Dominic Stone
Muscle cells, also known as myocytes, are specialized for contraction and are dissimilar in other respects. There are four main categories of muscle cells in mammals: skeletal muscle cells, heart (cardiac) muscle cells, smooth muscle cells, and myoepithelial cells. While muscle cells play a crucial role in movement and contraction, a common question arises regarding their ability to undergo mitosis. Mitosis is a process where cells divide to repair or regenerate. In adult humans, muscle cells generally lose their ability to divide and undergo mitosis. However, certain types of muscle fibers, such as smooth muscle fibers, retain their capacity for division, and new smooth fibers can arise from pericytes, a type of stem cell associated with small veins and blood capillaries. Additionally, skeletal muscles contain satellite cells that can divide and fuse with existing muscle fibers to facilitate repair and regeneration.
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What You'll Learn
Muscle cells in adult humans cannot divide by mitosis
Mitosis is the process by which somatic, or non-reproductive, cells divide. Somatic cells make up most of the body's tissues and organs, including skin, muscles, lungs, gut, and hair cells. During mitosis, the parent cell divides into two daughter cells, which have the same chromosomes and DNA as the parent cell. These daughter cells are called diploid cells, and each contains two complete sets of chromosomes.
In adult humans, muscle cells, or myocytes, do not have the ability to divide by mitosis. This means that they cannot repair themselves, and their cell cycle remains in the interphase. Muscle cells lose their ability to divide after maturation, when multiple myoblasts fuse to form skeletal muscle fibers. Once this fusion occurs, the ability to divide is lost. However, a few myoblasts persist in mature skeletal muscle as satellite cells, which can fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers.
Smooth muscle fibers, such as those in the uterus, retain their capacity for division. New smooth muscle fibers can also arise from pericytes, which are stem cells associated with small veins and blood capillaries. Additionally, studies have shown that bone marrow stem cells can be used to regenerate damaged heart tissue.
While muscle cells cannot divide by mitosis, they can, in some cases, be regenerated through other mechanisms, such as the activation of satellite cells or the use of stem cells.
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Skeletal muscle cells are responsible for practically all voluntary movements
Skeletal muscles are a type of muscle that humans can contract voluntarily, meaning that we can control how and when they move and work. They are attached to bones via tendons and are found throughout the body, including in the tongue, diaphragm, eye socket, and upper oesophagus. They are also present in the shoulder, back, and intercostal muscles.
Skeletal muscles are responsible for practically all voluntary movements. They contract in response to a stimulus, producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. They are composed of flexible muscle fibres that usually span the length of the muscle and can range from less than half an inch to just over three inches in diameter. Each muscle can contain thousands of these fibres, which are woven together like a quilt that covers the body.
The fibres contract (tighten), allowing the muscles to move bones so that we can perform a wide range of movements. Some muscle fibres contract quickly and use short bursts of energy (fast-twitch muscles), while others move slowly, like back muscles that help with posture. These fibres are either red or white. They are called striated or striped muscles because they look striated.
Skeletal muscles are highly susceptible to various conditions, from mild injuries to serious or even life-threatening myopathies. Muscular dystrophies, for example, are a group of diseases that cause the progressive degeneration of skeletal muscle fibres due to an abnormal gene and can be inherited. Myasthenia gravis (MG) is another condition that affects skeletal muscles. It is an autoimmune disease that prevents muscles and nerves from communicating properly, leading to severe muscle weakness and fatigue.
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Smooth muscle cells can arise from pericytes
Muscle cells, or myocytes, in adult humans do not have the ability to divide by mitosis. However, there are stem cells within skeletal and smooth muscles that can proliferate in response to damage.
Pericytes and smooth muscle cells (SMCs) originate from a common endothelial and mesenchymal cell precursor, the mesenchymoangioblast (MB). MBs differentiate into primitive PDGFRβ+ CD271+CD73− mesenchymal progenitors, which give rise to proliferative pericytes, SMCs, and mesenchymal stem/stromal cells. MB-derived pericytes can be further specified into CD274+ capillary and DLK1+ arteriolar pericytes with a proinflammatory and contractile phenotype, respectively.
The embryonic heart appears to have developed an efficient method of distributing smooth muscle progenitors. Pericytes in the heart travel very little along a vessel once they establish their location during development. Studies in the adult heart and other adult tissues have found that perivascular cells are a heterogeneous population with mesenchymal stem cell properties that can contribute to tissue fibrosis.
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Cardiac muscle cells cannot divide on their own
Muscle cells in adult humans cannot divide by mitosis and therefore cannot repair themselves. Most muscle cells lose their ability to divide after maturation. However, in cases of injury or disease, lost cells may be replaced. For example, there are stem cells within skeletal and smooth muscles that proliferate in response to damage.
Cardiac muscle cells, however, cannot divide on their own. After birth, cardiomyocytes (heart muscle cells) lose their ability to divide. This is due to the suppression of mitotic activity in these highly differentiated cells. The division and proliferation of ventricular heart muscle cells in adult mammals are not activated after injury, such as a myocardial infarction. This inability to divide and multiply means that the human heart is unable to produce new heart cells to replace those lost after a heart attack.
Scientists have been trying to find a way to get human adult cells to divide and regenerate tissue, inspired by organisms such as fish and salamanders that possess this ability. A team has developed a method to make adult cells divide and repair hearts in animal models. Additionally, studies have shown that bone marrow stem cells can be used to regenerate damaged heart tissue.
Further research has identified four genes that, when combined, cause mature cardiomyocytes to re-enter the cell cycle, resulting in cell division and rapid reproduction. This technique has been tested in animal models and cardiomyocytes derived from human stem cells, showing promise for cardiac regeneration in the future.
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Muscle fibres can lay down new protein and enlarge
Muscle cells, or myocytes, in adult humans are unable to divide by mitosis and therefore cannot repair themselves. Their cell cycle remains in the interphase. However, muscle fibres can lay down new protein and enlarge. This is because there are stem cells within skeletal and smooth muscles that proliferate in response to damage. For example, each skeletal muscle fibre arises from the fusion of multiple myoblasts during embryonic development. Similarly, new smooth fibres can arise from pericytes, stem cells present in small veins and blood capillaries.
In the case of injury or disease, muscle cells that are lost may be replaced by new cells. This is possible because, unlike neurons, muscle cells do not permanently lose their ability to divide. While they lose this ability after maturation, they can still divide in certain cases, such as disease or damage.
The formation of muscle fibres occurs through the successive fusion of mononucleated myoblasts into ribbon-like cells, containing hundreds of nuclei. After this fusion, the nuclei within the fibres do not synthesise DNA or undergo mitosis. Instead, they are restricted in RNA synthesis.
Overall, while muscle cells in adult humans cannot divide by mitosis under normal circumstances, they can lay down new protein and enlarge through the action of stem cells and the division of muscle cells in specific cases such as disease or damage.
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Frequently asked questions
No, muscle cells in adult humans do not have the ability to divide by mitosis.
Muscle cells lose their ability to divide after maturation.
While muscle cells cannot repair themselves, there are stem cells within skeletal and smooth muscles that proliferate in response to damage.
