 Elastin in the form of fenestrated sheets or lamellae between the muscle cell layers. These lamellae are arranged in concentric layers (Figs. 13.17a, 13.14, and Plate 33, page 434). As noted, fenestrations in the lamellae facilitate the diffusion of substances within the arterial wall. The number and thickness of these lamellae are related to blood pressure and age. At birth, the aorta is almost devoid of lamellae; in the adult, the aorta has 40 to 70 lamellae. In individuals with hypertension, both the number and the thickness of the lamellae are increased.
FIGURE 13.14 • Diagram and photomicrograph of an elastic artery. a. This schematic diagram of a typical elastic artery shows its cellular and extracellular components. Note the organization of smooth muscle cells in the tunica media and the distribution of elastic lamellae. The internal elastic membrane is not well defined and is represented by the innermost elastic lamellae of the arterial wall.
b. This low magnification photomicrograph shows the section of the wall of the human aorta stained with Weigert’s resorcin-fuchsin elastic stain to visualize elastic lamellae interspersed with the smooth muscle cells of the tunica media. Only the tunica media, which is the thickest of the three layers of the elastic arteries, is labeled on this image. Note that elastic lamellae, collagen fibrils, and blood vessels are present in the tunica adventitia. 48.
 Smooth muscle cells are arranged in layers. The smooth muscle cells are arranged in a low-pitch spiral relative to the long axis of the vessel; thus, in cross sections of the artery they appear in a circular array. The smooth muscle cells are spindle-shaped with an elongated nucleus. They are invested with an external (basal) lamina except where they are joined by gap junctions. Fibroblasts are not present in the tunica media. Smooth muscle cells synthesize the collagen, elastin, and other molecules of the extracellular matrix. In addition, in response to growth factors (i.e., PDGF, FGF) produced by endothelial cells, smooth muscle cells may proliferate and migrate to the adjacent intima. This characteristic is important in normal repair of the vascular wall and in pathologic processes similar to those occurring in atherosclerosis.
 Collagen fibers and ground substance (proteoglycans) are synthesized and secreted by the smooth muscle cells.
The tunica adventitia in the elastic artery is a relatively thin connective tissue layer.
In elastic arteries, the tunica adventitia is usually less than half the thickness of the tunica media. It consists of the following.
 Collagen fibers and elastic fibers form a loose network of elastic fibers (not lamellae) that are less organized than those in the tunica media. The collagen fibers help prevent the expansion of the arterial wall beyond physiologic limits during systole of the cardiac cycle.
 Fibroblasts and macrophages are the principal cells of the tunica adventitia.
 The vasa vasorum (blood vessels) and nervi vascularis (nerves) include arterial branches and veins similar to that of vasculature in general and provide nutrients and remove waste products from the outer portion of the arterial wall. They may partially enter the outer part of the tunica media. The inner part of the wall is supplied by nutrients from the lumen of the vessel. The hemodynamic impact (i.e., increased blood pressure) on the function of the vasa vasorum may play a role in the pathogenesis of atheromatous plaques.
Muscular arteries have more smooth muscle and less elastin in the tunica media than do elastic arteries.