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The human body is made up of three major muscle types: skeletal, cardiac, and smooth muscle. The formation of muscle tissues is known as myogenesis, which starts during embryogenesis when the embryo is just a flat, two-layered creature. The para-axial mesoderm undergoes stepwise differentiation to generate muscle tissue, forming the somites, which then differentiate into a dermomyotome and sclerotome. The dermomyotome migrates to form a myotome, which then differentiates to form the skeletal muscles in the body. The development of skeletal muscles depends on the expression of certain proteins and transcription factors. These embryonic myoblasts further differentiate to form primary muscle fibres and eventually secondary myofibers. Each skeletal muscle consists of thousands of muscle fibres wrapped together by connective tissue sheaths.
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What You'll Learn
Embryonic development
The embryonic development of muscles involves several processes and stages. Firstly, the embryo itself is formed through a process called embryogenesis, where human embryonic stem cells (hESCs) differentiate into three types of germ layers: the ectoderm, mesoderm, and endoderm. The mesoderm layer is crucial for muscle development, as it gives rise to muscle tissues, including skeletal muscle cells, cardiac muscle, and smooth muscle.
During early embryonic development, mesodermal progenitor cells divide rostro-caudally into structures called somites. These somites are found on each side of the neural tube and notochord, and they play a vital role in muscle formation. The somites will eventually contribute to the cartilage and bone of the vertebral column and ribs, as well as the skeletal muscles of the body and limbs. The ventral part of the somite, called the sclerotome, will form the cartilage and bone structures, while the dorsal part, known as the dermomyotome, gives rise to the dermis of the back and the skeletal muscles.
As development progresses, the dermomyotome differentiates into dermatome cells and myotome cells. The myotome cells are of particular importance as they will form the skeletal muscles. Before becoming skeletal muscles, myotome cells differentiate into myoblasts (embryonic muscle cells) through the elongation of their nuclei and cell bodies. Myoblasts are a type of embryonic progenitor cell that plays a crucial role in muscle formation. They proliferate and fuse together to form multinucleated, cylindrical muscle fibers, also known as myotubes.
The formation of muscle fibers, or myogenesis, occurs in two stages: primary and secondary myogenesis. Primary myogenesis takes place during the embryonic stage, early in gestation, and results in the formation of smaller muscle fibers. The process involves the proliferation and fusion of myoblasts, influenced by growth factors such as fibroblast growth factor (FGF) and transcription factors like Myf-5 and MyoD. When FGF is abundant, myoblasts proliferate, but when it runs out, they cease division and prepare for the next stage. The second stage involves the alignment of myoblasts into myotubes, followed by cell fusion, which is facilitated by calcium ions and enhanced by Myocyte Enhance Factors (MEFs).
The development of limb muscles follows a slightly different pattern of regulation compared to trunk muscles, but the essential stages remain the same. The formation of muscle progenitor cells, their migration, and the subsequent development of muscle masses in the limbs are all crucial steps in embryonic muscle development.
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Muscle stem cells
During embryogenesis, the para-axial mesoderm undergoes stepwise differentiation to generate muscle tissue. The para-axial mesoderm on either side of the neural tube starts to differentiate and undergoes segmentation to form somites, which are stimulated by myogenic regulatory factors to differentiate into a dermomyotome and sclerotome. The lateral aspect of the dermomyotome undergoes epithelial-to-mesenchymal transition as it migrates ventrally to form a myotome below the dermatome. The myotome then differentiates to form the skeletal muscles in the body, with the dorsomedial aspect giving rise to back muscles and the ventrolateral aspect giving rise to muscles of the body wall.
Satellite cells are typically found in a quiescent state but can enter the cell cycle following injury to regenerate skeletal muscle tissue and replenish the stem cell pool. This regenerative capacity has made them a strategic therapeutic target in treating acute muscle injuries and chronic diseases such as muscular dystrophies. The ability of satellite cells to fuse into the myofibre syncytium makes them favourable vectors for delivering corrective gene therapy. However, long-term engraftment remains a challenge, and specific sub-populations of satellite cells capable of self-renewal and engraftment are being explored for transplantation.
The pattern of expression of skeletal muscle stem cell genes differs among tissues, suggesting that satellite cells have heterogeneous phenotypes and functions. Muscle stem cells express a number of genes that are notable for their developmental and functional relevance, including the myogenic basic helix-loop-helix proteins Myf-5 and MyoD, and the paired box transcription factors Pax-3 and Pax-7.
