Muscle development

The source of origin of all skeletal, striated muscles of the body in humans, as in animals, is the middle germ layer - the mesoderm. However, the development of muscles within the trunk, head and limbs has a number of features that are easier to understand by tracing the initial stages of embryogenesis. The muscles of the trunk develop mainly from the dorsal, paraximal (near-axial) section of the mesoderm, which forms the primary segments of the body - the somites. The somites are located on the sides of the axial organs of the embryo - the neural tube and the dorsal cord. In the 4th week of development, there are about 40 pairs of somites: from 3 to 5 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4-5 caudal. Then each somite is subdivided into 3 parts: sclerotome, dermatome and myotome; the muscles of the trunk develop from the latter.

Initially, the myotome occupies the dorsomedial part of the somite and has a cavity (myocoel). As it grows, it loses much of its multilayered character and turns into a syncytial mass, its cavity disappearing. In the process of further development, the cellular mass differentiates into transversely striated contractile fibers. As a result, the entire mass of the myotome is divided into cylindrical sections consisting of muscle fibers that still retain a metameric position. Myotomes grow in the dorsal and ventral directions. From the dorsal parts of the myotomes, deep, so-called proper muscles of the back subsequently develop. From the ventral parts of the myotomes, deep muscles of the chest and the musculature of the anterior and lateral walls of the abdomen originate. The deep muscles of the back, chest and abdominal muscles, which are laid down and remain throughout the entire length within the body, are called autochthonous (own) muscles (from the Greek dutos - oneself, that very one; chton - earth, autochtonos - native, local).

Very early, at the stage of somite division into parts, myotomes receive a connection with the nervous system. Each myotome corresponds to a certain section of the neural tube - a neuromere, from which nerve fibers of the future spinal nerves approach it. In this case, the dorsal muscles receive innervation from the dorsal branches of the spinal nerves, while the ventral muscles are innervated by the ventral branches of these nerves. It is important to note that each nerve follows the muscle in the process of its movements and changes in ontogenesis. Therefore, the level of departure of the nerve to a given muscle can indicate the place of its formation. An example is the diaphragm, which develops from the cervical myotomes and is innervated by the phrenic nerve, which is a branch of the cervical plexus. The diaphragm develops from the 4th-5th cervical myotome and subsequently descends to the lower aperture of the chest. During the formation of some muscles, partial replacement of muscle fibers with connective tissue occurs, resulting in the formation of muscle aponeuroses (for example, oblique muscles, transverse abdominal muscle, etc.).

The muscles of the head (facial, chewing) and some muscles of the neck develop due to the ventral unsegmented section of the mesoderm in the head end of the embryo's body, in the location of the visceral muscles. The chewing muscles and some muscles of the neck (for example, the mylohyoid muscle, etc.) are formed as a result of the transformation of the anlage of the first visceral arch. These muscles are attached to the bones of the facial (visceral) skull, where the head end of the digestive tract is located. The facial muscles develop from the general anlage of the muscles of the second visceral arch. The trapezius and sternocleidomastoid muscles develop on the basis of the anlage of the muscles of the branchial arches. Some muscles of the perineum (for example, the muscle that lifts the anus) also belong to the visceral muscles.

In the head region there are also muscles that develop from the myotomes of the head somites. These include muscles that ensure the movement of the eyeball (innervated by the III, IV, VI cranial nerves). The muscles of the tongue, innervated by the hypoglossal nerve, are formed from the displaced occipital myotomes.

The muscles that connect the limbs to the body undergo complex development processes. There are muscles that are laid down in the mesenchymal rudiment of the limb, which then “move” to the body with their proximal ends and are attached to its bones. These are the so-called trunk-petal muscles (from the Latin truncus - body, petere - to direct, to start; directed to the body). The trunk-petal muscles include the pectoralis major and minor, the latissimus dorsi. There is one trunk-petal muscle on the lower limb - the lumbar major. Some other muscles, developing from the ventral sections of the trunk myotomes and on the basis of the gill muscles, move with their distal ends from the body and skull to the limbs and are attached to its bones. These muscles are called truncofugal (from the Latin truncus - trunk, fugere - to run; running away from the trunk). The truncofugal muscles include the trapezius, sternocleidomastoid, large and small rhomboids, anterior serratus, omohyoid, subclavian muscles, and also the muscle that lifts the scapula. Those muscles that are laid within the limb rudiments from the mesenchyme and remain within the limbs are called autochthonous (native) muscles of the limbs.

Muscle Variations and Anomalies

Variants and anomalies of skeletal muscle development in the form of changes in their position, size, and shape are more often found on both sides of the body simultaneously. Some muscles may be absent (for example, the large and small teres muscles). Some muscles develop new heads or fiber bundles (coracobrachialis, brachialis muscles) or one head is missing (biceps brachii). Divisions of one muscle into several independent muscles (digital flexor) have been described. Variants and anomalies of muscles are more often found on the upper limbs, especially in groups of more differentiated muscles (on the forearm and hand).

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