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Individual skeletal muscle fibers (also known as myofibers) are very long (up to 30 cm), cylindrical, typically arranged in parallel, and have diameters from 10 to 100 μm (Figures 3.4 and 3.5). They are multinucleated (more than one nucleus) as the result of the fusion of the mononucleated myoblasts that are muscle cell precursors. The nuclei are located just beneath the sarcolemma on the periphery of the sarcoplasm. The variation in muscle fiber diameters depends on many factors, such as age, gender, state of nutrition, physical training, or muscle damage. Physical training typically results in their enlargement of diameter and volume (hypertrophy), while their damage can lead to loss of cell volume and atrophy.

Muscle also contains satellite cells that are myogenic precursor cells (Mauro, 1961). Satellite cells are normally located beneath the basal lamina and adjacent to the sarcolemma of muscle fibers (Mauro, 1961; White & Esser, 1989). The satellite cells of young animals are referred to as myoblasts, are active, and initiate the process of muscle differentiation (Ishikawa, 1966; Snow, 1977). Yet, even in mature mammals in which satellite cells are typically quiescent, a muscle injury activates the satellite cells, driving them out of their quiescent state and initiating their proliferation (Charge & Rudnicki, 2004). Thereafter, they undergo differentiation into myocytes and fuse either with each other or with existing myofibers for the repair of injured muscles (Charge & Rudnicki, 2004). A minor fraction of satellite cells regenerate themselves or self‐renew and eventually return to a quiescent state (Collins et al., 2005). Bone marrow–derived stem cells that possess the ability to contribute to regeneration of injured skeletal muscle tissue have also been identified in human skeletal muscle (Stromberg et al., 2013). Thus, both satellite cells and bone marrow–derived stem cells play key roles in muscle growth and repair and are instrumental in the adaptation of muscle to various stimuli (Exeter & Connell, 2010). In pathological conditions and during muscle wound healing, inflammatory cells (neutrophils, macrophages, and lymphocytes) and myofibroblasts can be observed in muscle tissues (Barbe et al., 2021).

Table 3.2 Summary of Cells, Subtypes, Extracellular Matrix (ECM), and Function of Skeletal Muscle Under Normal Conditions

Based on Gillies, A. R., & Lieber, R. L. (2011). Structure and function of the skeletal muscle extracellular matrix. Muscle Nerve 44(3), 318–331. doi:10.1002/mus.22094.

Characteristic	Description
Tissue type	Contractile
Cells	Main cell types: Individual muscle fibers (myofibers), myoblasts, satellite cells, bone marrow–derived stem cellsAdditional cell types: Resident macrophages, endothelial cells associated with blood vessels throughout muscles, fibroblasts in sheaths, peripheral glial cells associated with nerve endings and neuromuscular junction
Subtypes	Type I (slow‐twitch/red), Type IIb (fast‐twitch/white), Type IIa (intermediate), Type IIx
ECM	Main composition: Collagen type I and glycoproteins in muscle proper, collagen IV in basement membraneAdditional components: Collagen III, collagen V, and elastin in fascial sheaths
Function	Contraction and then movement of the endoskeleton to which the muscle is attached (bones and cartilage)

Figure 3.4 Single muscle cells (fiber) showing multinucleated nature and striations.

From Tortora, G. J., & Derrickson, B. H. (Eds.), (2010). Muscle. In Introduction to the human body, 11th ed., Wiley.

Figure 3.5 Distribution of different myosin heavy chains detected by immunofluorescence in a rat flexor digitorum muscle. This muscle is composed predominantly of type IIb (myosin heavy‐chain type II) fast fibers. The banding seen is the myosin chains. A smaller number of fibers are type I, slow fibers. This muscle contains several intermediate fibers that stain for both types I and II myosin heavy chains. There are also some type IIx fibers that are unstained.

The connective tissue sheaths of muscles, described further below, contain fibroblasts, endothelial cells associated with blood vessels, nerve axons, and sometimes adipose cells and myofibroblasts. Muscles also contain resident macrophages that secrete various growth factors for the maintenance of the muscle fibers. The sites of neuromuscular junctions are the sites where nerve axons terminate on muscle fibers, and muscle spindles and Golgi tendon apparati are sensory receptors located within muscles and myotendinous junctions, respectively. These sites contain glial cells, the Schwann cells, and glial satellite cells, in additional to nerve axon terminals.