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Smooth muscle is one of the three types of muscle tissue, the other two being skeletal and cardiac. Smooth muscle is stiffer than its adjacent epithelium and often serves its morphogenetic function by physically constraining the growth of a proliferating epithelial layer. Smooth muscle is made up of cells that contain a single central nucleus and are connected by specialized cell junctions called gap junctions. They are located in the walls of hollow visceral organs, like the liver, pancreas, and intestines, and contribute to digestive mobility. Smooth muscle is also present in the lungs and oviducts of mammals, playing a role in epithelial bifurcation during branching morphogenesis.
| Characteristics | Values |
|---|---|
| Muscle tissue type | Smooth muscle |
| Muscle cell shape | Spindle-shaped |
| Muscle cell structure | Single central nucleus |
| Muscle cell junctions | Gap junctions |
| Muscle cell appearance | Non-striated |
| Muscle cell composition | Actin and myosin fibres |
| Muscle function | Involuntary |
| Muscle location | Walls of hollow visceral organs (e.g. liver, pancreas, intestines) |
| Muscle layers | Inner circular layer, outer longitudinal layer |
| Muscle stiffness | Higher stiffness than adjacent epithelium |
| Muscle role in epithelial morphogenesis | Constrains growth of epithelial layer, leading to mechanical instabilities and epithelial folding |
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What You'll Learn
Smooth muscle is stiffer than its adjacent epithelium
Smooth muscle is a mesenchymal tissue that surrounds the epithelia of organs, including the gut, blood vessels, lungs, bladder, ureter, uterus, oviduct, and epididymis. It is made up of cells that contain a single central nucleus, which stick together and are connected by specialised cell junctions, called gap junctions. The cells are spindle-shaped, and the nucleus is central.
Computational models have suggested that epithelial folding can be driven by a stiff, static smooth muscle layer surrounding the growing epithelium. Therefore, the feature of smooth muscle most critical for directing epithelial morphogenesis appears to be its high mechanical stiffness relative to the neighbouring epithelium.
In the adult chicken gut, the mucosa and its epithelium are folded into thousands of projections known as villi that increase the intestinal absorptive surface area. The mucosa is, in turn, surrounded by two major layers of smooth muscle that contribute to digestive mobility: an inner circular layer and an outer longitudinal layer. There is also a thin layer of smooth muscle known as the muscularis mucosa, which sits at the boundary of the mucosa and the submucosa and has less clear functional importance in the adult.
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Smooth muscle stiffness drives epithelial morphogenesis
Smooth muscle is a mesenchymal tissue that surrounds the epithelia of organs, including the gut, blood vessels, lungs, bladder, ureter, uterus, oviduct, and epididymis. It is made up of cells that contain a single central nucleus, with the cells sticking together and connected by specialised cell junctions called gap junctions. Smooth muscle is stiffer than its adjacent epithelium and often serves its morphogenetic function by physically constraining the growth of a proliferating epithelial layer. This constraint leads to mechanical instabilities and epithelial morphogenesis through buckling.
Smooth muscle stiffness, without smooth muscle cell shortening, is sufficient to drive epithelial morphogenesis. However, fully understanding the development of organs that use smooth muscle stiffness as a driver of morphogenesis requires investigating how smooth muscle develops. This includes distinguishing smooth muscle-like tissues from one another in vivo and in culture. Smooth muscle-like tissues include myofibroblasts and myoepithelial cells.
Computational models have suggested that epithelial folding can be driven by a stiff, static smooth muscle layer surrounding the growing epithelium. The contractile apparatus of smooth muscle consists of a meshwork of actin and myosin fibres that undergo cross-bridge cycling upon activation of the cell. This meshwork is interconnected with the cytoskeletal network, including intermediate filaments, cell-matrix, and cell-cell adhesions. The highly developed cytoskeletal network and their ability to maintain stiffness over a variety of cell lengths make smooth muscle cells well-suited for directing epithelial morphogenesis.
In the adult chicken gut, the mucosa and its epithelium are folded into thousands of projections known as villi, increasing the intestinal absorptive surface area. The mucosa is surrounded by two major layers of smooth muscle that contribute to digestive mobility: an inner circular layer and an outer longitudinal layer. Smooth muscle is critical for the morphogenesis of the gut in chicken and may play a secondary role in mice.
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Smooth muscle cells are spindle-shaped and contain a single central nucleus
Smooth muscle cells are spindle-shaped and contain a single, central nucleus. They range from 10 to 600 μm (micrometres) in length and are the smallest type of muscle cell. Smooth muscle cells are elastic and are responsible for involuntary contractions. They are found in the walls of blood vessels and hollow organs such as the gastrointestinal tract, uterus, bladder, and the eye. They are also present in the airways and reproductive systems.
Smooth muscle cells are not striated, unlike skeletal and cardiac muscle cells. This is because the actin and myosin in these muscles are more randomly arranged. In skeletal muscle, the highly regular configurations of actin and myosin cannot interact if the cell is stretched or compressed too far. The actin and myosin filaments in smooth muscle cells do not align in the cytoplasm, hence the name "smooth".
