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60-400: A multitude of functions can be performed by the cytoskeleton. Its primary function is to give the cell its shape and mechanical resistance to deformation, and through association with extracellular connective tissue and other cells it stabilizes entire tissues. The cytoskeleton can also contract, thereby deforming the cell and the cell's environment and allowing cells to migrate . Moreover, it

120-452: A WACA-proteins, which are mostly found in prokaryotes, is MinD . Examples for intermediate filaments, which have almost exclusively been found in animals (i.e. eukaryotes) are the lamins , keratins , vimentin , neurofilaments , and desmin . Although tubulin-like proteins share some amino acid sequence similarity, their equivalence in protein-fold and the similarity in the GTP binding site

180-454: A localized attachment site for other proteins , and preventing the diffusion of certain molecules from one cell compartment to another. In yeast cells, they build scaffolding to provide structural support during cell division and compartmentalize parts of the cell. Recent research in human cells suggests that septins build cages around bacterial pathogens, immobilizing the harmful microbes and preventing them from invading other cells. Spectrin

240-452: A role of microtubule vibrations in neurons in the origin of consciousness . Accessory proteins including motor proteins regulate and link the filaments to other cell compounds and each other and are essential for controlled assembly of cytoskeletal filaments in particular locations. A number of small-molecule cytoskeletal drugs have been discovered that interact with actin and microtubules. These compounds have proven useful in studying

300-467: A role). This generates forces, which play an important role in informing the cell of its microenvironment. Specifically, forces such as tension, stiffness, and shear forces have all been shown to influence cell fate, differentiation, migration, and motility. Through a process called “mechanotransduction,” the cell remodels its cytoskeleton to sense and respond to these forces. Mechanotransduction relies heavily on focal adhesions , which essentially connect

360-400: A very dynamic behavior, binding GTP for polymerization. They are commonly organized by the centrosome . In nine triplet sets (star-shaped), they form the centrioles , and in nine doublets oriented about two additional microtubules (wheel-shaped), they form cilia and flagella. The latter formation is commonly referred to as a "9+2" arrangement, wherein each doublet is connected to another by

420-415: Is a cytoskeletal protein that lines the intracellular side of the plasma membrane in eukaryotic cells. Spectrin forms pentagonal or hexagonal arrangements, forming a scaffolding and playing an important role in maintenance of plasma membrane integrity and cytoskeletal structure. In budding yeast (an important model organism ), actin forms cortical patches, actin cables, and a cytokinetic ring and

480-464: Is a major functional component of tendons , ligaments and aponeuroses , and is also found in highly specialized organs such as the cornea . Elastic fibers , made from elastin and fibrillin , also provide resistance to stretch forces. They are found in the walls of large blood vessels and in certain ligaments, particularly in the ligamenta flava . In hematopoietic and lymphatic tissues, reticular fibers made by reticular cells provide

540-676: Is a mix of fibrous and areolar tissue . Fibromuscular tissue is made up of fibrous tissue and muscular tissue . New vascularised connective tissue that forms in the process of wound healing is termed granulation tissue . All of the special connective tissue types have been included as a subset of fascia in the fascial system , with blood and lymph classed as liquid fascia . Bone and cartilage can be further classified as supportive connective tissue . Blood and lymph can also be categorized as fluid connective tissue , and liquid fascia . Membranes can be either of connective tissue or epithelial tissue . Connective tissue membranes include

600-459: Is characterized by collagen fibers arranged in an orderly parallel fashion, giving it tensile strength in one direction. Dense irregular connective tissue provides strength in multiple directions by its dense bundles of fibers arranged in all directions. Special connective tissue consists of cartilage , bone , blood and lymph . Other kinds of connective tissues include fibrous, elastic, and lymphoid connective tissues. Fibroareolar tissue

660-516: Is currently unclear. Additionally, curvature could be described by the displacement of crescentic filaments, after the disruption of peptidoglycan synthesis. The cytoskeleton is a highly anisotropic and dynamic network, constantly remodeling itself in response to the changing cellular microenvironment. The network influences cell mechanics and dynamics by differentially polymerizing and depolymerizing its constituent filaments (primarily actin and myosin, but microtubules and intermediate filaments also play

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720-404: Is involved in many cell signaling pathways and in the uptake of extracellular material ( endocytosis ), the segregation of chromosomes during cellular division , the cytokinesis stage of cell division, as scaffolding to organize the contents of the cell in space and in intracellular transport (for example, the movement of vesicles and organelles within the cell) and can be a template for

