Embryogenesis and Cellular Differentiation
Skeletal striated muscle tissue originates from the mesoderm. The primary source is the paraxial mesoderm, which forms somites. Each somite contains a myotome, from which future skeletal muscles develop. Meanwhile, adjacent regions give rise to other tissues: the dermatome forms the dermis, and the sclerotome forms the axial skeleton. Part of the trunk and limb musculature develops from the anterolateral mesoderm.
At the cellular level, the process begins with myoblasts. These cells fuse to form multinucleated structures called myotubes. It is at this stage that myofibrils and the characteristic cross-striations appear within the primordium. Subsequently, the sarcolemma, T-tubule system, sarcoplasmic reticulum, and motor endplates form, ultimately transforming the myotube into a mature muscle fiber.
Migration of Primordia and Formation of Muscle Groups
Initially, muscle primordia appear as segmented myotome arrays. In the trunk and neck regions, they divide into dorsal (future extensors) and ventral (future flexors) groups. Similarly, distinct dorsal and ventral arrays segregate within the limb buds.
As the fetus grows, these muscle bundles actively migrate, fuse, and span across joints, altering their lines of pull. These complex migrations account for the intricate spatial topography of the adult musculature.
The muscles of the head and neck follow a specialized developmental pathway. Facial expression muscles (mimetic muscles) derive from the pharyngeal (branchial) arches. Because these muscles are historically closely associated with the skin, they lack pronounced aponeuroses and possess only very thin fascial layers. Masticatory muscles, as well as the musculature of the pharynx, soft palate, and hyoid apparatus, also originate from the pharyngeal arches and retain segmental branchial innervation by cranial nerves.
Development of Accessory Structures and Innervation
The development of muscle fibers is closely linked to the formation of the connective tissue framework (endomysium, perimysium, and epimysium), which arises from the surrounding mesenchyme.
- Tendons develop as dense cords at the sites of attachment between muscle tissue and bone.
- Aponeuroses form as broad sheets that redirect the vectors of muscle pull (e.g., in the palms or soles).
- Fasciae organize intermuscular septa, defining isolated compartments for various muscle groups.
- Synovial bursae arise in areas of high pressure or friction where tendons glide over bones.
The innervation of developing muscles is strictly segmental. Motor neuroblasts grow into the initial myotome. When the muscle primordium migrates to its final destination, it literally "drags" the branch of its corresponding nerve along, preserving its original connection.
Age-Related Changes and Growth
A crucial principle of postnatal development is that after birth, the number of muscle fibers practically does not increase. Muscle growth occurs exclusively through hypertrophy—an increase in the length and diameter of existing fibers, alongside an increase in the number of nuclei and myofibrils.
Muscle mass increases heterochronously. Respiratory, masticatory, and trunk muscles mature first, while distal limb muscles catch up later. During puberty, under the influence of sex hormones, a sharp growth spurt in muscle mass occurs (especially in males), establishing a clear specialization into fast glycolytic and slow oxidative fibers depending on physical demands.
During aging, reverse mechanisms take over. Sarcopenia is observed: a decrease in the number of recruited motor units, impaired blood supply, and a 25–30% reduction in skeletal muscle mass compared to young adulthood.