Muscular System

With the exception of some smooth muscle tissue (see below), the muscular system develops from the mesodermal germ layer and consists of skeletal, smooth, and cardiac muscle. Skeletal muscle is derived from paraxial mesoderm, which forms somites from the occipital to the sacral regions and somitomeres in the head. Smooth muscle differentiates from splanchnic mesoderm surrounding the gut and its derivatives and from ectoderm (pupillary, mammary gland, and sweat gland muscles). Cardiac muscle is derived from splanchnic mesoderm surrounding the heart tube.

Striated Skeletal Musculature

Somites and somitomeres form the musculature of the axial skeleton, body wall, limbs, and head. From the occipital region caudally, somites form and differentiate into the sclerotome, dermatome, and two muscle-forming regions (Fig. 9.1). One of these is in the dorsolateral region of the somite. It expresses the muscle-specific gene MYO-D and migrates to provide progenitor cells for limb and body wall (hypomeric) musculature (Figs. 9.1 and 9.2). The other region lies dorsomedially, migrates ventral to cells that form the dermatome, and forms the myotome. This region, which expresses the muscle-specific gene MYF5, forms epimeric musculature (Figs. 9.1 and 9.2). During differentiation, precursor cells, the myoblasts, fuse and form long, multinucleated

Dermatome Sclerotome Myotome

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Figure 9.1 Stages in the development of a somite. A. Mesoderm cells are arranged around a small cavity. B. Cells of the ventral and medial walls of the somite lose their epithelial arrangement and migrate in the direction of the notochord. These cells collectively constitute the sclerotome. Cells at the dorsolateral portion of the somite migrate as precursors to limb and body wall musculature. Dorsomedial cells migrate beneath the remaining dorsal epithelium of the somite to form the myotome. C. Cells forming the myotome continue to extend beneath the dorsal epithelium. D. After ventral extension of the myotome, dermatome cells lose their epithelial configuration and spread out under the overlying ectoderm to form dermis.

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Figure 9.1 Stages in the development of a somite. A. Mesoderm cells are arranged around a small cavity. B. Cells of the ventral and medial walls of the somite lose their epithelial arrangement and migrate in the direction of the notochord. These cells collectively constitute the sclerotome. Cells at the dorsolateral portion of the somite migrate as precursors to limb and body wall musculature. Dorsomedial cells migrate beneath the remaining dorsal epithelium of the somite to form the myotome. C. Cells forming the myotome continue to extend beneath the dorsal epithelium. D. After ventral extension of the myotome, dermatome cells lose their epithelial configuration and spread out under the overlying ectoderm to form dermis.

Figure 9.2 Expression patterns of genes that regulate somite differentiation. Sonic hedgehog (SHH), secreted by the notochord and floor plate of the neural tube, causes the ventral part of the somite to form sclerotome and to express PAX1, which in turn controls chondrogenesis and vertebral formation. WNT proteins from the dorsal neural tube activate PAX3, which demarcates the dermomyotome. WNT proteins also direct the dorsomedial portion of the somite to form epaxial (back) muscles and to express the muscle-specific gene MYF5. The middorsal portion of the somite is directed to become dermis by neurotrophin 3 (NT-3) expressed by the dorsal neural tube. Hypaxial (limb and body wall) musculature is derived from the dorsolateral portion of the somite under the combined influence of activating WNT proteins and inhibitory BMP-4 protein, which together activate MYO-D expression.

muscle fibers. Myofibrils soon appear in the cytoplasm, and by the end of the third month, cross-striations typical of skeletal muscle appear. A similar process occurs in the seven somitomeres in the head region rostral to the occipital somites. Somitomeres remain loosely organized structures, however, never segregating into sclerotome and dermomyotome segments.

Molecular Regulation of Muscle Development

Genes regulating muscle development have recently been identified. BMP4 and probably FGFs from lateral plate mesoderm, together with WNT proteins from adjacent ectoderm, signal the dorsolateral cells of the somite to express the muscle-specific gene MYO-D. BMP4 secreted by overlying ectoderm induces production of WNT proteins by the dorsal neural tube, and these proteins cause dorsomedial cells of the somite to activate MYF5, another muscle-specific gene (Fig. 9.2). Both of these genes are members of the MYO-D muscle-specific family, which also includes the myogenin and MRF4 genes. MYO-D and MYF5 proteins activate the genes for myogenin and MRF5, which in turn promote formation of myotubes and myofibers. All MYO-D family members have DNA binding sites and act as transcription factors to regulate downstream genes in the muscle differentiation pathway.

Patterning of Muscles

Patterns of muscle formation are controlled by connective tissue into which myoblasts migrate. In the head region these connective tissues are derived from neural crest cells; in cervical and occipital regions they differentiate from somitic mesoderm; and in the body wall and limbs they originate from somatic mesoderm.

Derivatives of Precursor Muscle Cells

By the end of the fifth week prospective muscle cells are collected into two parts: a small dorsal portion, the epimere, formed from the dorsomedial cells of the somite that reorganized as myotomes; and a larger ventral part, the hypomere, formed by migration of dorsolateral cells of the somite (Figs. 9.1 B and 9.3 A). Nerves innervating segmental muscles are also divided into a dorsal primary ramus for the epimere and a ventral primary ramus for the hypomere (Fig. 9.3B) and these nerves will remain with their original muscle segment throughout its migration.

Myoblasts of the epimeres form the extensor muscles of the vertebral column, and those of the hypomeres give rise to muscles of the limbs and body wall (Fig. 9.3B). Myoblasts from cervical hypomeres form the scalene, geniohyoid, and prevertebral muscles. Those from thoracic segments split into three layers, which in the thorax are represented by the external intercostal, internal intercostal, and innermost intercostal or transversus thoracis muscle (Fig. 9.3B). In the abdominal wall these three muscle layers consist of the external oblique, the internal oblique, and the transversus abdominis muscles. The ribs cause the muscles in the wall of the thorax to maintain their segmental character, whereas muscles in the various segments of the abdominal wall fuse to form large sheets of muscle tissue. Myoblasts from the hypoblast of lumbar segments form the quadratus lumborum muscle, and those from sacral and coccygeal regions form the pelvic diaphragm and striated muscles of the anus.

In addition to the three ventrolateral muscle layers, a ventral longitudinal column arises at the ventral tip of the hypomeres (Fig. 9.3B). This column

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Responses

  • leilani
    What causes differentiation of myotome, dermatome, and sclerotome?
    8 years ago

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