Embryogenesis and Cellular Differentiation
Skeletal striated muscle tissue originates from the mesoderm. The primary source is the paraxial mesoderm, which forms somites. Within each such somite, a myotome is distinguished, from which future skeletal muscles develop. Meanwhile, adjacent regions give rise to other tissues: the dermatome gives rise to the skin, and the sclerotome to the axial skeleton. Part of the trunk and limb musculature originates from the anterolateral mesoderm.
At the cellular level, the process begins with myoblasts. These cells fuse with one another to form multinucleated structures called myotubes. It is at this stage that myofibrils and the characteristic cross-striation appear within the primordium. Subsequently, the sarcolemma, T-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 are segmented arrays of myotomes. In the region of the trunk and neck, 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 bundles actively migrate, fuse, and cross over joints, changing the direction of pull. These complex migrations explain the spatially challenging topography of the adult human musculature.
The muscles of the head and neck undergo a distinct pathway of development. Facial musculature is formed from derivatives of the visceral (pharyngeal) arches. Because these muscles are historically closely associated with the skin, they have poorly developed aponeuroses and only very thin fasciae. The muscles of mastication, as well as the musculature of the pharynx, soft palate, and hyoid apparatus, also have a "pharyngeal" origin and retain segmental-arch innervation by cranial nerves.
Development of the Accessory Apparatus and Innervation
The development of muscle fibers is continuously 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 where muscle tissue attaches to bones.
- Aponeuroses form as broad sheets that redirect the vector of muscle pull (for example, on the palms or soles).
- Fasciae organize intermuscular septa, establishing isolated compartments for various muscle groups.
- Synovial bursae arise in areas of increased pressure or friction of tendons against bones.
The innervation of developing muscles is strictly segmental. Motor neuroblasts grow into the initial myotome. When the muscle primordium migrates to its final location, it literally "drags" the branch of the corresponding nerve along with it, preserving its original connection.
Age-Related Changes and Growth
A crucial principle of postnatal development: 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 the fibers themselves, as well as an increase in the number of nuclei and myofibrils.
Mass increase occurs heterochronically. Respiratory, masticatory, and trunk muscles mature first, while the distal limb segments catch up later. During puberty, under the influence of sex hormones, there is a sharp growth spurt in muscle mass (especially in boys), and a clear specialization into fast glycolytic and slow oxidative fibers is established depending on the type of load.
During aging, reverse mechanisms are triggered. Sarcopenia is observed: a decrease in the number of recruited motor units, impaired blood supply, and a drop in skeletal muscle mass by approximately 25–30% compared to youth.
Briefly: answers to topical questions
From which embryonic sources does the smooth musculature of internal organs develop?
The smooth musculature of internal organs develops from mesenchyme. Mesenchyme is an embryonic tissue filling intermediate spaces, the cells of which migrate from various parts of the mesoderm (primarily from somites and splanchnotomes). The following structures form from this source:
- Smooth muscles of the respiratory system — in the walls of the larynx, trachea, and bronchi.
- Smooth muscles of the digestive tract — in the muscularis mucosae and the muscular coat of the stomach.
From which embryonic primordium does cardiac striated muscle tissue develop?
Cardiac striated muscle tissue develops from the myoepicardial mantle. This embryonic primordium is a thickening of the visceral layer of the splanchnotome (splanchnopleura), which is the most lateral unsegmented part of the mesoderm. During embryogenesis, the myoepicardial mantle forms the following layers of the heart wall:
- Myocardium — the middle muscular layer.
- Epicardium — the outer coat.
What is the percentage of skeletal muscle mass relative to total body weight in newborns and adults?
The percentage of skeletal muscle mass relative to total body weight increases significantly during ontogenesis.
| Age period | Share of skeletal muscles in body weight |
|---|---|
| Neonates | ~20–23 % |
| Adults | up to 40% |
The growth of muscle mass in the postnatal period is uneven. Up to 15 years of age, mass increases insignificantly; then, from 15 to 17 years, a rapid growth spurt occurs. The final development of the muscular system is completed by 25–30 years of age.
To easily remember the derivatives of the somite, use the mnemonic: Myotome (muscular system), Dermatome (skin), Sclerotome (bones and axial skeleton).
Class and exam questions
Does the number of muscle fibers increase after birth?
No, after birth the number of fibers hardly increases at all. Muscle growth is driven by hypertrophy—the thickening of existing cells and an increase in the number of contractile elements within them.
Which muscles mature first during embryogenesis?
The muscles vital to an infant immediately after birth develop first. These include the diaphragm, intercostal muscles, extraocular muscles, as well as the muscles of the lips and tongue required for suckling and swallowing.
Why does the innervation of certain muscles seem counterintuitive in terms of their final topography?
A muscle always maintains a segmental connection with the nerve of its original myotome. During complex embryonic development, the primordium may migrate far from its initial site of origin, dragging the nerve along with it.
What else to study on this topic
- Stages of formation of the sarcolemma, T-system, and sarcoplasmic reticulum
- Timelines and mechanisms of myelination of motor neurons for distal muscles
- Formation of synovial sheaths in the osteofibrous canals of the hand and foot
- Features of vascular network development: perforating and reticular anastomoses
- Effect of prolonged hypokinesia on the muscle-tendon complex
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