Figure 22–21 Diagram of a small artery in cross section. the endothelial cells form the endothelial lining, which although inconspicuous, is the fundamental component. Compare with the capillary in Figure 22–22.
Figure 22–22 Capillaries. electron micrograph (left) of a cross section of a small capillary in the pancreas. the wall is formed by a single endothelial cell surrounded by a basal lamina, as seen most clearly in the drawing to the right. (From R.p. bolender, J. Cell Biol. 61:269– 287, 1974. with permission from the Rockefeller university press.)
Less obvious than the blood vessels are the lymphatic vessels. These carry no blood and have much thinner and more permeable walls than the blood vessels. They provide a drainage system for the fluid (lymph) that seeps out of the blood vessels, as well as an exit route for white blood cells that have migrated from blood vessels into the tissues. Less happily, they can also provide the path by which cancer cells escape from a primary tumor to invade other tissues. The lymphatics form a branching system of tributaries, all ultimately discharging into a single large lymphatic vessel, the thoracic duct, which opens into a large vein close to the heart. Like blood vessels, lymphatics are lined with endothelial cells.
Thus, endothelial cells line the entire blood and lymphatic vascular system, from the heart to the smallest capillary, and they control the passage of materials—and the transit of white blood cells—into and out of the bloodstream. Arteries, veins, capillaries, and lymphatics all develop from small vessels constructed primarily of endothelial cells and a basal lamina: connective tissue and smooth muscle are added later where required, under the influence of signals from the endothelial cells.
To understand how the vascular system comes into being and how it adapts to the changing needs of tissues, we have to understand endothelial cells. How do they become so widely distributed, and how do they form channels that connect in just the right way for blood to circulate through the tissues and for lymph to drain back to the bloodstream?
Endothelial cells originate at specific sites in the early embryo from precursors that also give rise to blood cells. From these sites, the early embryonic endothelial cells migrate, proliferate, and differentiate to form the first rudiments of blood vessels—a process called vasculogenesis. Subsequent growth and branching of the vessels throughout the body occurs mainly by proliferation and movement of the endothelial cells of these first vessels, in a process called angiogenesis.
Angiogenesis occurs in a broadly similar way in the young organism as it grows and in the adult during tissue repair and remodeling. We can watch the behavior of the cells in naturally transparent structures, such as the cornea of the eye or the fin of a tadpole, or in tissue culture, or in the embryo. The embryonic retina, which blood vessels invade according to a predictable timetable, provides a convenient example for experimental study. Each new vessel originates as a capillary sprout from the side of an existing capillary or small venule (Figure 22–24). At the tip of the sprout, leading the way, is an endothelial cell with a distinctive character. This tip cell has a pattern of gene expression somewhat different from that of the endothelial stalk cells following behind it, and while they divide, it does not. The tip cell’s most striking feature is that it puts out many long filopodia, resembling