Figure 126

Schematic diagram of cellular interactions in the formation of an atherosclerotic plaque. Endothelial cells express cell adhesion molecules that initiate monocyte migration through the endothelium. Platelet-derived growth factor (PDGF) and other growth factors (blue arrow) released from endothelial cells stimulate migration of the smooth muscle cells from the tunica media to the tunica intima. In the tunica intima, smooth muscle cells produce large amounts of extracellular matrix (proteoglycans, collagen) that increase the thickness of the tunica intima. Foam cells derived from both macrophages and smooth muscle cells accumulate LDLs, which cross the endothelial barrier (yellow arrows), and are oxidized by free radicals produced by the endothelial cells.

teristic feature in the formation of atherosclerotic plaques.

The tunica media of elastic arteries consists of multiple layers of smooth muscle cells separated by elastic lamellae

The tunica media is the thickest of the three layers of elastic arteries and consists of

• Elastin in the form of fenestrated sheets or lamellae be: tween the muscle cell layers. These lamellae are arranged in concentric layers (Fig. 12.7a). 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.

• Smooth muscle cells 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 they may proliferate and migrate to the adjacent intima. This characteristic is important in normal repair of the vascular wall as well in pathologic processes similar to those occurring in atherosclerosis.

• Collagen fibers and ground substance (proteoglycans), which 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

• Collagen and elastic fibers in the form of 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, the principal cells of the tunica adventitia.

• Blood vessels (vasa vasorum) and nerves (nervi vascularis). Branches of the vasa vasorum partially enter the tunica media and provide nutrients to the outer portion of the arterial wall. The inner part of the wall is supplied by nutrients from the lumen of the vessel.

endothelium

tunica intima endothelium

-tunica media

-tunica adventitia a

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