Chapter 63. Development of The Muscular System - Review of Medical Embryology Book

Introduction: at day 17, the 3 germ layers are seen (ectoderm above, entoderm below, and mesoderm between). At day 19, the lateral mesodermal plate cleaves and the intraembryonic coelom appears, and differentiation of a somite plate is seen on the side of the neural tube. Metamerization begins at days 20 to 21 with embryonic flexion. Segmentation proceeds caudally, resulting in 42 to 44 pairs of somites by the end of week 5. Each somite develops a myocele. The sclerotome forms on its ventromedial portion and migrates to the notochord where it gives rise to fibroblasts, chondroblasts, and osteoblasts according to location. The rest of the somite, its dorsolateral part, is called the dermomyotome, which forms a dermatome (spreads under the surface ectoderm to form the subcutaneous tissue) and a myotome which forms the skeletal muscles
Skeletal muscles are derived from mesenchymal myoblasts which originate in the myotome portion of the dermomyotome. Skeletal muscles may also arise from mesenchyme in the branchial arches and somatic mesoderm

THE MYOBLASTS elongate, combine to form parallel bundles, and fuse to form multinucleated cells. The central nuclei move to the periphery, and during fetal life myofibrils are seen in the cytoplasm. By month 3, cross-striations are also visible
THE MYOTOME: most myotome development occurs in the thoracic region by week 5. Each myotome divides into a small dorsal epaxial division (epimere) and a larger ventral hypaxial division (hypomere). Each spinal nerve also divides, sending branches to each division, a dorsal primary ramus to the epimere and a ventral primary ramus to the hypomere. Most myotomes migrate to form nonsegmented muscles; some remain segmentally arranged like the somites (e.g., the intercostals of the thorax)

Epaxial derivatives: these myoblasts form the extensor muscles of the neck, vertebral column, and lumbar region. Extensors from the caudal sacral and coccygeal myotomes degenerate and become the adult dorsal sacrococcygeal ligament
Hypaxial derivatives: myoblasts of cervical myotomes form the scalene, prevertebral, infrahyoid, and geniohyoid muscles. Thoracic myotomes become the lateral and ventral flexors of the vertebral column. Lumbar myotomes become the quadratus lumborum muscl The sacrococcygeal myotomes form the muscles of the pelvic diaphragm, anus, and sex organs

BRANCHIAL ARCH MUSCLES: myoblasts from the arches migrate to form the muscles of mastication, of facial expression, and muscles of the pharynx and larynx. They are innervated by branchial arch nerves V, VII, IX, and X, respectively
OCULAR MUSCLES are probably derived from mesenchymal cells around the prochordal plate which gives rise to 3 preoptic myotomes. Groups of myoblasts with cranial nerves III, IV, and VI form the extrinsic muscles of the eyeball
TONGUE MUSCLES: 4 occipital myotomes are seen first, but the first pair disappears. The last 3 pairs form the tongue muscles, innervated by cranial nerve XII
LIMB MUSCLES develop in situ from mesenchyme around the developing limb bones. The mesenchyme comes from the somatic layer of the lateral plate mesoderm

SMOOTH MUSCLE forms from splanchnic mesenchyme around the primitive gut and its derivatives. Elsewhere, it forms from local mesenchyme

Muscles of the iris (sphincter and dilator pupillae) and the myoepithelial cells of the breast and sweat glands come from mesenchymal cells of ectodermal origin

CARDIAC (HEART) MUSCLE: forms from splanchnic mesenchyme around the embryonic heart

Special bundles of muscle cells develop with few myofibrils. These atypical cells form the Purkinje fibers of the conduction system of the heart

Congenital malformations of muscle may be due to muscle failure to develop or a pathologic process affecting the muscle or nerve during embryonic development