Podcast
Questions and Answers
What is cytosol?
What is cytosol?
Region of the cytoplasm between and surrounding the organelles, a gel-like substance with 20~30% proteins content.
What are the main functions of the cytoskeleton? (Select all that apply)
What are the main functions of the cytoskeleton? (Select all that apply)
The cytoskeleton is a complex network of interconnected ______ and tubules.
The cytoskeleton is a complex network of interconnected ______ and tubules.
filaments
What are the three major structural elements of the cytoskeleton?
What are the three major structural elements of the cytoskeleton?
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Microtubules are the smallest of the cytoskeletal elements.
Microtubules are the smallest of the cytoskeletal elements.
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What are axonemal microtubules associated with?
What are axonemal microtubules associated with?
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Describe the structure of microtubules.
Describe the structure of microtubules.
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What phase follows the lag phase in microtubule assembly?
What phase follows the lag phase in microtubule assembly?
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What happens during the plateau phase of microtubule assembly?
What happens during the plateau phase of microtubule assembly?
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What is the rapidly growing end of a microtubule called?
What is the rapidly growing end of a microtubule called?
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The minus ends of microtubules are often anchored at the centrosome.
The minus ends of microtubules are often anchored at the centrosome.
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What phenomenon occurs when assembly happens at the plus end while disassembly occurs at the minus end?
What phenomenon occurs when assembly happens at the plus end while disassembly occurs at the minus end?
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What protein is the building block of microfilaments?
What protein is the building block of microfilaments?
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What characteristic do muscle-specific actins have?
What characteristic do muscle-specific actins have?
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What structural role do intermediate filaments (IFs) play in cells?
What structural role do intermediate filaments (IFs) play in cells?
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Microtubules are generally thought to resist bending when a cell is ______.
Microtubules are generally thought to resist bending when a cell is ______.
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What is the extracellular matrix (ECM)?
What is the extracellular matrix (ECM)?
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Which of the following types of cell-cell junctions are found in animals?
Which of the following types of cell-cell junctions are found in animals?
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What are adhesive glycoproteins that allow cells to attach to the matrix?
What are adhesive glycoproteins that allow cells to attach to the matrix?
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What is the major component of the extracellular matrix (ECM)?
What is the major component of the extracellular matrix (ECM)?
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Which cells secrete collagen?
Which cells secrete collagen?
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Collagen fibers provide elasticity to the ECM.
Collagen fibers provide elasticity to the ECM.
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What function do proteoglycans serve in the extracellular matrix?
What function do proteoglycans serve in the extracellular matrix?
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What characteristics do glycosaminoglycans (GAGs) have?
What characteristics do glycosaminoglycans (GAGs) have?
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Where is hyaluronate most abundant in the body?
Where is hyaluronate most abundant in the body?
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What is the function of integrins?
What is the function of integrins?
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The basal lamina is thick and lacks structural support.
The basal lamina is thick and lacks structural support.
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What role does the ECM serve in regards to cells?
What role does the ECM serve in regards to cells?
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What are adhesive glycoproteins that allow cells to attach to the matrix?
What are adhesive glycoproteins that allow cells to attach to the matrix?
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Collagen is the single most abundant protein in the animal kingdom.
Collagen is the single most abundant protein in the animal kingdom.
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What protein is secreted by fibroblasts and osteoblasts?
What protein is secreted by fibroblasts and osteoblasts?
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What do elastins provide to the extracellular matrix?
What do elastins provide to the extracellular matrix?
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What are proteoglycans formed of?
What are proteoglycans formed of?
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Glycosaminoglycans (GAGs) are hydrophobic molecules.
Glycosaminoglycans (GAGs) are hydrophobic molecules.
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What is the role of hyaluronate in the extracellular matrix?
What is the role of hyaluronate in the extracellular matrix?
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What is the thickness of the basal lamina?
What is the thickness of the basal lamina?
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What purpose do integrins serve in cells?
What purpose do integrins serve in cells?
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What complex interactions occur between ECM and cells?
What complex interactions occur between ECM and cells?
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What is the cytosol?
What is the cytosol?
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Which of the following are functions of the cytoskeleton? (Select all that apply)
Which of the following are functions of the cytoskeleton? (Select all that apply)
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Microtubules are the ______ of the cytoskeletal elements.
Microtubules are the ______ of the cytoskeletal elements.
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What are the two types of microtubules mentioned?
What are the two types of microtubules mentioned?
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Intermediate filaments are the largest of the cytoskeletal elements.
Intermediate filaments are the largest of the cytoskeletal elements.
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What are the subunits that make up protofilaments in microtubules?
What are the subunits that make up protofilaments in microtubules?
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During microtubule assembly, the phase where formation is initially slow is called the ______ phase.
During microtubule assembly, the phase where formation is initially slow is called the ______ phase.
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What happens during the elongation phase of microtubule assembly?
What happens during the elongation phase of microtubule assembly?
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What is the plateau phase in microtubule dynamics?
