2. Portions of successive myotomes commonly fuse to form a composite single muscle (the erector spinae and rectus abdominis muscles). The latter is formed by the fusion of the ventral portions of the last six or seven thoracic myotomes. Only a few muscles are derivatives of single myotomes (the intercostals and some deep, short vertebral column muscles).
3. A myotome, or branchial arch muscle primordium, may split longitudinally into two or more parts that become separate muscles (the sternohyoid and omohyoid muscles and the trapezius and sternocleidomastoid muscles).
4. The original myotome masses may split tangentially into two or more layers (the external and internal intercostal and abdominal oblique and transverse abdominal muscles).
5. A portion or all of a muscle segment may degenerate. The degenerated muscle leaves connective tissue that becomes a sheet known as an aponeurosis (the epicranial aponeurosis [galea aponeurotica], which connects the frontal and occipital portions of the occipitofrontalis muscle).
6. Finally, muscle primordia may migrate, wholly or in part, to regions more or less remote from their original site of formation. An example is the formation of certain muscles of the upper limb that arise from cervical myotomes. The serratus anterior muscle migrates to the thoracic region, to attach ultimately to the scapula and the upper eight or nine ribs, taking along its fifth, sixth, and seventh cervical spinal nerve innervation. The trapezius muscle, along with the upper five cervical spinal nerves, migrates to attach ultimately to the skull, the nuchal ligament, and the spinous processes of the seventh cervical to twelfth thoracic vertebrae. The migration of the latissimus dorsi muscle is even more extensive; it carries with it its seventh and eighth cervical spinal nerve innervation to attach ultimately to the humerus, the lower thoracic and lumbar vertebrae, the last three or four ribs, and the iliac crest of the pelvis.
As these migrating upper limb muscles acquire their attachments to the trunk, they are all superficial to the underlying muscles of the body wall. The muscles of facial expression are also good examples of muscle migration. They arise from the mesenchyme of the second or hyoid branchial arch of the future neck and migrate with their facial (VII) nerve innervation to their final positions around the mouth, nose, and eyes.
A wide range of normal variations in skeletal muscle morphology result from one or more of the six fundamental processes going awry. Usually, the variations do not interfere with an individual’s normal functional ability, except when a greater part or all of a muscle is absent due to an initial failure to form, or when the usual amount of degeneration of a muscle segment is excessive. Some unusual muscle variations can be explained as genetic atavisms or muscles that were typical in one of the human’s vertebrate ancestors.
Skeletal muscle can undergo limited regeneration. When damaged, macrophages enter the necrotic area and remove the dead material. The damaged muscle fibers on each side of the necrotic area, which are actually open-ended syncytial tubes, form growth buds on their ends that grow toward each other, meet, and fuse. This reestablishes muscle fiber continuity across the damaged area and may be sufficient for the repair of a small muscle injury. When there is more extensive damage, the repair process is similar to the embryologic process of muscle fiber formation. Undifferentiated mononucleated cells normally present within the damaged muscle (named satellite cells) become myoblasts that divide and then fuse together to become new multinucleated syncytial myotubes go on to differentiate into typical muscle fibers. Even so, when large areas are damaged, the muscle regeneration may be so limited that the missing muscle is replaced chiefly with connective tissue.
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