780-489: Is mediated in eukaryotes by actin, but in prokaryotes usually by tubulin-like (often FtsZ-ring) proteins and sometimes ( Thermoproteota ) ESCRT-III , which in eukaryotes still has a role in the last step of division. Cytoplasmic streaming , also known as cyclosis, is the active movement of a cell's contents along the components of the cytoskeleton. While mainly seen in plants, all cell types use this process for transportation of waste, nutrients, and organelles to other parts of

840-476: Is more striking. The same holds true for the actin-like proteins and their structure and ATP binding domain. Cytoskeletal proteins are usually correlated with cell shape, DNA segregation and cell division in prokaryotes and eukaryotes. Which proteins fulfill which task is very different. For example, DNA segregation in all eukaryotes happens through use of tubulin, but in prokaryotes either WACA proteins, actin-like or tubulin-like proteins can be used. Cell division

900-708: Is one of the four primary types of animal tissue , along with epithelial tissue , muscle tissue , and nervous tissue . It develops mostly from the mesenchyme , derived from the mesoderm , the middle embryonic germ layer . Connective tissue is found in between other tissues everywhere in the body, including the nervous system . The three meninges , membranes that envelop the brain and spinal cord , are composed of connective tissue. Most types of connective tissue consists of three main components: elastic and collagen fibers , ground substance , and cells . Blood , and lymph are classed as specialized fluid connective tissues that do not contain fiber. All are immersed in

960-463: The Rho family of small GTP-binding proteins such as Rho itself for contractile acto-myosin filaments ("stress fibers"), Rac for lamellipodia and Cdc42 for filopodia. Functions include: Intermediate filaments are a part of the cytoskeleton of many eukaryotic cells. These filaments, averaging 10 nanometers in diameter, are more stable (strongly bound) than microfilaments, and heterogeneous constituents of

1020-626: The body water . The cells of connective tissue include fibroblasts , adipocytes , macrophages , mast cells and leukocytes . The term "connective tissue" (in German, Bindegewebe ) was introduced in 1830 by Johannes Peter Müller . The tissue was already recognized as a distinct class in the 18th century. Connective tissue can be broadly classified into connective tissue proper, and special connective tissue. Connective tissue proper includes loose connective tissue, and dense connective tissue. Loose and dense connective tissue are distinguished by

1080-509: The fruit fly do not have any cytoplasmic intermediate filaments. In those animals that express cytoplasmic intermediate filaments, these are tissue specific. Keratin intermediate filaments in epithelial cells provide protection for different mechanical stresses the skin may endure. They also provide protection for organs against metabolic, oxidative, and chemical stresses. Strengthening of epithelial cells with these intermediate filaments may prevent onset of apoptosis , or cell death, by reducing

1140-662: The meninges (the three membranes covering the brain and spinal cord ) and synovial membranes that line joint cavities. Mucous membranes and serous membranes are epithelial with an underlying layer of loose connective tissue. Fiber types found in the extracellular matrix are collagen fibers , elastic fibers , and reticular fibers . Ground substance is a clear, colorless, and viscous fluid containing glycosaminoglycans and proteoglycans allowing fixation of Collagen fibers in intercellular spaces. Examples of non-fibrous connective tissue include adipose tissue (fat) and blood . Adipose tissue gives "mechanical cushioning" to

1200-461: The sarcoplasmic reticulum . Increases in calcium in the cytosol allows muscle contraction to begin with the help of two proteins, tropomyosin and troponin . Tropomyosin inhibits the interaction between actin and myosin, while troponin senses the increase in calcium and releases the inhibition. This action contracts the muscle cell, and through the synchronous process in many muscle cells, the entire muscle. In 1903, Nikolai K. Koltsov proposed that

1260-420: The stroma —or structural support—for the parenchyma (that is, the bulk of functional substance) of the organ. Mesenchyme is a type of connective tissue found in developing organs of embryos that is capable of differentiation into all types of mature connective tissue. Another type of relatively undifferentiated connective tissue is the mucous connective tissue known as Wharton's jelly , found inside

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1320-486: The umbilical cord . This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body. Various types of specialized tissues and cells are classified under the spectrum of connective tissue, and are as diverse as brown and white adipose tissue , blood , cartilage and bone . Cells of the immune system—such as macrophages , mast cells , plasma cells , and eosinophils —are found scattered in loose connective tissue, providing

1380-503: The 'excluded volume' of a molecule is the volume that is inaccessible to other molecules in the system as a result of the presence of the first molecule. The excluded volume of a hard sphere is eight times its volume—however, for a two-molecule system, this volume is distributed among the two particles, giving the conventional result of four times the volume; this is an important quantity in the Van der Waals equation of state . The calculation of