What is the plateau phase in microtubule dynamics?
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What are the two ends of microtubules called?
What are the two ends of microtubules called?
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What does treadmilling refer to in the context of microtubules?
What does treadmilling refer to in the context of microtubules?
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The minus ends of microtubules are often anchored at the centrosome.
The minus ends of microtubules are often anchored at the centrosome.
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What are actin filaments primarily composed of?
What are actin filaments primarily composed of?
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What roles do microfilaments play in cells?
What roles do microfilaments play in cells?
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What is the diameter of microfilaments?
What is the diameter of microfilaments?
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What purpose do myosins serve?
What purpose do myosins serve?
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What are intermediate filaments primarily composed of?
What are intermediate filaments primarily composed of?
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Intermediate filaments are the least stable constituents of the cytoskeleton.
Intermediate filaments are the least stable constituents of the cytoskeleton.
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What is the extracellular matrix (ECM) important for?
What is the extracellular matrix (ECM) important for?
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What are the three classes of molecules that make up ECM?
What are the three classes of molecules that make up ECM?
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What type of junctions are involved in cell-cell adhesion in animals?
What type of junctions are involved in cell-cell adhesion in animals?
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Study Notes
Cytosol
- Region of the cytoplasm between and surrounding the organelles
- Gel-like substance
- Contains 20~30% proteins
Cytoskeleton
- Complex network within the cytosol
- High level of internal organization
- Dynamic and changeable
- Interconnected filaments and tubules
- Extends from the nucleus to the inner surface of the plasma membrane
- Enables cells to carry out cellular processes and maintain complex shapes
Functions of the Cytoskeleton
- Maintenance of cell shape
- Cell movement
- Cell division
- Positioning and active movement of membrane-bounded organelles
- Provides matrix for enzyme attachment
- Cell signaling
- Cell-cell adhesion
Major Structural Elements
- Microtubules
- Microfilaments
- Intermediate filaments
- Each element has a characteristic size, structure, and intracellular distribution
- Each element is formed by the polymerization of a different kind of protein subunit
Properties of Structural Elements
- Microtubules (MT)
- Diameter: 25nm
- Subunits: α- and β-tubulin heterodimers
- Function: Maintenance of cell shape, intracellular transport, cell motility
- Microfilaments (MF)
- Diameter: 7nm
- Subunits: Actin monomers
- Function: Muscle contraction, cell movement, maintenance of cell shape
- Intermediate filaments (IF)
- Diameter: 10nm
- Subunits: Diverse proteins (Keratin, vimentin, lamin, neurofilaments)
- Function: Structural support, anchorage of the nucleus and other organelles
Microtubules: The Largest of the Cytoskeletal Elements
- Axonemal microtubules
- Highly organized microtubules
- Associated with cellular movement
- Only found in cells that have structures like cilia and flagella
- Cytosolic microtubules
- Loosely organized, dynamic network
- Formation of mitotic and meiotic spindles
- Maintaining or altering cell shape
- Placement and movement of vesicles
- Found in all animal cells and plant cells
Microtubule Structure
- Straight, hollow cylinders
- Vary greatly in length
- Consists of longitudinal arrays
- Protofilaments: Built by heterodimers (α- and β-tubulin subunits)
- All the dimers in the protofilaments are oriented the same way (the plus (+) and minus (-) ends)
- Can form as singlets, doublets, and triplets
Microtubule Assembly
- Lag Phase: MT formation is initially slow due to the relatively slow process of MT nucleation → aggregation of tubulin dimers into clusters, i.e., oligomers
- Elongation Phase: MT grows by addition of tubulin dimers at either ends (relatively fast)
Microtubule Dynamics
- Microtubules (MTs) are dynamic structures that undergo continuous cycles of assembly and disassembly
- The rate of MT assembly is dependent on the concentration of free tubulin dimers
- During the "plateau phase", MT assembly is balanced by disassembly when the concentration of free tubulin becomes limiting
- MTs have two ends: a plus end which grows rapidly, and a minus end which grows slowly
- The minus ends of MTs are often anchored at the centrosome
- The different growth rates of the plus and minus ends of MTs reflect the different critical concentrations required
- If the free tubulin concentration is higher than the critical concentration for the plus end but lower than the critical concentration for the minus end, assembly will occur at the plus end while disassembly takes place at the minus end
- This simultaneous assembly and disassembly at the plus and minus ends of MTs is known as "treadmilling"
Microtubule Polarity
- In animal cells, the distribution of most microtubules is determined by microtubule-organizing centers (MTOCs)
- MT orientation in a cell may vary with that cell's function
Microtubules in Cell Division
- MTs in a dividing cell are oriented with their minus ends anchored in the centrosome and their plus ends pointing away from the centrosome
- Cell division is preceded by the division of the centrosome
