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Chapter 7: Muscular System by John F. Neas Except for the muscles of the iris, the arrector pili muscles attached to hairs, and the myoepithelial cells of the lacrimal, salivary, sweat and mammary glands, all muscles of the body are derived from mesoderm. Cardiac and visceral smooth muscles develop from splanchnic mesoderm. Cardiac muscle develops form mesodermal cells that migrate to and envelop the primitive heart tubes. Smooth muscle develops from mesodermal cells that migrate to and envelop the developing gastrointestinal tract and viscera. The smooth muscle of blood vessels develops from local mesenchyme. All skeletal muscles develop from the mesoderm of somites, except for that of the head and extremities that develop from loosely organized mesenchyme. By 4 weeks of development, the mesoderm on either side of the notochord and developing nervous system forms dense columns that become segmented into a series of blocks of cells called somites. The first pair of somites appears on the twentieth day and 44 pairs of somites eventually form by the thirtieth day. At six weeks, the cells of a somite are differentiated into a (1) myotome (paraxial mesoderm), the medial portion of the somite, that forms most of the skeletal muscles, (2) dermatome that forms the connective tissues, including the dermis, and (3) sclerotome that produces the vertebrae. Spinal nerves arise from the spinal cord and exit between vertebrae to innervate developing muscles in the adjacent myotomes. (The ventral mesoderm does not form segmental masses but remains as a sheet called the lateral plate. A cavity, or coelom, appears within the lateral plate of the chest and abdomen. Formation of the coelom divides the lateral plate into an inner visceral layer and an outer parietal layer.) The process of muscle fiber development occurs within the myotomes, in the embryonic trunk area, and from loosely organized masses of mesoderm in the head and appendage areas. Skeletal muscle tissue formation begins during the fourth week of embryonic development as specialized mesodermal cells, called myoblasts, begin rapid mitotic division. New cells continue to proliferate while the myoblast cells fuse into syncytial myotubes. (A syncytium is a multinucleated protoplasmic mass that forms by the secondary union of originally separate cells.) Thus, each muscle fiber develops from several embryonic myoblast cells. At nine weeks, primitive myofilaments course through the myotubes, and the nuclei of the myoblasts are centrally located. Growth in length continues by addition of myoblasts. It is unclear when skeletal muscle is sufficiently developed to sustain contractions. However, in the seventh week, fetal movements known as quickening are strong enough for the mother to recognize. At this time, the individual muscle fibers have thickened, the nuclei have moved peripherally, and the myofilaments have differentiated into alternating thin (actin) and thick (myosin) bands. Muscles continue to grow in length through addition of myoblasts. The formation of myoblasts ends and all of the muscle cells of a person are determined shortly before birth. Histology--Myogenesis The transverse section of skeletal muscle cut shows the extreme peripheral location of the nuclei of skeletal muscle fibers. Note the minute capillary (C) in the endomysial space. Note in the longitudinal section the regular cross striations that are a characteristic feature of skeletal muscle fibers. The muscles of the body axis include the (1) muscles of head and neck not associated with the spinal column, including those that move the face, tongue, and larynx; (2) muscles of the spine, including numerous flexors and extensors of the head, neck, and vertebral column; (3) oblique and rectus muscles that form the muscular walls of the thoracic and abdominopelvic cavities, including the diaphragm (see Chapter 24—Respiratory System); and (4) muscles of the pelvic floor that form the muscular perineum. By 6 weeks, the myotomal muscles organize into epaxial and hypaxial masses of muscle that respectively lie above and below the axis of the embryo, or notochord. The myotomes produce. The epaxial muscles from the cervical to lumbar myotomes remain arranged in segments and become the muscles of the dorsal neck and trunk. The deep epaxial muscles include the short intervertebral muscles (transversospinalis muscles). Superficial epaxial muscles form the superficial extensor muscles (iliocostocervicalis muscles) of the vertebral column major. Hypaxial mesoderm in the trunk grows around the body wall toward the sternum in company with the ribs, producing a mesodermal layer that extends from the chin to the pelvic girdle. The hypaxial musculature becomes the muscles of the ventral body wall that are essentially flexors of the vertebral column. (The musculature of the ventral body wall and limbs probably include a contribution from the unsegmented somatic mesoderm.) Hypaxial muscles vary at different segmental levels so that regional trunk muscles develop in the cervical, thoracic, lumbar, sacral, and coccygeal regions. The oblique, transverse, and rectus muscle groups develop from the hypaxial layer. The hypaxial mesoderm near the sacrum migrates caudally to generate the muscles of the pelvic floor. Specifically, the myotomes (and the muscles they probably produce) are as follows: preotic (extrinsic muscles of the eye), occipital (tongue muscles, sternomastoid and trapezius), cervical (prevertebral muscles of neck, scalenes, geniohyoid, infrahyoid muscles, portions of the diaphragm), thoracic (intercostals, transversus thoracis), lumbar (psoas, obliqui, transversus abdominis, quadratus lumborum, rectus abdominis), sacral and first coccygeal (coccygeus, levator ani, anal sphincters, genital muscles). The tongue muscles possibly arise in situ from mesenchyme. The sternomastoid and trapezius possibly develop in situ from pharyngeal arch mesoderm. There are very few somites in the head region of the embryo. Thus, most of the skeletal muscles there develop from the general mesoderm in the head region. Muscles of the pharyngeal arches include, in part, the muscles of mastication (mandibular arch, arch I), facial expression (hyoid arch, or arch II), palate and pharynx (arches III and IV), intrinsic laryngeal muscles, esophagus (arch VI), and possibly the trapezius and sternomastoid (post arch VI?). These muscles arise from either head mesoderm or, less likely, from splanchnic mesoderm. Pharyngeal myoblasts form a superficial layer that later subdivides to produce the trapezius and sternocleidomastoid muscles (the trapezius and sternomastoid may possibly develop from occipital myotomes). Perineal Muscles Muscles Mesoderm from the parietal portion of the somatic mesoderm (local body wall mesenchyme), and the adjacent myotome forms the limb buds by about the fourth week. The four limb buds each have a flattened distal tip with a thickened apical ectodermal ridge. The limb muscles develop either from the hypaxial component of the myotome (upper limb: C3 to T1; lower limb: L2 to S3) or from somatic mesoderm, or a combination of these. The muscles of the pectoral and pelvic girdles, however, are clearly of myotomic origin. Evidence suggests that the adjacent spinal cord stimulates growth of somitic muscle. As the myotomes develop, they elongate anteriorly toward the midline of the body or distally into the developing limbs. Migration of myoblasts over the dorsal surface of the trunk produces the extensor muscles of the limb; migration of ventral myoblasts produces the flexor muscles. Rotation of the arm and leg buds changes the position of these masses in relationship to the body axis. Additional myoblasts invade the limb from nearby myotomal segments as the limb buds enlarge. The muscles of the entire muscular system are differentiated and in correct position by the eighth week. The orientation of the developing skeletal muscles is preceded and influenced by cartilaginous models of bones in embryonic limb buds (origins of future extremities).
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