During the course of embryonic development, mesenchymal progenitor cells originating from the somites, undergo a multistep differentiation process to form muscle fibers and muscle mass. Each somitic area (occipital, cervical, thoracic, lumbar and sacral) contributes to the formation of muscles. Muscle satellite cells are formed during embryonic development as well, and persist in a quiescent state in the adult muscles, ensuring restoration of muscle cells following any type of muscle injury.
The head mesoderm constitutes an additional source of progenitor cells, which contribute to the formation of head muscles, or craniofacial muscles. These progenitor cells are primarily contributed from the unsegmented cranial paraxial and splanchnic mesoderm, which bear a somewhat different genetic program, when compared to their counterparts in the somites. The prechordal mesoderm cells, which reside in the most anterior part of the forming head, represent another population of progenitor cells. These cells eventually give rise to the extraoccular muscles (EOM, eye muscles). The formation of vertebrate head muscles allow for vision, mastication and food uptake, facial expression and breathing in humans.
Skeletal muscle, also called striated muscle, is a dense, fibrous contractile tissue which exists throughout the entire body, and functions to allow body movement by applying force to bones and joints, via contraction. In humans, there are approximately 640 muscles and almost all are symmetrically distributed between the left and right sides of the body. Size and shape of different muscles are highly variable depending on their functions throughout the body. Muscles are attached to specific bones by collagenous fibers called tendons. Skeletal muscle contraction is voluntary and is controlled by the somatic nervous system. The contraction is caused by neural impulses delivered to the muscle by motor neurons originating from the central nervous system (CNS).
Skeletal muscles can be grouped according to their location in the body: head and neck muscles, trunk muscles and upper and lower limbs muscles. Trunk muscles are then subdivided by their source of innervations: hypaxial muscles, located mostly ventrally, are innervated by the ventral ramus of spinal nerves, while epaxial muscles are innervated by the dorsal ramus.
The skeletal muscle features a highly organized structure. Each muscle consists of a large number of long, cylindrical, multinucleated cells called multinuclear myocytes or muscle fibers, held together by connective tissue. Each myofiber is composed of myofibrils, which, in turn, are comprised of sarcomeres – the smallest functional units of the muscle, built of actin and myosin filaments precisely aligned with each other. The muscle contraction is triggered by neural impulses and results from actin and myosin filaments “sliding” along each other.
Muscle fibers are grouped into two main types, primarily defined by their contraction speed. Type I and Type II, the slow and fast fibers, respectively, differ in their myosin content, energy source and myoglobin content. Type I fibers contain high amounts of myoglobin, which provide them with their red color. These fibers use oxidative metabolism to generate ATP as an energy source, and are therefore slow and most appropriate for prolonged contractions. Type II fibers have low myoglobin content and thus appear white, and use both aerobic and anaerobic metabolism to generate energy, depending on the specific muscle. Type IIb fibers use only anaerobic metabolism, and undergo the highest rate of contraction. In general, Type II muscle fibers are capable of creating short bursts of speed and power, but fatigue relatively quickly.