Smooth muscle cells are arranged together in sheets, allowing them to contract simultaneously. They have poorly developed sarcoplasmic reticulums and do not contain T-tubules due to their restricted size. However, they do contain other normal cell organelles such as sarcosomes, but in lower numbers.
The contractile apparatus of smooth muscle consists of a meshwork of actin and myosin fibres that undergo cross-bridge cycling upon activation of the cell. This meshwork is interconnected with the cytoskeletal network, including intermediate filaments, cell-matrix, and cell-cell adhesions. The highly developed cytoskeletal network and their ability to maintain stiffness over a variety of cell lengths make smooth muscle cells well-suited for directing epithelial morphogenesis.
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Smooth muscle is located in the walls of hollow visceral organs
Smooth muscle is a type of tissue found in the walls of hollow visceral organs, such as the liver, pancreas, stomach, bladder, uterus, and intestines. It is also present in the walls of passageways, including arteries and veins of the cardiovascular system, as well as in the tracts of the urinary, respiratory, and reproductive systems. Smooth muscle is derived from both mesoderm and neural crest cells, which contribute to its widespread presence throughout the body.
Smooth muscle has a distinct structure, consisting of thick and thin filaments that are not arranged into sarcomeres, resulting in a non-striated pattern. It is composed of spindle-shaped cells that contain a single central nucleus and are connected by specialized cell junctions called gap junctions. The actin and myosin filaments in smooth muscle are more randomly arranged compared to skeletal muscle, allowing for interaction and contraction over a range of cell lengths. This structural arrangement contributes to the ability of smooth muscle to maintain stiffness and direct epithelial morphogenesis.
The presence of smooth muscle in hollow visceral organs serves various functions. For example, in the gastrointestinal tract, it aids in the propulsion of food. In the cardiovascular system, smooth muscle helps regulate blood flow and pressure by controlling the diameter of blood vessels. Smooth muscle can also be found in the ducts of exocrine glands, where it plays a role in sealing orifices or transporting substances through wavelike contractions.
The contractile apparatus of smooth muscle consists of a meshwork of actin and myosin fibres that undergo cross-bridge cycling upon activation. This meshwork is interconnected with the cytoskeletal network, including intermediate filaments, cell-matrix, and cell-cell adhesions. The calcium-containing sarcoplasmic reticulum is another important structure that aids in sustaining contraction. Smooth muscle cells contract slower than skeletal muscle cells but are stronger, more sustained, and require less energy.
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Smooth muscle is a sculptor of epithelial shapes
Smooth muscle is increasingly recognized as a key mechanical sculptor of epithelia during embryonic development. Smooth muscle is a mesenchymal tissue that surrounds the epithelia of organs, including the gut, blood vessels, lungs, bladder, ureter, uterus, oviduct, and epididymis. Smooth muscle is stiffer than its adjacent epithelium and often serves its morphogenetic function by physically constraining the growth of a proliferating epithelial layer. This constraint leads to mechanical instabilities and epithelial morphogenesis through buckling.
Smooth muscle stiffness alone, without smooth muscle cell shortening, seems to be sufficient to drive epithelial morphogenesis. Fully understanding the development of organs that use smooth muscle stiffness as a driver of morphogenesis requires investigating how smooth muscle develops, a key aspect of which is distinguishing smooth muscle-like tissues from one another in vivo and in culture. This necessitates a comprehensive appreciation of the genetic, anatomical, and functional markers that are used to distinguish the different subtypes of smooth muscle (for example, vascular versus visceral) from similar cell types (including myofibroblasts and myoepithelial cells).
The contractile apparatus of smooth muscle consists of a meshwork of actin and myosin fibres that undergo cross-bridge cycling upon activation of the cell. This meshwork is interconnected with the cytoskeletal network, including intermediate filaments, cell–matrix, and cell–cell adhesions. The highly developed cytoskeletal network and their ability to maintain stiffness over a variety of cell lengths make smooth muscle cells especially well poised for directing epithelial morphogenesis.
In the adult chicken gut, the mucosa and its epithelium are folded into thousands of projections known as villi that increase the intestinal absorptive surface area. The mucosa is, in turn, surrounded by two major layers of smooth muscle that contribute to digestive mobility: an inner circular layer and an outer longitudinal layer. There is also a thin layer of smooth muscle known as the muscularis mucosa, which sits at the boundary of the mucosa and the submucosa and has less clear functional importance in the adult. This anatomy is similar to that of the mammalian gut.
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Frequently asked questions
Smooth muscle is stiffer than its adjacent epithelium and often serves its morphogenetic function by physically constraining the growth of a proliferating epithelial layer. This constraint leads to mechanical instabilities and epithelial morphogenesis through buckling.
Smooth muscle is made up of cells that contain a single central nucleus. The cells stick together and are connected by specialized cell junctions, called gap junctions. The cells are spindle-shaped, and the nucleus is central. Smooth muscle is located in the walls of hollow visceral organs (such as the liver, pancreas, and intestines), except the heart, and is under involuntary control.
The highly developed cytoskeletal network and their ability to maintain stiffness over a variety of cell lengths make smooth muscle cells well-suited for directing epithelial morphogenesis. Smooth muscle stiffness alone seems to be sufficient to drive epithelial morphogenesis.
