Information

18.2: Cytoskeletal Components - Biology

18.2: Cytoskeletal Components - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Most eukaryotic cells look like a membrane-bound sac of cytoplasm containing a nucleus and assorted organelles in a light microscope. In the late 19th century, microscopists described a dramatic structural re-organization of dividing cells. In mitosis, duplicated chromosomes (i.e., chromatids) condense in the nucleus just as the nuclear membrane dissolves. Spindle fibers emerge and then seem to pull the chromatids apart to opposite poles of the cell. Spindle fibers turn out to be bundles of microtubules, each of which is a polymer of tubulin proteins. Let’s look below at that fluorescence micrograph of a mitosing metaphase cell again; most of the cell other than what is fluorescing is not visible in the micrograph.

To get this image, antibodies were made against purified microtubule, kinetochore and chromosomal proteins (or DNA), and then linked to different fluorophores (organic molecular fluorescent tags). When the fluorophores were added to dividing cells in metaphase, they bound to their respective fibers. Upon UV light irradiation, the fluorophores emit different colors of visible light, visible in a fluorescence microscope. Microtubules are green, metaphase chromosomes are blue and kinetochores are red in the micrograph.

Both mitosis and meiosis are very visible examples of movements within cells, both already described by the late 19th century. As for movement in whole organisms, mid20th century studies focused on what the striations (or stripes) seen in skeletal muscle in the light microscope might have to do with muscle contraction. The striations turned out to be composed of a protein complex originally named actomyosin (acto for active; myosin for muscle). Electron microscopy later revealed that actomyosin (or actinomyosin) is composed of thin filaments (actin) and thick filaments (myosin) that slide past one another during muscle contraction.

Electron microscopy also hinted at a more complex cytoplasmic structure of cells in general. The cytoskeleton consists of fine rods and tubes in more or less organized states that permeate the cell. The most abundant of these cytoskeletal components are microfilaments, microtubules and intermediate filaments. But, even myosin is present in non-muscle cells, albeit at relatively low concentrations. Microtubules account for chromosome movements of mitosis and meiosis, while together with microfilaments (i.e., actin), they enable organelle movement inside cells (you may have seen cytoplasmic streaming of Elodea chloroplasts in a biology lab exercise). Microtubules also underlie cilia- and flagella-based motility of whole cells such as paramecium, amoeba, phagocytes, etc., while actin microfilaments and myosin enable muscle and thus higher animal movement! Finally, the cytoskeleton is a dynamic structure. Its fibers not only account for the movements of cell division, but they also give cells mechanical strength and unique shapes. All of the fibers can disassemble, reassemble and rearrange, allowing cells to change shape, for example, creating pseudopods in amoeboid cells and spindle fibers of mitosis and meiosis. In this chapter we look at the molecular basis of cell structure and different forms of cell motility.


All cells have a cytoskeleton, but usually the cytoskeleton of eukaryotic cells is what is meant when discussing the cytoskeleton. Eukaryotic cells are complex cells that have a nucleus and organelles. Plants, animals, fungi, and protists have eukaryotic cells. Prokaryotic cells are less complex, with no true nucleus or organelles except ribosomes, and they are found in the single-celled organisms bacteria and archaea. The cytoskeleton of prokaryotic cells was originally thought not to exist it was not discovered until the early 1990s.

The eukaryotic cytoskeleton consists of three types of filaments, which are elongated chains of proteins: microfilaments, intermediate filaments, and microtubules.


The microfilaments of this cell are shown in red, while microtubules are shown in green. The blue dots are nuclei.

Microfilaments

Microfilaments are also called actin filaments because they are mostly composed of the protein actin their structure is two strands of actin wound in a spiral. They are about 7 nanometers thick, making them the thinnest filaments in the cytoskeleton. Microfilaments have many functions. They aid in cytokinesis, which is the division of a cytoplasm of a cell when it is dividing into two daughter cells. They aid in cell motility and allow single-celled organisms like amoebas to move. They are also involved in cytoplasmic streaming, which is the flowing of cytosol (the liquid part of the cytoplasm) throughout the cell. Cytoplasmic streaming transports nutrients and cell organelles. Microfilaments are also part of muscle cells and allow these cells to contract, along with myosin. Actin and myosin are the two main components of muscle contractile elements.

