Capillaries form blood vascular networks that allow fluids containing gases, metabolites, and waste products to move through their thin walls. The human body contains approximately 50,000 miles of capillaries. Each consists of a single layer of endothelial cells and their basal lamina. The endothelial cells form a tube just large enough to allow the passage of red blood cells one at a time. In many capillaries the lumen is so narrow that the red cells literally fold on themselves to pass through the vessel (Fig. 13.20). The passing red blood cells fill virtually the entire capillary lumen, minimizing the diffusion path for gases and nutrients between the capillary and the extravascular tissue. In cross sections and with the TEM, the tube appears to be formed by only one cell or portions of several cells. Because of their thin walls and close physical association with metabolically active cells and tissues, capillaries are particularly well suited for the exchange of gases and metabolites between cells and the bloodstream. The ratios of capillary volume to endothelial surface area and thickness also favor movement of substances across the vessel wall.
Classification of Capillaries
Capillary structure varies in different tissues and organs. On the basis of their morphology, three types of capillaries are described: continuous capillaries, fenestrated capillaries, and discontinuous capillaries.
Continuous capillaries are typically found in muscle, lung, and the CNS. With the TEM, they appear in cross sections as two plasma membranes enclosing a ribbon of cytoplasm that may include the nucleus (Fig. 13.21). Occluding junctions can be seen in the typical cross section of a continuous capillary. They only allow the passage of relatively small molecules (less than 10,000 daltons) between adjacent endothelial cells.
FIGURE 13.20 • Photomicrograph of the capillary network in the retina. This is a whole-mount preparation of retinal capillaries. After mild enzymatic digestion, retina was spread on a glass slide, stained with the periodic acid–Schiff (PAS) procedure, and counterstained with hematoxylin. Vertically crossing the image is an arterial (A) with a clearly visible layer of circularly arranged smooth muscle cells (SM). The arterial is crossed perpendicularly by a venule (V). Note the extensive network of capillaries connecting both vessels. Nuclei of endothelial cells (E) are clearly visible within capillaries. At this magnification, pericytes are difficult to discern. 560. (Courtesy Mr. Denifield W. Player.)
Numerous pinocytotic vesicles underlie both the luminal and basal plasma membrane surfaces. The vesicles are approximately 70 nm in diameter and function in the transport of larger molecules between the lumen and the connective tissue and vice versa.
In some continuous capillaries and postcapillary venules, pericytes (historically known as Rouget cells) may be associated with the endothelium (see Figs. 13.20 and 13.21). The pericyte, when present, intimately surrounds the capillary, with branching cytoplasmic processes, and is enclosed by a basal lamina that is continuous with that of the endothelium. Pericytes are contractile and are controlled by NO produced by endothelial cells. They provide vascular support and promote stability of capillaries and postcapillary venules through physical and chemical signaling with vascular endothelial cells. Histologically, pericytes display features of undifferentiated mesenchymal stem cells with large nuclei rich in heterochromatin. During embryonic development or angiogenesis (i.e., wound healing), pericytes give rise to both endothelial cells and smooth muscle cells. Pericytes are directly involved in the pathogenesis of vascular driven diseases (i.e., diabetic retinopathy and tumor angiogenesis). In addition, uncontrolled divisions of pericytes give rise to the hemangiopericytoma, a rare vascular tumor that can originate in the body anywhere there are capillaries.