Development of muscles

The source of the entire skeletal, striated muscle of the body in humans, as well as in animals, is the middle embryonic leaf - the mesoderm. However, the development of muscles within the trunk, head and limbs has a number of characteristics, which are easier to understand, following the initial stages of embryogenesis. Musculature of the trunk develops mainly from the dorsal, para-axial (near-axis) section of the mesoderm, which forms the primary segments of the body - the somite. Somites are located on the sides of the axial organs of the embryo - the neural tube and spinal cord. At 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 tail. Further, each somite is divided into 3 parts: sclerotome, dermatome and myotome; from the latter develop the muscles of the trunk.

Initially, the myotome occupies the dorsomedial section of the somite and has a cavity (miocel). Growing, much loses the character of multilayered formation and turns into a syncytial mass, its cavity disappears. In the process of further development, the cell mass is differentiated into striated fibers. As a result, the entire mass of the myotome is divided into sections of a cylindrical shape, consisting of muscle fibers, which still retain the metameric position. Myotomies proliferate in the dorsal and ventral directions. From the dorsal parts of the myotomes further deep, so-called own back muscles develop. From the ventral parts of the myotomes deep chest muscles and muscles of the anterior and lateral walls of the abdomen occur. Deep muscles of the back, chest and abdominal muscles, which are laid and remain all over the trunk, are called autochthonous (own) muscles (from the Greek dutos - himself, the same, chton - earth, autochtonos - native, local).

Very early, at the stage of separation of somites into parts, the myotomes get a connection with the nervous system. Each neuron corresponds to a certain region of the neural tube, from which nerve fibers of the future spinal nerves approach it. In this case, the dorsal muscles get innervation from the dorsal branches of the spinal nerves, whereas the ventral muscles are innervated by the ventral branches of these nerves. It is important to note that each nerve follows the muscle during its movements and changes in ontogeny. Therefore, the level of separation of the nerve to this muscle can indicate the place of its laying. An example is the diaphragm, which develops from the cervical myotomes and is innervated by the diaphragmatic nerve, which is the branch of the cervical plexus. The diaphragm develops from the 4th-5th cervical myotome and then descends to the lower aperture of the thorax. In the process of forming some muscles, a partial replacement of muscle fibers with connective tissue occurs, resulting in aponeurosis of muscles (for example, oblique muscles, transverse abdominal muscle, etc.).

The head muscles (mimic, chewing) and some neck muscles develop due to the ventral, non-segmented mesoderm at the head end of the embryo body, at the location of the visceral musculature. Chewing muscles and some neck muscles (for example, the maxillo-hyoid muscle, etc.) are formed as a result of the transformation of the bookmark of the first visceral arch. These muscles are attached to the bones of the facial (visceral) skull, where the head end of the digestive tube is located. Mimic muscles develop from the general bookmark of the musculature of the second visceral arch. On the basis of the musculature of the gill arches, the trapezoidal and sternocleidomastoid muscles develop. Some muscles of the perineum also belong to the visceral musculature (for example, the muscle lifting the anus).

In the region of the head there are also such muscles that develop from the myotomes of the head somites. These include muscles that provide movement of the eyeball (innervated by III, IV, VI cranial nerves). From the shifted occipital myotms, the muscles of the tongue are formed innervated by the hyoid nerve.

Complex musculature, which connects the limbs with the trunk, undergoes complex developmental processes. Here there are muscles laid in the mesenchymal bud of the limb, which then by their proximal ends "move" on the trunk and attach to its bones. These are the so-called trunk-pectoral muscles (from the Latin truncus - trunk, petere - to guide, start, heading towards the trunk). The truncate muscles include the large and small pectoral muscles, the broadest muscle of the back. On the lower limb there is one trunk-crooked muscle - a large lumbar. Some other muscles that develop from the ventral parts of the trunk myotomes and on the basis of the gill musculature, with their distal ends pass from the trunk and skull to the limbs and attach to its bones. These muscles are called trunkfugal (from Latin truncus - trunk, fugere - to run, escaping from the trunk). The trunk-focal muscles are trapezoidal, sternocleid-mastoid, large and small rhomboid, anterior dentate, scapular-hyoid, subclavian muscles, and also a muscle that lifts the scapula. Those muscles that are embedded within the limbs of the limbs from the mesenchyme and remain within the limbs are called autochthonous (indigenous) muscles of the limbs.

Variants and anomalies of muscles

Variants and anomalies in the development of skeletal muscles in the form of changes in their position, size and shape are more common on both sides of the body at the same time. Some muscles may be absent (for example, large and small round muscles). Some muscles have new heads or bundles of fibers (beak-brachial, brachial muscle) or missing one head (biceps brachialis muscle). The division of one muscle into several independent muscles (the flexor of the fingers) is described. More often variants and abnormalities of muscles are found on the upper extremities, especially in groups of more differentiated muscles (on the forearm and hand).

[1], [2], [3], [4]