media reveals nuclei of smooth muscle cells dispersed between the elastic lamellae. The inset shows the intima at higher magnification. Note the nuclei of the endothelial cells (EnC) at the luminal surface. The remainder of the intima consists mostly of collagenous fibers (stained blue) with occasional elastic fibers (EF) identified by their darker coloration. The nuclei of the fibroblasts and occasional smooth muscle cells (SMC) appear randomly arranged.
KEY A, adventitia BV, blood vessel CF, collagenous fibers EF, elastic fibers En, endothelium EnC, endothelial cells I, intima LCT, loose connective tissue M, media SMC, smooth muscle cells
Muscular arteries have more smooth muscle and less elastin in the tunica media than do elastic arteries. Thus, as the arterial tree is traced fur-ther from the heart, the elastic tissue is considerably reduced and smooth muscle becomes the predominant component of the tunica media. The muscular arteries are characterized, however, by a refractile internal elastic membrane separating the tunica intima from the tunica media and, usually, by an external elastic membrane separating the tunica media from the tunica adventitia. Muscular arteries, or arteries of medium caliber, constitute the majority of the named arteries in the body. Veins usually accompany arteries as they travel in the loose connective tissue. The veins have the same three layers in their walls, but the tunica media is thinner than in the accompanying artery, and the tunica adventitia is the predominant layer in the wall. The veins usually have the same name as the artery they accompany.
Muscular artery and medium vein, monkey, H&E ×365.
In this photomicrograph, the lumen of the artery is at the left, the lumen of the vein is at the right. The arterial endothelium (AEn) is clearly seen on the corrugated surface of the tunica intima, whereas the venous endothelium (VEn) is somewhat harder to distinguish. The internal elastic membrane (IEM) is seen as a thin clear zone immediately beneath the endothelial layer, separating the tunica intima from the underlying smooth muscle (SM) of the tunica media (TM). It is evident here that the tunica media is almost twice as thick as the tunica adventitia (TA ).
Muscular artery, monkey, H&E ×545. elastic membrane (IEM), which rests directly on the most luminal layer of smooth muscle cells (SM) of the thick tunica media (TM). The thinner tu-
This is a higher-magnification micrograph of the portion of nica adventitia (TA ).
the figure above outlined by the rectangle turned 90°. At this magnification, it is evident that the flattened endothelial cells (EN) follow the contours of the refractile, corrugated internal
M USCU LAR ARTERIES AND MEDIUM VEINS KEY AEn, arterial endothelium C, collagen bundles EF, elastic fibers EN, endothelial cells IEM, internal elastic membrane N, nuclei SM, smooth muscle SSm, small smooth muscle TA' tunica adventitia of artery TA, tunica adventitia of accompanying vein TI, tunica intima TM, tunica media VEn, venous endothelium
Medium vein, monkey, H&E ×600.