Generally, in the region of transition between elastic arteries and large muscular arteries, the amount of elastic material decreases, and smooth muscle cells become the predominant endothelial cells cytoplasm of endothelial cells basal lamina junctional complex ba FIGURE 13.15 • Diagram and scanning electron micrograph of the endothelium. a. This schematic drawing shows the luminal surface of the endothelium. The cells are elongated with their long axis parallel to the direction of blood flow. Nuclei of endothelial cells are also elongated in the direction of blood flow. b. Scanning electron micrograph of a small vein, showing the cells of the endothelial lining. Note the spindle shape with the long axis of the cells running parallel to the vessel. 1,100.
FIGURE 13.16 • Diagram depicting segments of two adjacent endothelial cells. This diagram shows cell-to-cell and cellto-extracellular matrix junctions in the endothelial cells. Junctional complex near the luminal surface (dashed line box) comprises the zonula occludens and the zonula adherens, and a desmosome represents a group of cell-to-cell junctions that maintain selective permeability barrier. Cell-to-cell communication junctions are represented by an area of gap junctions (green). Anchoring junctions (cell-to-extracellular matrix) are represented here by hemidesmosomes and focal adhesions. Observe the organization of the cytoplasm and cytoplasmic inclusions, the Weibel-Palade bodies, that are characteristic of endothelial cells. Pinocytotic vesicles in the cell on the left have been positioned to suggest the pathway of vesicles from the lumen of the blood vessel to the basal cell membrane or to the lateral cell membrane as indicated by the dashed arrows. Various markers have been traced through pinocytotic pathways across the endothelial cell. Luminal surface of the endothelial cells expresses variety of endothelial cell-surface receptors.
FIGURE 13.17 • 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, the tunica media, and the 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. 300. b. Photomicrograph of a cross section through a muscular artery in a routine H&E 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 refractile. 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. 360.