Intermediate Filaments

Intermediate filaments are about 8-12 nm wide they are called intermediate because they are in-between the size of microfilaments and microtubules. Intermediate filaments are made of different proteins such as keratin (found in hair and nails, and also in animals with scales, horns, or hooves), vimentin, desmin, and lamin. All intermediate filaments are found in the cytoplasm except for lamins, which are found in the nucleus and help support the nuclear envelope that surrounds the nucleus. The intermediate filaments in the cytoplasm maintain the cell’s shape, bear tension, and provide structural support to the cell.

Microtubules

Microtubules are the largest of the cytoskeleton’s fibers at about 23 nm. They are hollow tubes made of alpha and beta tubulin. Microtubules form structures like flagella, which are “tails” that propel a cell forward. They are also found in structures like cilia, which are appendages that increase a cell’s surface area and in some cases allow the cell to move. Most of the microtubules in an animal cell come from a cell organelle called the centrosome, which is a microtubule organizing center (MTOC). The centrosome is found near the middle of the cell, and microtubules radiate outward from it. Microtubules are important in forming the spindle apparatus (or mitotic spindle), which separates sister chromatids so that one copy can go to each daughter cell during cell division. They are also involved in transporting molecules within the cell and in the formation of the cell wall in plant cells.


Intermediate Filaments

Intermediate filaments are made of several strands of fibrous proteins that are wound together (Figure). These elements of the cytoskeleton get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules.

Intermediate filaments consist of several intertwined strands of fibrous proteins.

Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the shape of the cell, and anchor the nucleus and other organelles in place. Figure shows how intermediate filaments create a supportive scaffolding inside the cell.

The intermediate filaments are the most diverse group of cytoskeletal elements. Several types of fibrous proteins are found in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the epidermis of the skin.


Biology 171

By the end of this section, you will be able to do the following:

  • Describe the cytoskeleton
  • Compare the roles of microfilaments, intermediate filaments, and microtubules
  • Compare and contrast cilia and flagella
  • Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the cell’s shape, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, scientists call this network of protein fibers the cytoskeleton . There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules ((Figure)). Here, we will examine each.


Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are comprised of two globular protein intertwined strands, which we call actin ((Figure)). For this reason, we also call microfilaments actin filaments.


ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein we call myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells and cytoplasmic streaming, which is the cell cytoplasm’s circular movement in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to an infection site and phagocytize the pathogen.

To see an example of a white blood cell in action, watch white blood cell chases bacteria a short time-lapse of the cell capturing two bacteria. It engulfs one and then moves on to the other.

Intermediate Filaments

Several strands of fibrous proteins that are wound together comprise intermediate filaments ((Figure)). Cytoskeleton elements get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules.


Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cell’s shape, and anchor the nucleus and other organelles in place. (Figure) shows how intermediate filaments create a supportive scaffolding inside the cell.

The intermediate filaments are the most diverse group of cytoskeletal elements. Several fibrous protein types are in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the skin’s epidermis.

Microtubules

As their name implies, microtubules are small hollow tubes. Polymerized dimers of α-tubulin and β-tubulin, two globular proteins, comprise the microtubule’s walls ((Figure)). With a diameter of about 25 nm, microtubules are cytoskeletons’ widest components. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly.


Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosome’s two perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as we discuss below.

Flagella and Cilia

The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (for example, sperm, Euglena, and some prokaryotes). When present, the cell has just one flagellum or a few flagella. However, when cilia (singular = cilium) are present, many of them extend along the plasma membrane’s entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cell’s outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.)

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center ((Figure)).


You have now completed a broad survey of prokaryotic and eukaryotic cell components. For a summary of cellular components in prokaryotic and eukaryotic cells, see (Figure).

Components of Prokaryotic and Eukaryotic Cells
Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment controls passage of organic molecules, ions, water, oxygen, and wastes into and out of cell Yes Yes Yes
Cytoplasm Provides turgor pressure to plant cells as fluid inside the central vacuole site of many metabolic reactions medium in which organelles are found Yes Yes Yes
Nucleolus Darkened area within the nucleus where ribosomal subunits are synthesized. No Yes Yes
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidize and thus break down fatty acids and amino acids, and detoxify poisons No Yes Yes
Vesicles and vacuoles Storage and transport digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells microtubule source in animal cells No Yes No
Lysosomes Digestion of macromolecules recycling of worn-out organelles No Yes Some
Cell wall Protection, structural support, and maintenance of cell shape Yes, primarily peptidoglycan No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm cells
Cilia Cellular locomotion, movement of particles along plasma membrane’s extracellular surface, and filtration Some Some No

Section Summary

The cytoskeleton has three different protein element types. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Biologists often associate microfilaments with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia.

