Figure 1210

Photomicrographs of an atherosclerotic lesion, a. This specimen is from a human aorta stained by the Masson trichrome method. The lesion, referred to as a fibrous plaque, consists of connective tissue fibers, smooth muscle cells, fat-containing macrophages (foam cells), and a necrotic material. It occupies the site of the tunica intima (Tl), which is greatly expanded in thickness. TM, tunica media; TA, tunica adventitia. x40. b. A higher power of the area in the rectangle in a. On the right, some of the fibrous connective tis-

ticularly LDL (Fig. 12.10). Progression of the lesion is marked by accumulation of lipid and loss of integrity of the endothelium. In advanced lesions, blood stasis and clotting (thrombosis) may lead to occlusion of the vessel. Other changes seen in advanced lesions include thinning of the tunica media, calcifications, and a necrotic mass within the lesion. Progression from simple to complicated lesions can be found in some people in their 20s and in most individuals by age 50 or 60 years.

sue of the plaque is evident. The arrows point to smooth muscle cell nuclei that have produced the collagen fibers of the fibrous plaque. Also evident are the foam cells (FC) and the characteristic cholesterol clefts (CC). The latter are spaces occupied previously by cholesterol crystals that have been dissolved during specimen preparation. The remainder of the plaque consists of necrotic material and lipid. x240.

v capillaries

Capillaries are the smallest diameter blood vessels, often smaller than the diameter of an erythrocyte

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 upon themselves to pass through the vessel. 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. Based on their morphology, three types of capillaries are

pinoçytotic

' \;ocir-loc cell junction /m basal lamina '

pericyte

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