Logan Steele
The extracellular matrix (ECM) is a complex meshwork consisting of collagens, glycoproteins, proteoglycans, and elastin. It is a kind of connective tissue in the cell microenvironment, which is of great significance to tissue development. The ECM in muscle fiber niche consists of three layers: the epimysium, the perimysium, and the endomysium. These layers of connective tissue structure maintain the morphology of skeletal muscle and play an important role in the physiological functions of muscle cells, such as the transmission of mechanical force, muscle growth and repair, and the regeneration of muscle fiber. The ECM also contains molecules that support blood vessel formation and immune cell recruitment.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of small fibres woven together |
| Muscle fibre width | 3-8 micrometres |
| Muscle fibre breadth | 18-200 micrometres |
| Muscle fibre length | 0.5-3 inches |
| Muscle fibre nuclei | Hundreds to thousands |
| Types of muscle tissue | Skeletal, cardiac, smooth |
| Skeletal muscle composition | Skeletal muscle fibres, blood vessels, nerve fibres, connective tissue |
| Number of connective tissue layers | 3 |
| Connective tissue layers | Endomysium, perimysium, epimysium |
| Skeletal muscle fibre types | Type I (slow-twitch), Type II (fast-twitch) |
| Cardiac muscle location | Walls of the heart |
| Sternocleidomastoid muscle function | Rotation of the head to the opposite side, flexion of the neck |
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What You'll Learn
The role of ECM in muscle repair and regeneration
Skeletal muscle is a highly coordinated tissue composed of myofibers formed by myogenic progenitor cell fusion. It is primarily responsible for skeletal support, body movement, and temperature regulation. Skeletal muscle (SM) also has an impressive ability to regenerate after injury, and this ability depends on resident muscle satellite (stem) cells (MSCs) located in unique anatomical sites along the margins of myofibers.
The Extracellular Matrix (ECM) in skeletal muscle plays an integral role in force transmission, structural maintenance, and the regulation of the stem cell niche. ECM interacts with stem cells either directly by binding cell surface receptors or indirectly through growth factor presentation, and maintains a balance between their quiescence, self-renewal, and differentiation. The ECM of skeletal muscles is a complex meshwork consisting of collagens, glycoproteins, proteoglycans, and elastin. Collagens form a network of intramuscular connective tissue (IMCT), i.e., the central, fibrous components of the ECM. The IMCT is typically depicted to be organized in three layers: the endomysium, perimysium, and epimysium.
ECM deposition can be observed within a week post-injury and is primarily due to the activity of fibroblasts in response to locally produced mediators such as transforming growth factor beta 1 (TGF-β1). In chronic muscle injuries, the inflammatory phase persists for weeks, and the deposition of ECM proceeds more rapidly than myogenesis. A major impediment to the complete recovery of skeletal muscle post-injury is the development of fibrosis, defined as an abnormal and chronic over-proliferation of ECM components. If unresolved, fibrotic tissue can interfere with muscle contractility and impair muscle regeneration.
ECM scaffolds are three-dimensional biodegradable structures that mimic natural cellular environments and have been shown to support the growth and differentiation of stem cells into muscle cells. These scaffolds, which are primarily composed of ECM proteins such as COLs, laminins, and FN, can provide a supportive environment for stem cells to differentiate into muscle cells and facilitate their integration into host tissue. To fully harness the therapeutic capabilities of ECM scaffolds, extensive research is required to devise novel scaffolding technologies that surpass current limitations and facilitate the advancement of more effective strategies for muscle repair and regeneration.
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The composition of ECM
The extracellular matrix (ECM) is a dynamic 3-dimensional network of macromolecules that provides structural support for cells and tissues. It is composed of water, proteins, and polysaccharides, with each tissue exhibiting a unique ECM composition. The ECM is responsible for the physical maintenance of all cells and plays a crucial role in tissue development, influencing cellular activity and responses.
The major elements that form the ECM include collagens, elastin, and fibronectin, which together create a '3D amorphous gel'. Collagens constitute the majority of fibrous proteins within the matrix, while fibronectin dictates the organisation of the matrix structure. Other components of the ECM include laminins, tenascins, proteoglycans, glycosaminoglycans, and hyaluronan.
