Figure 127

Photomicrographs of the wall of elastic and muscular types of arteries. a. This photomicrograph shows a cross section of the human aorta stained with resorcin-fuchsin to demonstrate elastic material. Three layers can be recognized: the tunica intima, tunica media, and tunica adventitia. The tunica intima consists of a lining of endothelial cells that rest on a thin layer of connective tissue containing smooth muscle cells, occasional macrophages, and collagen and elastic fibers. The boundary between it and the tissue below, the tunica media, is not sharply defined. The tunica media contains an abundance of smooth muscle cells (note the blue staining nuclei) and numerous elastic fenestrated membranes (the red, wavy lamellae). The tunica adventitia, the outermost part, lacks elastic laminae, consists mainly of connective tissue, and contains the blood vessels and nerves that supply the aortic wall. x300. b. Photomicrograph of a cross section endothelium

tunica intima

-tunica media i f internal elastic membrane ^

tunica adventitia ^

V b through a muscular artery in a routine H9E preparation shows that the wall of the muscular artery is also divided into the same three layers as in the elastic artery. The tunica intima consists of an endothelial lining, a small amount of connective tissue, and the internal elastic membrane. This structure has a scalloped appearance when the vessel is constricted and is highly retractile. Constriction also causes the endothelial cell nuclei to appear rounded. The tunica media consists mainly of circularly arranged smooth muscle cells and collagen and elastic fibers. The nuclei of the smooth muscle cells, when contracted, have a corkscrew appearance. The tunica adventitia consists mostly of connective tissue. A well-defined external elastic membrane is not apparent in this vessel, but profiles of elastic material (arrows) are present. x360.

Muscular Arteries

Muscular arteries have more smooth muscle and less elastin in the tunica media than do elastic arteries

There is no sharp dividing line between elastic arteries and large muscular arteries (Fig. 12.8). Some of these arteries are difficult to classify because they have features that are intermediate between the two types. Generally, in the region of transition, the amount of elastic material decreases, and smooth muscle cells become the predominant constituent of the tunica media. Also, a prominent internal elastic membrane becomes apparent, helping to distinguish muscular arteries from elastic arteries. In many instances, a recognizable external elastic membrane is also evident.

The tunica intima is thinner in muscular arteries and contains a prominent internal elastic membrane

The tunica intima is relatively thinner in muscular arteries than in elastic arteries and consists of an endothelial lining with its basal lamina, a sparse subendothelial layer of connective tissue, and a prominent internal elastic membrane. In some muscular arteries, the subendothelial layer is so scanty that the basal lamina of the endothelium appears to make contact with the internal elastic membrane.

In histologic sections, the internal elastic membrane usually appears as a well-defined, undulating or wavy structure because of contraction of the smooth muscle (Fig. 12.7b).

The thickness of the tunica intima varies with age and other factors. In young children, it is very thin. In muscular arteries of young adults, the tunica intima accounts for about one sixth of the total wall thickness. In older adults, the tunica intima may be expanded by lipid deposits, often in the form of irregular "fatty streaks."

The tunica media of muscular arteries is composed almost entirely of smooth muscle, with little elastic material

The tunica media of muscular arteries consists of smooth muscle cells amid collagen fibers and relatively little elastic material. The smooth muscle cells are arranged in a spiral fashion in the arterial wall. Their contraction helps maintain blood pressure. As in elastic arteries, there are no fibroblasts in this layer. The smooth muscle cells possess an external (basal) lamina except at the sites of gap junctions and produce extracellular collagen, elastin, and ground substance.

The tunica adventitia of muscular arteries is relatively thick and is often separated from the tunica media by a recognizable external elastic membrane

The tunica adventitia of muscular arteries consists of fibroblasts, collagen fibers, elastic fibers, and in some vessels scattered adipose cells. Compared with elastic arteries, the tunica adventitia of muscular arteries is relatively thick, about the same thickness as the tunica media. Collagen fibers are the principal extracellular component. However, a concentration of elastic material immediately adjacent to the tunica media is often present and as such constitutes the external elastic membrane. Nerves and small vessels travel in the adventitia and give off branches that penetrate into the tunica media in the large muscular arteries as the vasa vasorum.

Small Arteries and Arterioles

Small arteries and arterioles are distinguished from one another by the number of smooth muscle cell layers in the tunica media

By definition, arterioles have only one or two layers of smooth muscle in their tunica media (Fig. 12.9); a small artery may have up to about eight layers. Typically, the tunica intima of a small artery has an internal elastic membrane, whereas this layer may or may not be present in the arteriole. The endothelium in both is essentially similar to endothelium in other arteries, except that at the EM level, gap junctions may be found between endothelial cells and the smooth muscle cells of the tunica media. Lastly, the tunica adventitia is a thin, ill-defined sheath of connective tissue that blends with the connective tissue in which these vessels travel.

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