Skeletal Muscle

A. Molecular events

1. Mesodermal (mesenchymal) cells within somites become committed to a muscle-forming cell line (through a poorly understood mechanism) to form myogenic cells.

2. Stimulated by fibroblast growth factor (FGF) and transforming growth factor (TGF-P), myogenic cells enter the cell cycle (i.e., undergo mitosis).

3. Myogenic cells begin to express MyoD (a helix-loop-helix transcription factor), which removes the myogenic cells from the cell cycle (i.e., micosis stops) and switches on muscle-specific genes to form postmitotic myoblasts.

4. Myoblasts begin to synthesize actin and myosin; in addition, myoblasts fuse with each other to form multinucleated myotubes.

5. Myotubes synthesize actin, myosin, troponin, tropomyosin, and other muscle proteins; these proteins aggregate into myofibrils, at which stage the cells are called muscle fibers.

6. Because muscle fibers are postmitotic, further growth is accomplished by means of satellite cells, through a poorly understood mechanism.

B. Paraxial mesoderm is a thick plate of mesoderm on each side of the midline. This thick plate becomes organized into segments known as somitomeres, which form in a craniocaudal sequence.

1. Somitomeres 1-7 do not form somites but contribute mesoderm to the head and neck region (pharyngeal arches).

2. The remaining somitomeres further condense in a craniocaudal sequence to form 42-44 pairs of somites of the trunk region. The somites closest to the caudal end eventually disappear to give a final count of approximately 35 pairs of somites.

3. Somites further differentiate into the sclerotome (cartilage and bone component), myotome (muscle component), and dermatome (dermis of skin component).

C. Head and neck musculature (see Chapter 11) is derived from somitomeres 1-7 of the head and neck region, which participate in the formation of the pharyngeal arches.

1. Extraocular muscles are derived from somitomeres 1, 2, 3, and 5.

a. Somitomeres 1, 2, and 3 are called preotic myotomes.

b. Extraocular muscles are innervated by cranial nerve (CN) III, CN IV, and CN VI.

2. Tongue muscles are derived from occipital myotomes innervated by CN XII.

D. Trunk musculature (Figure 18-1) is derived from myotomes in the trunk region.

Each myotome partitions into a dorsal epimere and a ventral hypomere.

1- Epimeres develop into the intrinsic back muscles (e.g., erector spinae). They are innervated by the dorsal ramus of a spinal nerve.

2. Hypomeres develop into the prevertebral, intercostal, and abdominal muscles. They are innervated by the ventral ramus of a spinal nerve.

E. Limb musculature (see Figure 18-1) is derived from myotomes (somites) in the upper and lower limb bud regions. This mesoderm migrates into the limb bud and forms a posterior condensation and an anterior condensation.

1. The posterior condensation develops into the extensor and supinator musculature of the upper limb and the extensor and abductor musculature of the lower limb.

2. The anterior condensation develops into the flexor and pronator musculature of the upper limb and the flexor and adductor musculature of the lower limb.

II. SMOOTH MUSCLE of the gastrointestinal tract and the tunica media of blood vessels is derived from mesoderm.

III. CARDIAC MUSCLE is derived from mesoderm that surrounds the primitive heart tube and becomes the myocardium.

Epimere

Dorsal primary ramus

Hypomere

Limb Bud Somitomeres

Posterior condensation

Ventral primary ramus condensation

Figure 1.8-1.. Drawing of a transverse section through the thorax and limb bud, showing the muscles of the epimere, hypomere, and limb bud. The limb bud musculature develops from mesodenn of various myotomes. The epimeric muscles are innervated by dorsal primary rami, and the hypomeric and limb muscles are innervated by ventral primary rami of spinal nerves. (From Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 214.)

Posterior condensation condensation

Epimere

Dorsal primary ramus

Hypomere

Ventral primary ramus

Figure 1.8-1.. Drawing of a transverse section through the thorax and limb bud, showing the muscles of the epimere, hypomere, and limb bud. The limb bud musculature develops from mesodenn of various myotomes. The epimeric muscles are innervated by dorsal primary rami, and the hypomeric and limb muscles are innervated by ventral primary rami of spinal nerves. (From Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 214.)

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