the culture is stained with a fluorescent antibody (green) against skeletal muscle myosin, which marks differentiated muscle cells, and with a dna-specific dye (blue) to show cell nuclei. (a) a short time after a change to a culture medium that favors differentiation, just two of the many myoblasts in the field of view have switched on myosin production and have fused to form a muscle cell with two nuclei (upper right). (b) Somewhat later, almost all the cells have differentiated and fused.
(C) high-magnification view, showing characteristic striations (fine transverse stripes) in two of the multinucleate muscle cells. (Courtesy of Jacqueline Gross and terence partridge.) damage to muscle fber
Figure 22–20 Satellite cells repair skeletal muscle fibers. (a) the specimen is stained with an antibody (red) against a muscle cadherin, m-cadherin, which is present on both the satellite cell and the muscle fiber and is concentrated at the site where their membranes are in contact. the nuclei of the muscle fiber are stained green, and the nucleus of the satellite cell is stained blue.
(b) Schematic of the repair of a damaged muscle fiber by proliferation and fusion of satellite cells. (a, courtesy of terence partridge.)
Skeletal muscle fibers are one of four main categories of vertebrate cells specialized for contraction, and they are responsible for all voluntary movement. Each skeletal muscle fiber is a syncytium and develops by the fusion of many myoblasts. Myoblasts proliferate extensively, but once they have fused, they can no longer divide. Fusion generally follows the onset of myoblast differentiation, in which many genes encoding muscle-specific proteins are switched on coordinately. Some myoblasts persist in a quiescent state as satellite cells in adult muscle; when a muscle is damaged, these cells are reactivated to proliferate and to fuse in order to replace the muscle cells that have been lost. They are the stem cells of skeletal muscle, and exhaustion of their regenerative capacity is responsible for some forms of muscular dystrophy as well as for the decline of muscle mass in old age.
blood veSSelS, lymphatiCS, and endothelial CellS
Almost all tissues depend on a blood supply, and the blood supply depends on endothelial cells, which form the linings of the blood vessels. Endothelial cells have a remarkable capacity to adjust their number and arrangement to suit local requirements. They create an adaptable life-support system, extending by cell migration into almost every region of the body. If it were not for endothelial cells extending and remodeling the network of blood vessels, tissue growth and repair would be impossible. Cancerous tissue is as dependent on a blood supply as is normal tissue, and this has led to a surge of interest in endothelial cell biology, in the hope that it may be possible to block the growth of tumors by attacking the endothelial cells that bring them nourishment.
The largest blood vessels are arteries and veins, which have a thick, tough wall of connective tissue and many layers of smooth muscle cells (Figure 22–21). The inner wall is lined by an exceedingly thin single sheet of endothelial cells, the endothelium, separated from the surrounding outer layers by a basal lamina. The amounts of connective tissue and smooth muscle in the vessel wall vary according to the vessel’s diameter and function, but the endothelial lining is always present. In the finest branches of the vascular tree—the capillaries and sinusoids—the walls consist of nothing but endothelial cells and a basal lamina (Figure 22–22), together with a few scattered pericytes. Related to vascular smooth muscle cells, pericytes wrap themselves around the small vessels and strengthen them (Figure 22–23).
basal lamina nucleus of endothelial cell basal lamina lumen of capillary tight junction 2 µm loose connective elastic lamina