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the tissue constituting the bulk of the mass of muscles and accounting for the contractile function of muscles. The following types are distinguished: striated (skeletal and heart), smooth, and doubly oblique striated.
Almost all skeletal muscle tissue in vertebrates develops from myotomes, the paired metameric rudiments of the body musculature. Myoblasts, which are mononuclear cells, divide rapidly in ontogenesis and upon fusing together give rise to bundles, which are then transformed into differentiated muscle fibers, the main structural element of striated muscle. The striation of the fibers is due to the alternation of numerous contractile strands known as myofibrils, which are structures differing in physico-chemical and optical properties.
Smooth muscle tissues of the skin, of walls of organs along the gastrointestinal and urogenital tracts, and of walls of blood vessels develop from mesenchymal cells. The smooth muscle cells of salivary, sweat, and mammary glands derive from the ectoderm. All smooth muscle cells are mononuclear and spindle-shaped.
Muscle tissue with doubly oblique striation is comparatively rare. It is found in some worms and in the obturator muscles of bivalve mollusks. Striation appears at an acute angle (10° to 15°) to the longitudinal axis of the muscle fiber where the dark and light disks of adjacent myofibrils coincide with some regular displacement. This muscle tissue is characterized by slow contraction and the ability to remain contracted over many hours with only a slight loss of energy.