critical for vessel wall tension at low systolic pressures. As the elastic fibers become taut at higher pressures, the load is transferred to collagen fibers. The collagen fibers mediate resistance to vessel wall deformation at physiologic pressures (14). Collagen is also critical for vessel wall integrity, because vessels treated with collagenase are prone to rupture (15). The extracellular collagen network is composed primarily of Type I and Type III collagen. After production of procollagen by smooth muscles and fibroblasts, posttranslational processing leads to the formation of triple helix strands of collagen alpha chains. These chains provide the framework for vessel strength during exposure to high intraluminal pressure (16).

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