constituent of the tunica media (Fig. 13.18 and Plate 34, page 436). 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. 13.17b).
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.
FIGURE 13.18 • Diagram and photomicrograph of a muscular artery. a. In this schematic diagram of a muscular artery, the cellular and extracellular components are labeled. Note the distribution of cellular components in all three tunics and the locations of external and internal elastic membranes. b. In this photomicrograph of a cross section through a muscular artery in Weigert’s resorcin-fuchsin elastic stain preparation, note two distinct layers of elastic tissue: a wavy-appearing inner layer of the internal elastic membrane and a well-defined outer layer of the external elastic membrane. The relatively thick tunica media enclosed by the internal and external elastic membranes consists mainly of circularly arranged smooth muscle cells, collagen, and fine elastic fibers. The tunica intima in this preparation is indiscernible; the tunica adventitia is well defined, consisting mostly of connective tissue with collagen and elastic fibers. 175.
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 are distinguished from one another by the number of smooth muscle cell layers in the tunica media.