Free Response

What are the similarities and differences between the structures of centrioles and flagella?

Centrioles and flagella are alike in that they are made up of microtubules. In centrioles, two rings of nine microtubule “triplets” are arranged at right angles to one another. This arrangement does not occur in flagella.

How do cilia and flagella differ?

Cilia and flagella are alike in that they are made up of microtubules. Cilia are short, hair-like structures that exist in large numbers and usually cover the entire surface of the plasma membrane. Flagella, in contrast, are long, hair-like structures when flagella are present, a cell has just one or two.

Describe how microfilaments and microtubules are involved in the phagocytosis and destruction of a pathogen by a macrophage.

A macrophage engulfs a pathogen by rearranging its actin microfilaments to bend the plasma membrane around the pathogen. Once the pathogen is sealed in an endosome inside the macrophage, the vesicle is walked along microtubules until it combines with a lysosome to digest the pathogen.

Compare and contrast the boundaries that plant, animal, and bacteria cells use to separate themselves from their surrounding environment.

All three cell types have a plasma membrane that borders the cytoplasm on its interior side. In animal cells, the exterior side of the plasma membrane is in contact with the extracellular environment. However, in plant and bacteria cells, a cell wall surrounds the outside of the plasma membrane. In plants, the cell wall is made of cellulose, while in bacteria the cell wall is made of peptidoglycan. Gram-negative bacteria also have an additional capsule made of lipopolysaccharides that surrounds their cell wall.

Glossary


Intermediate Filaments and Microtubules

Microtubules are part of the cell’s cytoskeleton, helping the cell resist compression, move vesicles, and separate chromosomes at mitosis.

Learning Objectives

Describe the roles of microtubules as part of the cell’s cytoskeleton

Key Takeaways

Key Points

  • Microtubules help the cell resist compression, provide a track along which vesicles can move throughout the cell, and are the components of cilia and flagella.
  • Cilia and flagella are hair-like structures that assist with locomotion in some cells, as well as line various structures to trap particles.
  • The structures of cilia and flagella are a 𔄡+2 array,” meaning that a ring of nine microtubules is surrounded by two more microtubules.
  • Microtubules attach to replicated chromosomes during cell division and pull them apart to opposite ends of the pole, allowing the cell to divide with a complete set of chromosomes in each daughter cell.

Key Terms

  • microtubule: Small tubes made of protein and found in cells part of the cytoskeleton
  • flagellum: a flagellum is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells
  • cytoskeleton: A cellular structure like a skeleton, contained within the cytoplasm.

Microtubules

As their name implies, microtubules are small hollow tubes. Microtubules, along with microfilaments and intermediate filaments, come under the class of organelles known as the cytoskeleton. The cytoskeleton is the framework of the cell which forms the structural supporting component. Microtubules are the largest element of the cytoskeleton. The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins. With a diameter of about 25 nm, microtubules are the widest components of the cytoskeleton. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can dissolve and reform quickly.

Micrtubule Structure: Microtubules are hollow, with walls consisting of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the molecular structure of the tube.

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome ). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes.

Stained Keratin Intermediate filaments: Keratin cytoskeletal intermediate filaments are concentrated around the edge of the cells and merge into the surface membrane. This network of intermediate filaments from cell to cell holds together tissues like skin.

Intermediate Filaments

Intermediate filaments (IFs) are cytoskeletal components found in animal cells. They are composed of a family of related proteins sharing common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin (microfilaments), and that of 25 nm microtubules, although they were initially designated ‘intermediate’ because their average diameter is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells. Intermediate filaments contribute to cellular structural elements and are often crucial in holding together tissues like skin.

Flagella and Cilia

Flagella (singular = flagellum ) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. When cilia (singular = cilium) are present, however, many of them extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells (such as paramecia) or substances along the outer surface of the cell (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils).

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a 𔄡 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets surrounding a single microtubule doublet in the center.

Microtubules are the structural component of flagella: This transmission electron micrograph of two flagella shows the 9 + 2 array of microtubules: nine microtubule doublets surround a single microtubule doublet.


