Logan Steele
Skeletal muscle cells are the only type of muscle cells that are multinucleated. They are long, cylindrical, and striated, and they make up the muscle tissues connected to the skeleton. These muscles are responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. Each muscle, such as the biceps brachii, contains hundreds of thousands of muscle fibers. The multinucleated condition of skeletal muscle cells results from multiple myoblasts fusing to produce each muscle fiber, with each myoblast contributing one nucleus.
| Characteristics | Values |
|---|---|
| Types of muscles that are multinucleated | Skeletal muscles |
| How skeletal muscles become multinucleated | Skeletal muscles arise in embryos as uninucleated entities. Later, myoblast cells fuse together to form multinucleated muscle fibers. |
| Why skeletal muscles are multinucleated | Skeletal muscles are multinucleated for functional and structural reasons. Each myonucleus takes care of a certain area of the large muscle cell. The presence of multiple nuclei also means that translated proteins can be synthesised close to their functional position, reducing the need for transport over large distances. |
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What You'll Learn
Skeletal muscle cells are multinucleated
Skeletal muscle cells are the only type of muscle cells that are multinucleated, meaning they have more than one nucleus. They are long, cylindrical, and striated, and they make up the muscle tissues connected to the skeleton. These muscles are attached to bones by tendons and are responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints.
The presence of multiple nuclei in skeletal muscle cells arises from the fusion of embryonic myoblasts, with each myoblast contributing a single nucleus to the newly formed muscle cell or myotube. This fusion process is facilitated by muscle-specific proteins called fusogens, specifically myomaker and myomerger. The multinucleated state of skeletal muscle cells serves several functional and structural purposes.
From a functional perspective, each nucleus (referred to as myonuclei) takes care of a specific area within the large muscle cell. Due to the muscle cell's significant size, ranging from its insertion to origin, having multiple nuclei enables better management of its various regions. Additionally, the multinucleated structure facilitates the synthesis of translated proteins close to their functional positions, reducing the need for long-distance protein transport.
The multinucleation also enhances the efficiency of energy production. Mitochondria, responsible for powering the cell, function optimally when energy is generated near its site of use. To maintain mitochondrial function along the length of the myotube, enzyme synthesis is required, and having multiple nuclei synthesizing these enzymes improves efficiency.
Furthermore, the multinucleated structure of skeletal muscle cells contributes to their ability to generate force for muscle contractions. The myofibrillar actin/myosin structure within the myotubes is organized into long fibres, enabling the generation of force between anchored positions, resulting in muscle contraction. The multinucleated structure supports the length and thinness required for the packing of these fibres.
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Muscle fibres are multinucleated
Muscle fibres, also known as myocytes, are the cells that make up muscle tissue. There are three types of muscle cells in the human body: cardiac, skeletal, and smooth muscle cells. Skeletal muscle fibres are multinucleated, meaning they have more than one nucleus. This is because they are formed from the fusion of embryonic myoblasts, with each myoblast contributing a nucleus to the newly formed muscle cell.
Skeletal muscle is found throughout the body and is responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilising joints. It comprises approximately 40% of human body weight and contains 50-75% of all body proteins. Skeletal muscle cells are long, cylindrical, and striated, with the cell membrane, or sarcolemma, combining with tendon fibres at the myotendinous junction.
The presence of multiple nuclei in skeletal muscle fibres serves several functional and structural purposes. Firstly, each nucleus, or myonucleus, is responsible for a specific area of the large muscle cell. Due to the size of the muscle cell, multiple nuclei are required to ensure adequate coverage. Additionally, multinucleation allows for the synthesis of proteins close to their functional position, reducing the need for long-distance transport.
The mitochondria, responsible for powering the cell, also benefit from the presence of multiple nuclei. Enzyme synthesis is required to support mitochondrial function, and having multiple nuclei synthesising these enzymes ensures that energy production occurs close to where it is needed. This enhances the efficiency of the system.
In summary, muscle fibres, specifically skeletal muscle fibres, are multinucleated. This multinucleation is a result of the fusion of embryonic myoblasts during the formation of skeletal muscle cells. The presence of multiple nuclei serves important functional and structural purposes, optimising the performance of skeletal muscle.