1440-423: The bacterial cytoskeleton may not have been identified as of yet. FtsZ was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of guanosine triphosphate (GTP), but these filaments do not group into tubules. During cell division , FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize

1500-452: The body, among other functions. Although there is no dense collagen network in adipose tissue, groups of adipose cells are kept together by collagen fibers and collagen sheets in order to keep fat tissue under compression in place (for example, the sole of the foot). Both the ground substance and proteins (fibers) create the matrix for connective tissue. Type I collagen is present in many forms of connective tissue, and makes up about 25% of

1560-463: The cap. Cortical patches are discrete actin bodies on the membrane and are vital for endocytosis , especially the recycling of glucan synthase which is important for cell wall synthesis. Actin cables are bundles of actin filaments and are involved in the transport of vesicles towards the cap (which contains a number of different proteins to polarize cell growth) and in the positioning of mitochondria. The cytokinetic ring forms and constricts around

1620-641: The cell to communicate through the desmosome of multiple cells to adjust structures of the tissue based on signals from the cells environment. Mutations in the IF proteins have been shown to cause serious medical issues such as premature aging, desmin mutations compromising organs, Alexander Disease , and muscular dystrophy . Different intermediate filaments are: Microtubules are hollow cylinders about 23 nm in diameter (lumen diameter of approximately 15 nm), most commonly comprising 13 protofilaments that, in turn, are polymers of alpha and beta tubulin . They have

1680-463: The cell.  Plant and algae cells are generally larger than many other cells; so cytoplasmic streaming is important in these types of cells. This is because the cell's extra volume requires cytoplasmic streaming in order to move organelles throughout the entire cell. Organelles move along microfilaments in the cytoskeleton driven by myosin motors binding and pushing along actin filament bundles.  Connective tissue Connective tissue

1740-408: The cell; processing signals from the other cells or the fluids surrounding it. Additionally, the microtubules control the beating (movement) of the cilia and flagella. Also, the dynein arms attached to the microtubules function as the molecular motors. The motion of the cilia and flagella is created by the microtubules sliding past one another, which requires ATP. They play key roles in: In addition to

1800-450: The construction of a cell wall . Furthermore, it can form specialized structures, such as flagella , cilia , lamellipodia and podosomes . The structure, function and dynamic behavior of the cytoskeleton can be very different, depending on organism and cell type. Even within one cell, the cytoskeleton can change through association with other proteins and the previous history of the network. A large-scale example of an action performed by

1860-660: The cytoplasm to another. Thus, it is necessary to have the cytoskeleton to organize the polymers and ensure that they can effectively communicate across the entirety of the cell. By definition, the cytoskeleton is composed of proteins that can form longitudinal arrays (fibres) in all organisms. These filament forming proteins have been classified into 4 classes. Tubulin -like, actin -like, Walker A cytoskeletal ATPases (WACA-proteins), and intermediate filaments . Tubulin-like proteins are tubulin in eukaryotes and FtsZ , TubZ, RepX in prokaryotes. Actin-like proteins are actin in eukaryotes and MreB , FtsA in prokaryotes. An example of

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1920-576: The cytoskeleton is muscle contraction . This is carried out by groups of highly specialized cells working together. A main component in the cytoskeleton that helps show the true function of this muscle contraction is the microfilament . Microfilaments are composed of the most abundant cellular protein known as actin. During contraction of a muscle , within each muscle cell, myosin molecular motors collectively exert forces on parallel actin filaments. Muscle contraction starts from nerve impulses which then causes increased amounts of calcium to be released from

1980-408: The cytoskeleton is affected in these diseases. Parkinson's disease is marked by the degradation of neurons, resulting in tremors, rigidity, and other non-motor symptoms. Research has shown that microtubule assembly and stability in the cytoskeleton is compromised causing the neurons to degrade over time. In Alzheimer's disease, tau proteins which stabilize microtubules malfunction in the progression of

2040-509: The cytoskeleton will induce a more significant response. In this way, the anisotropy of the cytoskeleton serves to more keenly direct cell responses to intra or extracellular signals. The specific pathways and mechanisms by which the cytoskeleton senses and responds to forces are still under investigation. However, the long-range order generated by the cytoskeleton is known to contribute to mechanotransduction. Cells, which are around 10–50 μm in diameter, are several thousand times larger than