Other Cytoskeletal Components

The other two main components of the eukaryotic cytoskeleton are microfilaments and intermediate filaments. Microfilaments are smaller than microtubules at about 7 nm in diameter. They aid in the division of cytoplasm during cell division, and also have a role in cytoplasmic streaming, which is the flow of cytosol (cell fluid) throughout the cell. Intermediate filaments are bigger than microfilaments, but smaller than microtubules. They help give the cell its shape and provide structural support.


19 The Cytoskeleton

By the end of this section, you will be able to do the following:

  • Describe the cytoskeleton
  • Compare the roles of microfilaments, intermediate filaments, and microtubules
  • Compare and contrast cilia and flagella
  • Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the cell’s shape, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, scientists call this network of protein fibers the cytoskeleton . There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules ((Figure)). Here, we will examine each.


Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are comprised of two globular protein intertwined strands, which we call actin ((Figure)). For this reason, we also call microfilaments actin filaments.


ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein we call myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells and cytoplasmic streaming, which is the cell cytoplasm’s circular movement in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to an infection site and phagocytize the pathogen.

To see an example of a white blood cell in action, watch a short time-lapse video of the cell capturing two bacteria. It engulfs one and then moves on to the other.

Intermediate Filaments

Several strands of fibrous proteins that are wound together comprise intermediate filaments ((Figure)). Cytoskeleton elements get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules.


Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cell’s shape, and anchor the nucleus and other organelles in place. (Figure) shows how intermediate filaments create a supportive scaffolding inside the cell.

The intermediate filaments are the most diverse group of cytoskeletal elements. Several fibrous protein types are in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the skin’s epidermis.

Microtubules

As their name implies, microtubules are small hollow tubes. Polymerized dimers of α-tubulin and β-tubulin, two globular proteins, comprise the microtubule’s walls ((Figure)). With a diameter of about 25 nm, microtubules are cytoskeletons’ widest components. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly.


Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosome’s two perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as we discuss below.

Flagella and Cilia

The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (for example, sperm, Euglena, and some prokaryotes). When present, the cell has just one flagellum or a few flagella. However, when cilia (singular = cilium) are present, many of them extend along the plasma membrane’s entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cell’s outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.)

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center ((Figure)).


You have now completed a broad survey of prokaryotic and eukaryotic cell components. For a summary of cellular components in prokaryotic and eukaryotic cells, see (Figure).

Components of Prokaryotic and Eukaryotic Cells
Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment controls passage of organic molecules, ions, water, oxygen, and wastes into and out of cell Yes Yes Yes
Cytoplasm Provides turgor pressure to plant cells as fluid inside the central vacuole site of many metabolic reactions medium in which organelles are found Yes Yes Yes
Nucleolus Darkened area within the nucleus where ribosomal subunits are synthesized. No Yes Yes
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidize and thus break down fatty acids and amino acids, and detoxify poisons No Yes Yes
Vesicles and vacuoles Storage and transport digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells microtubule source in animal cells No Yes No
Lysosomes Digestion of macromolecules recycling of worn-out organelles No Yes Some
Cell wall Protection, structural support, and maintenance of cell shape Yes, primarily peptidoglycan No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm cells
Cilia Cellular locomotion, movement of particles along plasma membrane’s extracellular surface, and filtration Some Some No

Section Summary

The cytoskeleton has three different protein element types. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Biologists often associate microfilaments with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia.

Review Questions

Which of the following have the ability to disassemble and reform quickly?


1. Introduction

The cytoskeletal components of endothelial cells play important roles in maintaining the fundamental structure of the thin inner layer of the blood vessels called the endothelium. Endothelial cells form a single cell layer on the surfaces of blood vessels and are therefore constantly subjected to both fluid shear stress and periodic strain caused by blood pressure induced by the pulsatile flow. Endothelial cells have been shown to possess many stress fibers both in vitro and in situ, and these cells are known to be capable of responding to the level of fluid shear stress by changing their shape [1, 2], distribution of cytoskeletal components [3𠄵], expression of signal transduction-associated proteins [6, 7], and expression of various genes [8, 9]. Endothelial cells in culture are known to respond to cyclic stretching [10] and hyperosmotic shock [7]. On exposure to uniaxial cyclic stretching, endothelial cells become aligned perpendicular to the axis of stretching [10, 11]. Cyclic stretching applied to cells in culture is a model of pulsatile stretching induced by blood pressure in vivo, and the effects differ from those of fluid shear stress generated by the blood flow. Cyclic stretching is experienced by both the apical and basal portions of the cell. Hemodynamic shear stress caused by blood flow occurs in combination with cyclic stretching caused by the pressure of pulsatile flow generated by blood pressure.

