Occipital Myotomes

Figure 9.3 A. Transverse section through the thoracic region of a 5-week embryo. The dorsal portion of the body wall musculature (epimere) and the ventral portion (hy-pomere) are innervated by a dorsal primary ramus and a ventral primary ramus, respectively. B. Similar to A later in development. The hypomere has formed three muscle layers and a ventral longitudinal muscle column.

is represented by the rectus abdominis muscle in the abdominal region and by the infrahyoid musculature in the cervical region. In the thorax, the longitudinal muscle normally disappears, but is occasionally represented by the sternalis muscle.

Head Musculature

All voluntary muscles of the head region are derived from paraxial mesoderm (somitomeres and somites), including musculature of the tongue, eye (except that of the iris, which is derived from optic cup ectoderm), and that associated with the pharyngeal (visceral) arches (Table 9.1). Patterns of muscle formation in the head are directed by connective tissue elements derived from neural crest cells.

Limb Musculature

The first indication of limb musculature is observed in the seventh week of development as a condensation of mesenchyme near the base of the limb buds (Fig. 9.4A). The mesenchyme is derived from dorsolateral cells of the table 9.1 Origins of the Craniofacial Muscles

Mesodermal Origin

Muscles

Innervation

Somitomeres 1,2 Somitomere 3 Somitomere 4 Somitomere 5 Somitomere 6 Somitomere 7 Somites 1,2 Somites 2-5a

Superior, medial, ventral recti Superior oblique Jaw closing Lateral rectus

Jaw opening, other 2nd arch Stylopharyngeus Intrinsic laryngeals Tongue

Oculomotor (III) Trochlear (IV) Trigeminal (V) Abducens (VI) Facial (VII)

Glossopharyngeal (IX) Vagus (X) Hypoglossal (XII)

aSomites 2-5 constitute the occipital group (somite 1 degenerates for the most part).

aSomites 2-5 constitute the occipital group (somite 1 degenerates for the most part).

Glossopharyngeal Somite

Figure 9.4 A. Myotomes in the head, neck, and thoracic region of a 7-week embryo. Note the location of the preotic and occipital myotomes and condensation of mesenchyme at the base of the limb bud, B. Transverse section through the region of attachment of the limb bud. Note the dorsal (extensor) and ventral (flexor) muscular components of the limb.

Figure 9.4 A. Myotomes in the head, neck, and thoracic region of a 7-week embryo. Note the location of the preotic and occipital myotomes and condensation of mesenchyme at the base of the limb bud, B. Transverse section through the region of attachment of the limb bud. Note the dorsal (extensor) and ventral (flexor) muscular components of the limb.

somites that migrate into the limb bud to form the muscles. As in other regions, connective tissue dictates the pattern of muscle formation, and this tissue is derived from somatic mesoderm, which also gives rise to the bones of the limb.

With elongation of the limb buds, the muscle tissue splits into flexor and extensor components (Fig. 9.4B). Although muscles of the limbs are segmental initially, with time they fuse and are then composed of tissue derived from several segments.

The upper limb buds lie opposite the lower five cervical and upper two thoracic segments (Fig. 9.5, A and B), and the lower limb buds lie opposite the lower four lumbar and upper two sacral segments (Fig. 9.5C). As soon as the buds form, ventral primary rami from the appropriate spinal nerves penetrate into the mesenchyme (Fig. 9.6). At first each ventral ramus enters with isolated dorsal and ventral branches, but soon these branches unite to form large dorsal

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