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Contraction
Skeletal muscles are voluntary muscles that we have direct control over through our nervous system. They can produce powerful, fast movements or small precision actions. They are also capable of stretching and contracting while still returning to their original shape.
The formation of muscle tissues is known as myogenesis. During embryonic development, myoblasts (progenitor cells of muscle tissue) divide mitotically to create more myoblasts or differentiate into myocytes (muscle cells). These myocytes are also known as muscle fibres.
Each skeletal muscle is made up of thousands of muscle fibres, bundled together and wrapped in a connective tissue covering. The outermost connective tissue sheath is called the epimysium. The epimysium contains fascicles, which are enclosed by a layer of connective tissue called the perimysium. Each muscle fibre is then enclosed by another layer of connective tissue called the endomysium.
The sarcomere is the fundamental contractile unit of a skeletal muscle. It is made up of myofibrils, which consist of an ordered arrangement of longitudinal myofilaments (thin actin filaments and thick myosin filaments). The actin and myosin filaments are the two most significant myofilaments, and they form distinctive bands on the skeletal muscle. The Z-line defines the lateral boundary of each sarcomere.
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Types of muscle fibres
The human body has three major muscle types: skeletal, cardiac, and smooth muscle. Skeletal muscles are made up of thousands of muscle fibres wrapped together by connective tissue sheaths. Each muscle fibre is a single cylindrical muscle cell, and when bundled together, the myofibrils get arranged in a unique striated pattern forming sarcomeres, which are the fundamental contractile unit of a skeletal muscle.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Skeletal muscles are voluntary muscles because we have direct control over them through our nervous system. Contractions can vary to produce powerful, fast movements or small precision actions. They can stretch or contract and still return to their original shape.
The stem cells that differentiate into mature muscle fibres are called satellite cells, which can be found between the basement membrane and the sarcolemma (the cell membrane surrounding the striated muscle fibre cell). When stimulated by growth factors, they differentiate and multiply to form new muscle fibre cells.
The sarcolemma forms a physical barrier against the external environment and also mediates signals between the exterior and the muscle cell. The sarcoplasm is the specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, and a modified endoplasmic reticulum.
The myofibrils are contractile units within the muscle cell that consist of an ordered arrangement of longitudinal myofilaments (thin actin filaments and thick myosin filaments). The interaction of myosin and actin is responsible for muscle contraction. Within a muscle, the muscle fibres are functionally organized as motor units.
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Skeletal muscle shape and size
Skeletal muscles vary in size, shape, and fibre arrangement. They can be extremely small strands, such as the stapedius muscle in the middle ear, or large masses like the thigh muscles. Some are broad, some are narrow, and some are oblique. The fibres in some muscles are parallel to the long axis, while in others they converge to a narrow attachment. Each skeletal muscle is made up of hundreds or thousands of muscle fibres, each a single cylindrical muscle cell. These are bundled together and wrapped in a connective tissue covering.
Each muscle fibre is made up of myofibrils, which contain multiple myofilaments. When bundled together, the myofibrils form a unique striated pattern, creating sarcomeres—the fundamental contractile unit of a skeletal muscle. The two most significant myofilaments are actin and myosin, which form bands on the skeletal muscle. The actin and myosin filaments interact to cause muscle contraction.
The outermost connective tissue sheath surrounding the entire muscle is the epimysium. The connective tissue sheath covering each bundle of muscle fibres (fasciculi) is the perimysium, and the innermost sheath surrounding individual muscle fibres is the endomysium. The epimysium anchors the muscle tissue to tendons at each end, protecting the muscle from friction against other muscles and bones.
The stem cells that differentiate into mature muscle fibres are called satellite cells. These can be found between the basement membrane and the sarcolemma (the cell membrane surrounding the striated muscle fibre cell). When stimulated by growth factors, they differentiate and multiply to form new muscle fibre cells.
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Frequently asked questions
Muscles are fibrous tissues that can stretch and contract, allowing for movement in the body. There are three major types of muscles in the human body: skeletal, cardiac, and smooth muscle.
The formation of muscle tissues is known as myogenesis. During embryonic development, myoblasts (progenitor cells of muscle tissue) divide to create more myoblasts or differentiate into myocytes (muscle cells). The myoblasts then differentiate to form skeletal muscles in the body.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