In muscle tissue, the ECM consists of three layers: the epimysium, the perimysium, and the endomysium (basal lamina). These layers of connective tissue maintain the morphology of skeletal muscle and play a vital role in muscle cell functions, such as force transmission, muscle fibre regeneration, and the formation of the neuromuscular junction.
The ECM is a highly dynamic structure that constantly undergoes remodelling, either enzymatically or non-enzymatically. Its molecular components are subject to various post-translational modifications, resulting in changes to its topological and viscoelastic properties.
The composition and structure of the ECM can be analysed using techniques such as confocal microscopy, electron microscopy, and Raman spectroscopy. These methods allow for the examination of the ECM's biomechanical properties, such as its stiffness and elasticity.
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ECM and muscle contraction
Extracellular matrix (ECM) is a type of connective tissue in the cell microenvironment that is essential for tissue development. ECM in muscle fibre niches consists of three layers: the epimysium, the perimysium, and the endomysium (basal lamina). These layers maintain the morphology of skeletal muscle and play a role in muscle cell functions such as the transmission of mechanical force, muscle fibre regeneration, and the formation of neuromuscular junctions.
ECM regulates muscle development, growth, and repair and is essential for effective muscle contraction and force transmission. It is highly malleable, and its texture and physiological roles may be affected by physical training, disuse, ageing, or various diseases. The ECM of skeletal muscles is a complex meshwork consisting of collagens, glycoproteins, proteoglycans, and elastin. Collagens form a network of intramuscular connective tissue (IMCT), the central fibrous components of the ECM. The IMCT is typically organised into three layers: the endomysium, the perimysium, and the epimysium. However, the traditional classification of these three layers may be simplistic, and a higher-order organisation of muscle ECM may exist that is yet to be defined.
Studies have shown that the ECM plays a crucial role in muscle development, growth, and repair, as well as the transmission of contractile force. For example, in skeletal muscle diseases, degenerative changes in muscle fibres are characterised by the gradual replacement of individual muscle fibres by connective tissue. This process involves the fragmentation of collagen fibres and the eventual encapsulation of the hollow basement membrane sheath by the extracellular matrix. Additionally, ECM can exert transverse stress on fibres and have axial strain. If the connection between ECM and muscle cells is insufficient, muscle fibres will lack mechanical support, and the force transmission pathway will be disrupted, leading to deformation beyond the physiological limit.
ECM remodelling induced by alternating electrical and mechanical stimulations has been found to increase the contraction of engineered skeletal muscle tissues. The application of these stimulations remodels the structure of ECM networks, influencing the mechanical properties of the ECM. The stiffness of the ECM is critical; the stiffness of the parallel ECM must be low to allow contraction, while the stiffness of the serial ECM must be high to transmit forces effectively. Furthermore, extracellular matrix stiffness has been shown to regulate human airway smooth muscle contraction by altering cell-cell coupling. The binding of integrins to the ECM exhibits catch bond characteristics, where the bond lifetime increases with force.
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The influence of physical training on ECM
The extracellular matrix (ECM) is a complex network of molecules that provides structural and functional support to the cells and tissues of the body. It is composed of proteins, proteoglycans, and glycoproteins, and fills the spaces between cells, providing a structural framework for tissues and organs. The ECM also plays a crucial role in cell signalling, differentiation, and migration, and is essential for maintaining tissue integrity and function.
In muscle tissue, the ECM is particularly important for muscle function and repair. It provides a structural framework for muscle cells and plays a key role in muscle contraction and force transmission. The ECM also contributes to muscle flexibility and resilience, and is involved in muscle growth and repair processes. Physical training, such as strength and endurance exercises, has been shown to have a significant impact on the ECM.
Exercise and physical training are known to induce structural and functional adaptations in skeletal muscle. These adaptations include changes in muscle fibre type composition, increases in muscle mass and strength, and improved metabolic function. Less understood are the effects of exercise on the extracellular matrix (ECM) of skeletal muscle, a critical component that provides structural support and regulates muscle function.
