Pericytes, also called adventitial cells or perivascular cells, are found around capillaries and venules (Fig. 6.24). Several observations support the interpretation that vascular pericytes are indeed mesenchymal stem cells. Experimental studies show that in response to external stimuli, pericytes express a cohort of proteins similar to those of stem cells in the bone marrow. Pericytes are surrounded by basal lamina material that is continuous with the basal lamina of the capillary endothelium; thus, they are not truly located in the connective tissue compartment. The pericyte is typically wrapped, at least partially, around the capillary, and its nucleus takes on a shape similar to that of endothelial cells (i.e., flattened but curved to conform to the tubular shape of the vessel).
TEM studies demonstrate that pericytes surrounding the smallest venules have cytoplasmic characteristics almost identical with those of the endothelial cells of the same vessel. Peri cytes associated with larger venules have characteristics of smooth muscle cells of the tunica media of small veins. In fortuitous sections cut parallel to the long axis of venules, the distal portion and proximal portion of the same pericyte exhibit characteristics of endothelial cells and smooth muscle cells, respectively. These studies suggest that during the development of new vessels, cells with characteristics of pericytes may differentiate into smooth muscle of the vessel wall. The role of pericytes as mesenchymal stem cells was confirmed experimentally in studies in which cultured pericytes from retinal capillaries were able to differentiate into a variety of cells, including osteoblasts, adipocytes, chondrocytes, and fibroblasts.
FIGURE 6.24 • Electron micrograph of a small blood vessel.
The nucleus at the upper left belongs to the endothelial cell that forms the wall of the vessel. At the right is another cell, a pericyte, that is in intimate relation to the endothelium. Note that the basal lamina (BL) covering the endothelial cell divides (arrows) to surround the pericyte. 11,000.
The fibroblasts and blood vessels within healing wounds develop from mesenchymal stem cells associated with the tunica adventitia of venules.
Autoradiographic studies of wound healing using parabiotic (crossed-circulation) pairs of animals have established that mesenchymal stem cells located in the tunica adventitia of venules and small veins are the primary source of new cells in healing wounds. In addition, fibroblasts, pericytes, and endothelial cells in portions of the connective tissue adjacent to the wound divide and give rise to additional cells that form new connective tissue and blood vessels.
Lymphocytes, Plasma Cells, and Other Cells of the Immune System
Lymphocytes are principally involved in immune responses.
Connective tissue lymphocytes are the smallest of the wandering cells in the connective tissue (see Fig. 6.23b). They have a thin rim of cytoplasm surrounding a deeply staining, heterochromatic nucleus. Often, the cytoplasm of connective tissue lymphocytes may not be visible. Normally, small numbers of lymphocytes are found in the connective tissue throughout the body. The number increases dramatically, however, at sites of tissue infammation caused by pathogenic agents. Lymphocytes are most numerous in the lamina propria of the respiratory and gastrointestinal tracts, where they are involved in immunosurveillance against pathogens and foreign substances that enter the body by crossing the epithelial lining of these systems.
Lymphocytes are a heterogeneous population of at least three major functional cell types: T cells, B cells, and natural killer (NK) cells.