SMOOTH MUSCLE

A. Histogenesis: Most smooth muscle cells differentiate from mesenchymal cells of mesodermal origin in the walls of developing hollow organs of cardiovascular, digestive, urinary, and reproductive systems. During differentiation, the cells elongate and accumulate myofilaments. Smooth muscles of the iris arise from ectoderm.

B. Smooth Muscle Cells: Mature smooth muscle fibers are spindle-shaped cells with a single central ovoid nucleus. The sarcoplasm at the nuclear poles contain abundant mitochondria, some RER, and a large Golgi complex. Each fiber produces its own basal lamina, consisting of proteoglycan-rich material and type III collagen fibers.

1. Myofilaments a. Thin filaments. The actin filaments of smooth muscle are like those of skeletal and cardiac muscle. They are always present in the cytoplasm and are anchored by alpha-actinin dense bodies associated with the plasma membrane. b. Thick filaments. The myosin filaments of smooth muscle are less stable than those in striated muscle cells; they are not always present in the cytoplasm but seem to form in response to a contractile stimulus. Unlike the thick filaments in striated muscle cells those in smooth muscle have heads along most of their length and bare areas at the ends of the filaments. c. Organization of the myofilaments. The filaments run mostly parallel to the long axis of smooth muscle fibers, but they overlap much more than those of striated muscle, accounting for the absence of cross striations. The greater overlap of thick and thin filaments results from the unique organization of the thick filaments and permits greater contraction. The ratio of thin to thick filaments in smooth muscle is about 12:1, and the arrangement of the filaments is less regular and crystalline than in striated muscle.

2. Sarcoplasmic reticulum.Smooth muscle cells contain a poorly organized sarcoplasmic reticulum that participates in the sequestration and release of Ca but does not divide the myofilaments into myofibrillar bundles. Abundant surface-associated membrane-limited ves icles termed caveolae appear to aid in Ca2+ uptake and release. The small size and slow contraction of these fibers make an elaborate stimulus-conducting system unnecessary; these fibers have no T tubules, dyads, or triads.

3. Types of smooth muscle fibers. Although smooth muscle cells exhibit similar morphology in histologic section, they can be classified according to developmental, biochemical, and functional differences.

a. Visceral smooth muscle derives from splanchnopleural mesenchyme and is found in the walls of the hollow thoracic, abdominal, and pelvic organs of the respiratory, digestive, urinary, and reprc;ductive systems. In addition to thick myosin and thin actin filaments, its sarcolemma-associated dense bodies are linked by desmin-containing intermediate filaments. Because of their poor nerve supply, the cells transmit contractile stimuli to one another through their abundant gap junctions, acting as a functional syncytium. Contraction is slow and in waves. Visceral smooth muscle is classed as unitary smooth muscle.

b. Vascular smooth muscle differentiates in situ from mesenchyme around developing blood vessels. Its cells have intermediate filaments containing vimentin as well as desmin. It has the same functional features as visceral smooth muscle and is also classed as unitary smooth muscle, although its waves of contraction are not sustained and are localized.