2100-407: The cytoskeleton, and several have clinical applications. Microfilaments, also known as actin filaments, are composed of linear polymers of G-actin proteins, and generate force when the growing (plus) end of the filament pushes against a barrier, such as the cell membrane. They also act as tracks for the movement of myosin molecules that affix to the microfilament and "walk" along them. In general,

2160-554: The cytoskeleton. Like actin filaments, they function in the maintenance of cell-shape by bearing tension ( microtubules , by contrast, resist compression but can also bear tension during mitosis and during the positioning of the centrosome). Intermediate filaments organize the internal tridimensional structure of the cell, anchoring organelles and serving as structural components of the nuclear lamina . They also participate in some cell-cell and cell-matrix junctions. Nuclear lamina exist in all animals and all tissues. Some animals like

2220-536: The day. It also led to the concept of the theta point , the set of conditions at which an experiment can be conducted that causes the excluded volume effect to be neutralized. At the theta point, the chain reverts to ideal chain characteristics. The long-range interactions arising from excluded volume are eliminated, allowing the experimenter to more easily measure short-range features such as structural geometry, bond rotation potentials, and steric interactions between near-neighboring groups. Flory correctly identified that

2280-407: The dividing daughter cells by a mechanism analogous to that used by microtubules during eukaryotic mitosis . The bacterium Caulobacter crescentus contains a third protein, crescentin , that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and vibrioid forms of bacteria, but the mechanism by which it does this

2340-472: The eukaryotic cytoskeleton have been found in prokaryotes . Harold Erickson notes that before 1992, only eukaryotes were believed to have cytoskeleton components. However, research in the early '90s suggested that bacteria and archaea had homologues of actin and tubulin, and that these were the basis of eukaryotic microtubules and microfilaments. Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone,

2400-411: The excluded volume for particles with non-spherical shapes is usually difficult, since it depends on the relative orientation of the particles. The distance of closest approach of hard ellipses and their excluded area has been recently considered. In polymer science, excluded volume refers to the idea that one part of a long chain molecule can not occupy space that is already occupied by another part of

2460-412: The ground for starting inflammatory and immune responses upon the detection of antigens . There are many types of connective tissue disorders, such as: Excluded volume The concept of excluded volume was introduced by Werner Kuhn in 1934 and applied to polymer molecules shortly thereafter by Paul Flory . Excluded volume gives rise to depletion forces . In liquid state theory,

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2520-480: The illness causing pathology of the cytoskeleton. Excess glutamine in the Huntington protein involved with linking vesicles onto the cytoskeleton is also proposed to be a factor in the development of Huntington's Disease. Amyotrophic lateral sclerosis results in a loss of movement caused by the degradation of motor neurons, and also involves defects of the cytoskeleton. Stuart Hameroff and Roger Penrose suggest

2580-438: The intermediate filaments are known as neurofilaments . Each type is formed by the polymerization of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution. Microfilaments are polymers of the protein actin and are 7 nm in diameter. Microtubules are composed of tubulin and are 25 nm in diameter. Intermediate filaments are composed of various proteins, depending on

2640-491: The intracellular cytoskeleton with the extracellular matrix (ECM). Through focal adhesions, the cell is able to integrate extracellular forces into intracellular ones as the proteins present at focal adhesions undergo conformational changes to initiate signaling cascades. Proteins such as focal adhesion kinase (FAK) and Src have been shown to transduce force signals in response to cellular activities such as proliferation and differentiation, and are hypothesized to be key sensors in

2700-456: The major component or protein of microfilaments are actin. The G-actin monomer combines to form a polymer which continues to form the microfilament (actin filament). These subunits then assemble into two chains that intertwine into what are called F-actin chains. Myosin motoring along F-actin filaments generates contractile forces in so-called actomyosin fibers, both in muscle as well as most non-muscle cell types. Actin structures are controlled by

2760-427: The mechanical properties of cells determine how far and where, directionally, a force will propagate throughout the cell and how it will change cell dynamics. A membrane protein that is not closely coupled to the cytoskeleton, for instance, will not produce a significant effect on the cortical actin network if it is subjected to a specifically directed force. However, membrane proteins that are more closely associated with

2820-505: The mechanotransduction pathway. As a result of mechanotransduction, the cytoskeleton changes its composition and/or orientation to accommodate the force stimulus and ensure the cell responds accordingly. The cytoskeleton changes the mechanics of the cell in response to detected forces. For example, increasing tension within the plasma membrane makes it more likely that ion channels will open, which increases ion conductance and makes cellular change ion influx or efflux much more likely. Moreover,