Previous in vivo experiments indicated that stress fibers in endothelial cells respond to fluid shear stress and show increases in both number and thickness in a manner related to the magnitude of shear stress (for review, see Katoh et al., 2008) [12]. Previously, we reported increases in number and thickness of stress fibers and focal adhesions in both the apical and basal portions of endothelial cells in an artificial coarctation zone in the abdominal aorta where fluid shear stress is significantly high in comparison to endothelial cells subjected to averaged shear stress [6]. The plaque-like vinculin-containing spots detected at the ends of stress fibers were enlarged in the coarctation area, especially in the apical portions of the cells [6]. In addition, stress fibers and their sites of association with the plasma membrane are closely attached to both the apical and basal portions of endothelial cells, and we suggested that they may play key roles in force transfer by fluid stress [13, 14]. Our findings also suggested that even in the case of endothelial cells in situ, the apical plaques (i.e., stress fiber-plasma membrane attachment sites with accumulation of focal adhesion-associated proteins) and their associated stress fibers are candidates for sensing and/or transferring mechanical signals of fluid shear stress applied to the laminar surface of endothelial cells [6, 14]. Apical plaques are enlarged in the apical portion of endothelial cells in the coarctation zone reflecting the response of the apical plaque and its associated stress fibers to mechanical stimuli generated by blood flow [6]. Such responses increase according to the magnitude of applied shear stress, in agreement with the observations in traditional in vitro cell culture systems [2, 15�].

Stress fibers are major higher-order structural components of the cytoskeleton in nonmuscle cells, which are composed of actomyosin filaments and show contractility both in vitro [19] and in situ [20]. We reported previously that stress fibers could be isolated from fibroblasts without loss of morphological or functional characteristics and that they represent a major part of the contractile apparatus within the cell [19]. The principal role of stress fibers is related to their contractility within the cell. We also reported that the stress fibers are located not only in the basal portion of the cell, but also in the apical portion in both cultured fibroblasts [13] and in guinea pig aortic endothelial cells [14], and we called these apically located stress fibers 𠇊pical stress fibers.” Some apical stress fibers not only connect to the apical plaques but also make direct connections with focal adhesions in the basal portion of endothelial cells, and the apical stress fibers have the ability to transfer mechanical forces from the apical to the basal portion of the cell. Apical stress fibers in endothelial cells in situ are directly subjected to fluid shear stress, and the mechanical stimuli generated by this fluid shear stress are applied directly to the apical stress fibers.

Blood vessels in the living animal are subjected to pulsatile stretches generated by the heart via the circulatory system. These pulsatile stretches seem to induce changes in endothelial cell shape and the formation of cytoskeletal components. Previous in vitro experiments showed that cyclic stretching applied to cells in culture causes the cells to become oriented perpendicular to the direction of stretching [10, 21, 22] consistent with in vivo results [23]. On the other hand, in cells exposed to unidirectional tension, the stress fibers become organized along the axis of tension [24].

In situ experiments indicated that guinea pig venous endothelial cells were elongated in the direction of blood flow to a greater extent than unperturbed aortic endothelial cells [25]. Moreover, thick stress fibers located at the basal side of venous endothelial cells were fewer in number than in aortic endothelial cells. The morphological differences between venous and aortic endothelial cells seem to be due to the sustained exposure of the former cell type to significantly lower levels of fluid shear stress than the latter. However, cell culture conditions preclude accurate observations because the cells have been artificially removed from the living animal. Therefore, analyses of the fundamental mechanisms involved in the responses to mechanical stimuli, such as fluid shear stress, pulsatile enlargement of blood vessel diameter, and/or stretching, should be performed in living intact blood vessels.

