New smooth muscle cells have been shown to differentiate from undifferentiated mesenchymal stem cells in the adventitia of blood vessels. Differentiation of smooth muscle progenitor cells is regulated by a variety of intracellular and environmental stimuli, and developing muscles exhibit a wide range of different phenotypes at different stages of their development. To date, no transcription factors have been identified that are characteristic for the smooth muscle cell lineage. However, serum response factor (RF), a member of the MADS–box transcription factor family, has been shown to regulate most smooth muscle differentiation marker genes. Smooth muscle cells have also been shown to develop from the division and differentiation of endothelial cells and pericytes during the repair process after vascular injury.
Vascular pericytes are located within the basal lamina of capillaries and postcapillary venules. They function as multi-potential mesenchymal progenitor cells. In capillaries, their cytoplasmic morphology is difficult to distinguish from that of the endothelial cell. In postcapillary venules and pericytic venules, they may form a nearly complete investment of the vessel with cells that resemble smooth muscle cells (see Chapter 13, Cardiovascular System).
Fibroblasts in healing wounds may develop morphologic and functional characteristics of smooth muscle cells (myofibroblasts; see page 179). Epithelial cells in numerous locations, particularly sweat glands, mammary glands, salivary glands, and the iris of the eye, may acquire the characteristics of smooth muscle cells (myoepithelial cells). Myoid cells of the testis have a contractile function in the seminiferous tubules, and cells of the perineurium, a concentric layer of connective tissue that surrounds groups of nerve fibers and partitions peripheral nerves into distinct fascicles, function as contractile cells as well as transport barrier cells.
This page intentionally left blank.
340 Muscle tissue is classified on the basis of the appearance of its contractile cells. Two major types are recognized: striated muscle, in which the cells exhibit a cross-striation pattern when observed at the light microscope level; and smooth muscle, in which the cells lack striations. Striated muscle is further subclassified based on location namely skeletal muscle, visceral striated muscle, and cardiac muscle. Skeletal mus-cle is attached to bone and is responsible for movement of the axial and apendicular skeleton, and for maintenance of body position and pos-ture. Visceral striated muscle is morphologically identical, but is restricted to soft tissues including the tongue, pharynx, upper part of the esophagus, and the diaphragm. Cardiac muscle is a type of striated muscle found in the heart and the base of the large veins that empty into the heart. The cross-striations in striated muscle are due to the organization of the contractile elements that occur in the muscle cell, namely thin fila-ments composed largely of the protein actin and thick filaments composed of the protein myosin II. The two types of myofilaments occupy bulk of the cytoplasm. The skeletal and visceral striated muscle cells, more commonly called fibers, are a multinucleated syncytium formed during development by the fusion of individual small muscle cells called myoblasts. Surrounding each fiber is a delicate mesh of collagen fibrils referred to as endomysium. In turn, bundles of muscle fibers that form func-tional units within a muscle are surrounded by a thicker connective tissue layer. This connective tissue is referred to as perimysium. Lastly, a sheath of dense connective tissue that surrounds the muscle, is referred to as epimysium. The force generated by individual muscle fibers is transferred to the collagenous elements of each of these connective tissue elements to end in a tendon.
Skeletal muscle, human, H&E, ×33. but are not individually discernable. However, the small blue dot-like structures are nuclei of the fibers. Between the fascicles, though difficult to see at