Exercise training can induce remodelling of the muscle ECM, influencing its composition, organisation, and function. One of the key effects is an increase in the production of collagen, the main structural protein of the ECM. This can lead to improved muscle stiffness and strength, contributing to increased force transmission and resistance to muscle damage. Additionally, exercise can stimulate the synthesis of other ECM components, such as proteoglycans and glycoproteins, which play important roles in muscle hydration, nutrient transport, and cell signalling.
The specific type of exercise performed appears to have a significant impact on ECM remodelling. For example, endurance exercises, such as long-distance running or swimming, tend to promote the synthesis of more elastic ECM components, resulting in increased muscle flexibility and fatigue resistance. On the other hand, resistance and strength training exercises induce the production of stiffer and stronger ECM, leading to enhanced muscle stiffness and strength. This adaptation is particularly important for improving muscle performance and force generation.
In addition to structural changes, physical training can also influence the ECM's regulatory functions. Exercise can stimulate the release of growth factors and cytokines, which modulate ECM remodelling and promote muscle regeneration and repair. Additionally, exercise-induced mechanical stress on the muscle can activate specific signalling pathways involved in ECM synthesis and degradation, further contributing to ECM remodelling and adaptation. Understanding how physical training influences the ECM provides valuable insights into optimising training programmes to enhance muscle performance and promote healthy muscle ageing.
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ECM and muscle disease
Extracellular matrix (ECM) is a type of connective tissue in the cell microenvironment that plays a significant role in tissue development. ECM in muscle fibre niches consists of three layers: the epimysium, the perimysium, and the endomysium (basal lamina). These layers maintain the morphology of skeletal muscle and play a crucial role in muscle cell physiological functions, including mechanical force transmission, muscle fibre regeneration, and neuromuscular junction formation. ECM regulates muscle development, growth, and repair and is vital for effective muscle contraction and force transmission. Its texture and physiological roles can be influenced by physical training, disuse, aging, and diseases such as diabetes.
In skeletal muscle diseases, degenerative changes in muscle fibres are marked by the gradual replacement of individual muscle fibres by connective tissue. This process involves the exfoliation of peripheral cytoplasm into the endomysium cavity, resulting in muscle fibre contraction and collagen fibre fragmentation. The degradation products of ECM biological scaffolds can promote the alternate activation and polarisation construction of M2 macrophages, facilitating the migration and myogenesis of skeletal muscle progenitor cells.
ECM has been widely applied in developmental biology, regenerative medicine, and bioengineering due to its regulatory role in cell physiological functions and unique biological characteristics. It serves as a biochemical and mechanical interface between muscle cells and their surroundings. The assembly of its collagenous scaffold is primarily promoted by the growth factors TGF-β and CTGF, which are regulated by proteoglycans like decorin and biglycan. Additionally, proteolytic enzymes (MMPs) and their inhibitors (TIMPs) are involved in ECM regulation.
The understanding of ECM in skeletal muscle is still evolving, and further research is needed to elucidate the contribution of various cells to the structure and remodelling of the intramuscular connective tissue (IMCT). While ECM plays a crucial role in muscle function and adaptation, its involvement in muscle pathology and potential as a key to treating muscular and metabolic disorders require further exploration.
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Frequently asked questions
CM does not stand for anything in relation to muscles. However, ECM stands for the extracellular matrix, which is a kind of connective tissue in the cell microenvironment.
The ECM is made up of approximately 300 proteins, referred to as the core matrisome, which is composed of 43 COL subtypes, 36 proteoglycans, and nearly 200 complex glycoproteins.
The ECM regulates muscle development, growth and repair and is essential for effective muscle contraction and force transmission.
The ECM can be categorised into three layers: the epimysium, the perimysium, and the endomysium.
The ECM is essential for muscle regeneration and repair. It provides instructive signals to cells during adult tissue regeneration and its ability to sequester growth factors and generate concentration gradients also has an indirect effect on this process.