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Myoblasts fuse to form multinucleated myotubes
Skeletal muscles are multinucleated. They are derived from the paraxial mesoderm, which first divides into segments called somitomeres. These eventually form the skeletal muscles. Mesodermal cells form myogenic cells, which undergo mitosis to form postmitotic myoblasts.
Myoblast fusion also occurs during muscle regeneration. When muscles are injured, satellite cells are activated and divide asymmetrically to generate a new pool of myoblasts. These myoblasts fuse with each other and with injured myotubes to repair the muscle.
Several proteins have been found to regulate myoblast fusion, including Myomaker and Myomerger–Minion. These proteins are expressed in the developing myotome during embryogenesis but are not present after muscle development has ceased. However, they are reactivated upon muscle injury.
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Multinucleation reduces the need for protein transport
Skeletal muscle fibres are the only muscle cells that are multinucleated, with the nuclei usually referred to as myonuclei. This multinucleated condition results from multiple myoblasts fusing to produce each muscle fibre, where each myoblast contributes one nucleus.
The process of multinucleation involves the fusion of myoblasts, with each myoblast contributing a nucleus to the newly formed muscle cell or myotube. This fusion depends on muscle-specific proteins known as fusogens, including myomaker and myomerger.
Multinucleation can reduce the need for protein transport within the cell. This is because the presence of multiple nuclei in close proximity to the cell's cytoskeleton allows for more efficient regulation of protein synthesis and distribution. With nuclei strategically located throughout the cell, the distances that proteins need to be transported are reduced, resulting in more efficient cellular functions.
In addition to reducing the need for protein transport, multinucleation also offers other advantages. For example, multinucleated cells can produce more ATP energy, which is essential for muscle contraction and movement. This increased energy production supports the function of skeletal muscles, which are responsible for producing movement, sustaining body posture, maintaining body temperature, storing nutrients, and stabilising joints.
While multinucleation is advantageous in skeletal muscle cells, it is important to note that abnormal multinucleation has been associated with certain diseases and cancers. For example, bacterial pathogens such as Mycobacterium tuberculosis and viruses like HIV can induce multinucleation in host cells, leading to cellular dysfunction and disease progression.
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Muscle cells are also known as myocytes
Muscle cells, also called myocytes, are specialised cells designed for contraction and force production. They are long, cylindrical, and have a striated (or striped) appearance due to their highly organised structure into functional units known as sarcomeres. Each myocyte contains thousands of myofibrils, and each myofibril is made up of numerous sarcomeres, the functional contractile region of a striated muscle.
Sarcomeres, in turn, are composed of myofilaments of myosin and actin, which interact using the sliding filament model (powered by molecular motors) and cross-bridge cycles to contract. The thin myofilaments are filaments of mostly actin, and the thick filaments are of mostly myosin. These filaments slide over each other to shorten the fibre length in a muscle contraction. The third type of myofilament is an elastic filament composed of titin, a very large protein.
The smallest contractile unit in the fibre is called the sarcomere, which is a repeating unit within two Z bands. The sarcoplasm, or the specialised cytoplasm of a muscle cell, contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum called the sarcoplasmic reticulum, myoglobin, and mitochondria. The sarcoplasmic reticulum forms a network around each myofibril of the muscle fibre, and this network is composed of groupings of two dilated end-sacs called terminal cisternae and a single T-tubule (transverse tubule).
Skeletal muscle is found throughout the body and functions to contract in response to a stimulus. It serves many purposes, including producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilising joints. Skeletal muscle is the only type of muscle that can be consciously activated via the intent to move. Skeletal muscle fibres are the only muscle cells that are multinucleated, with the nuclei usually referred to as myonuclei.
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Frequently asked questions
Skeletal muscles are multinucleated.
Skeletal muscle cells are multinucleated because they are formed from the fusion of embryonic myoblasts, each contributing a nucleus to the newly formed muscle cell or myotube.
Multinucleation means that translated proteins can be synthesised close to their functional position, reducing the need for transport over large distances. Additionally, as skeletal muscles are so large, having multiple nuclei allows each myonucleus to take care of a specific area of the muscle cell.
Skeletal muscles are attached to bones by tendons and are found throughout the body. Examples include the biceps brachii and the muscles in the trunk region.