2880-471: The molecules found within the cytoplasm that are essential to coordinate cellular activities. Because cells are so large in comparison to essential biomolecules, it is difficult, in the absence of an organizing network, for different parts of the cytoplasm to communicate. Moreover, biomolecules must polymerize to lengths comparable to the length of the cell, but resulting polymers can be highly disorganized and unable to effectively transmit signals from one part of

2940-567: The new cell wall between the dividing cells. Prokaryotic actin-like proteins, such as MreB , are involved in the maintenance of cell shape. All non-spherical bacteria have genes encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis . Some plasmids encode a separate system that involves an actin-like protein ParM . Filaments of ParM exhibit dynamic instability , and may partition plasmid DNA into

3000-432: The probability of stress. Intermediate filaments are most commonly known as the support system or "scaffolding" for the cell and nucleus while also playing a role in some cell functions. In combination with proteins and desmosomes , the intermediate filaments form cell-cell connections and anchor the cell-matrix junctions that are used in messaging between cells as well as vital functions of the cell. These connections allow

3060-424: The protein dynein . As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton. There are two types of cilia: motile and non-motile cilia. Cilia are short and more numerous than flagella. The motile cilia have a rhythmic waving or beating motion compared to the non-motile cilia which receive sensory information for

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3120-513: The purpose of the cytoskeleton and its components. Initially, it was thought that the cytoskeleton was exclusive to eukaryotes but in 1992 it was discovered to be present in prokaryotes as well. This discovery came after the realization that bacteria possess proteins that are homologous to tubulin and actin; the main components of the eukaryotic cytoskeleton. Eukaryotic cells contain three main kinds of cytoskeletal filaments: microfilaments , microtubules , and intermediate filaments . In neurons

3180-511: The ratio of ground substance to fibrous tissue. Loose connective tissue has much more ground substance and a relative lack of fibrous tissue, while the reverse is true of dense connective tissue. Loose connective tissue includes reticular connective tissue , and adipose tissue . Dense connective tissue also known as fibrous tissue is subdivided into dense regular and dense irregular connective tissue . Dense regular connective tissue, found in structures such as tendons and ligaments ,

3240-429: The roles described above, Stuart Hameroff and Roger Penrose have proposed that microtubules function in consciousness. Septins are a group of the highly conserved GTP binding proteins found in eukaryotes . Different septins form protein complexes with each other. These can assemble to filaments and rings. Therefore, septins can be considered part of the cytoskeleton. The function of septins in cells include serving as

3300-432: The same molecule. Excluded volume causes the ends of a polymer chain in a solution to be further apart (on average) than they would be were there no excluded volume (e.g. in case of ideal chain model). The recognition that excluded volume was an important factor in analyzing long-chain molecules in solutions provided an important conceptual breakthrough, and led to the explanation of several puzzling experimental results of

3360-570: The shape of cells was determined by a network of tubules that he termed the cytoskeleton. The concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was proposed by Rudolph Peters in 1929 while the term ( cytosquelette , in French) was first introduced by French embryologist Paul Wintrebert in 1931. When the cytoskeleton was first introduced, it was thought to be an uninteresting gel-like substance that helped organelles stay in place. Much research took place to try to understand

3420-438: The similarity of their three-dimensional structures and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous. Three laboratories independently discovered that FtsZ, a protein already known as a key player in bacterial cytokinesis, had the "tubulin signature sequence" present in all α-, β-, and γ-tubulins. However, some structures in

3480-419: The site of cell division . Prior to the work of Jones et al., 2001, the cell wall was believed to be the deciding factor for many bacterial cell shapes, including rods and spirals. When studied, many misshapen bacteria were found to have mutations linked to development of a cell envelope . The cytoskeleton was once thought to be a feature only of eukaryotic cells, but homologues to all the major proteins of

3540-612: The total protein content of the mammalian body. Connective tissue has a wide variety of functions that depend on the types of cells and the different classes of fibers involved. Loose and dense irregular connective tissue , formed mainly by fibroblasts and collagen fibers , have an important role in providing a medium for oxygen and nutrients to diffuse from capillaries to cells, and carbon dioxide and waste substances to diffuse from cells back into circulation. They also allow organs to resist stretching and tearing forces. Dense regular connective tissue , which forms organized structures,

3600-546: The type of cell in which they are found; they are normally 8-12 nm in diameter. The cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol , it adds to the level of macromolecular crowding in this compartment. Cytoskeletal elements interact extensively and intimately with cellular membranes. Research into neurodegenerative disorders such as Parkinson's disease , Alzheimer's disease , Huntington's disease , and amyotrophic lateral sclerosis (ALS) indicate that

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