In the basal portion of endothelial cells, stress fibers generally run along the axis of blood flow in typical aortic and venous endothelial cells. However, we reported previously that stress fibers in the apical portion of venous endothelial cells run perpendicular to the direction of blood flow [25]. These observations raised questions regarding whether the mechanism by which stress fibers run is independent of the direction of blood flow. Both the right and left renal arteries branch off from the abdominal aorta at an angle of 90° and carry blood to the kidneys. Approximately 1/3 of the blood from the heart is directed into the kidneys. Blood in the renal artery is filtered by the kidneys, and so the resistance to blood flow applied to the surface of the renal artery should be higher than that in most other arteries. Mechanical stress applied to the endothelial cells in the renal artery should be different from the straight portion of the abdominal aorta in situ, and therefore the distribution of cytoskeletal components, such as stress fibers and focal adhesions, should differ considerably between renal artery endothelial cells and endothelial cells experiencing unidirectional flow in situ. The observations outlined above prompted us to examine the detailed distributions of cytoskeletal components and associated proteins. Here, we carefully compared the cytoskeletal components of endothelial cells in the renal artery with those of unperturbed arterial and venous endothelial cells. The results indicated that the cytoskeletal components of endothelial cells in the renal artery showed quiet different distribution patterns from the stress fibers in unperturbed aortic and venous endothelial cells.


Microtubules

  • are straight, hollow cylinders whose wall is made up of a ring of 13 "protofilaments"
  • have a diameter of about 25 nm
  • are variable in length but can grow 1000 times as long as they are wide
  • are built by the assembly of dimers of alpha tubulin and beta tubulin
  • are found in both animal and plant cells. In plant cells, microtubules are created at many sites scattered through the cell. In animal cells, the microtubules originate at the centrosome.
  • The attached end is called the minus end the other end is the plus end.
  • grow at the plus end by the polymerization of tubulin dimers (powered by the hydrolysis of GTP), and
  • shrink by the release of tubulin dimers (depolymerization) at the same end.

Microtubules participate in a wide variety of cell activities. Most involve motion. The motion is provided by protein "motors" that use the energy of ATP to move along the microtubule.

Microtubule motors

  • kinesins (most of these move toward the plus end of the microtubules) and
  • dyneins (which move toward the minus end).
  • The rapid transport of organelles, like vesicles and mitochondria, along the axons of neurons takes place along microtubules with their plus ends pointed toward the end of the axon. The motors are kinesins.
  • The migration of chromosomes in mitosis and meiosis takes place on microtubules that make up the spindle fibers. Both kinesins and dyneins are used as motors [Link].
    • Vincristine, a drug found in the Madagascar periwinkle (a wildflower), binds to tubulin dimers preventing the assembly of microtubules. This halts cells in metaphase of mitosis.
    • Taxol®, a drug found in the bark of the Pacific yew, prevents depolymerization of the microtubules of the spindle fiber. This, in turn, stops chromosome movement, and thus prevents the completion of mitosis.

    Because the hallmark of cancer cells is uncontrolled mitosis, both vincristine and Taxol are used as anticancer drugs

    Cilia and Flagella

    Cilia and flagella are built from arrays of microtubules. They are discussed on a separate page. Link to it.


    The Function Of a Cytoskeleton

    Through a series of intercellular proteins, the cytoskeleton gives a cell its shape, offers support, and facilitates movement through three main components: microfilaments, intermediate filaments, and microtubules. The cytoskeleton helps the cell move in its environment and controls the movement of all of the cell's interior workings.

    Microfilaments are the smallest of the three parts of the cytoskeleton, as they are only around seven nanometers in diameter. These helically shaped filaments are made up of G-actin proteins. Intermediate filaments are slightly larger at eight to twelve nanometers around, and these keratin-based filaments are twisted around each other to form a cord shape. Microtubules are made of stronger proteins that form long, hollow cylinders. They are the largest of the three at twenty-five nanometers.

    The microtubules have three different functions which contribute to the job of the cytoskeleton. They make up the centrioles in a cell, they are the base of both the flagella and cilia of a cell, and they function as the pathway thatthe transport vesicles move along.


    Watch the video: THE CYTOSKELETON - MICROTUBULES, INTERMEDIATE FILAMENTS, MICROFILAMENTS (July 2022).


Comments:

  1. Parttyli

    I absolutely agree with you. This is a great idea. I am ready to support you.

  2. Jimiyu

    There is something in this. I will know, thank you for your help in this matter.

  3. Loyal

    In my opinion it is very interesting theme. I suggest all to take part in discussion more actively.

  4. Akram

    It is compliant, the admirable phrase

  5. Cleobis

    forgot to write about the loot !!!!!!!!!

  6. Kigakinos

    This answer, is matchless



Write a message