Axonemal Microtubules
- Axonemal MTs are the structural components of flagella and cilia
- Flagella are whip-like appendages that undulate to move cells and are longer than cilia
- Cilia are hair-like structures causing the movement of unicellular paramecium
- Cilia are also found in specialized linings in eukaryotes
Microfilaments
- Microfilaments (MFs) are the smallest cytoskeletal elements, with a diameter of about 7 nm
- MFs are involved in the development and maintenance of cell shape, cell migration, and a variety of cell movements
- MFs are involved in muscle contraction when interacting with myosin
- Actin is the protein building block of microfilaments
- Actin is an extremely abundant protein in virtually all eukaryotic cells, including those of plants, algae, and fungi
- Actin monomers are oriented in the same direction within a microfilament, giving it an inherent polarity
- MFs are less rigid than microtubules
- The plus (+) end of an MF grows faster than the minus (-) end
Architecture of Microfilaments
- Different arrangements of MFs allow cells to adopt different shapes and perform different functions
- Stress fibers help cells exert strong forces on their surroundings
- The cell cortex is a loosely organized meshwork of MFs
- Lamellipodia and filopodia at the leading edge of cells allow them to move along a surface
Polymerization of Actins
- Monomeric actin binds to ATP
- Upon polymerization, actin ATPase activity cleaves ATP to ADP
- ATP hydrolysis acts as a molecular "clock"
- Older actin filaments with ADP are unstable and disassemble from MF
Actin Binding Proteins
- Actin binding proteins regulate the polymerization, length, and organization of microfilaments
- These proteins can affect monomer availability, monomer addition, or growth of existing filaments
- Some proteins cap the ends of filaments to prevent further addition or loss of subunits
- Crosslinking proteins affect filament organization
"Pushing" Force
- Localized polymerization of actins at the leading edge of the cell drives forward protrusion of the plasma membrane
- This mechanism is involved in intracellular movement and cell-to-cell spreading of pathogens, like Listeria monocytogenes
Myosins
- Myosins are actin-based motor proteins that convert ATP hydrolysis into movement along actin filaments
- There are many different classes of myosins, some move cargoes, while others slide actin
- Actomyosin, a complex of actin filaments and myosin motor proteins, is responsible for force generation during muscle contraction
Intermediate Filaments
- Intermediate filaments (IFs) have a diameter of about 8–12 nm, making them intermediate in size between microtubules and microfilaments
- IFs are the most stable and least soluble constituents of the cytoskeleton
- Keratin is an important component of structures that grow from skin in animals, including hair, appendages, and the outermost layer of the skin
- IFs differ markedly in amino acid composition from tissue to tissue
IF Assembly
- IF assembly begins with a pair of IF polypeptides that twist around each other, with their N- and C-terminal ends aligned, forming a dimer
- Two dimers align laterally to form a tetrameric protofilament
- Protofilaments assemble into larger filaments by end-to-end and side-to-side alignment
- A fully assembled IF is thought to be eight protofilaments thick at any point
Mechanical Strength
- IFs are often found in areas of the cell that are subject to mechanical stress, playing a tension-bearing role
- IFs are less susceptible to chemical attack than microtubules and microfilaments
- Mutations in keratins can give rise to a blistering skin disease called epidermolysis bullosa simplex
The Cytoskeleton
- The cytoskeleton provides an intracellular scaffolding that organizes structures and shapes of cells
- Microtubules resist bending when a cell is compressed, while microfilaments generate tension
- Intermediate filaments are elastic and can withstand tensile forces
Motility
- Motility refers to the ability to move spontaneously and independently
- This applies to tissues, cells, and subcellular levels
Chemical Agents Used to Perturb the Cytoskeleton
- There are a range of chemical agents that can be used to perturb the cytoskeleton, including:
- Colchicine: inhibits microtubule assembly
- Taxol: stabilizes microtubules
- Cytochalasin D: inhibits polymerization of actin
- Phalloidin: stabilizes actin filaments
Extracellular Structures
- In order to understand how multicellular organisms are constructed, it is important to consider both connections between cells and the extracellular structures to which cells attach
- Animal cells have an extracellular matrix (ECM) that takes on a variety of forms and plays important roles in diverse cellular processes
- The ECM consists mainly of macromolecules that are secreted by the cell
Cell-Cell Junctions
- Multicellular organisms have specific means of joining cells in long-term associations to form tissues and organs
- The specialized structures where two cells come together are called cell-cell junctions
- In animals, the most common cell-cell junctions are:
- Adhesive junctions (including adherens junctions and desmosomes)
- Tight junctions
- Gap junctions
- Plant cells have special structures called plasmodesmata
Extracellular Matrix (ECM)
- ECM is a complex network of specific proteins and carbohydrates that fills the spaces between cells
- ECM plays a role in determining the shape and mechanical properties of organs and tissues
Examples of ECM-Enriched Tissues
- Bone consists mainly of a rigid extracellular matrix with a small number of interspersed cells
- Cartilage is another ECM-rich tissue with a more flexible matrix than bone
- Connective tissue surrounding glands and blood vessels has a gelatinous extracellular matrix containing numerous interspersed fibroblast cells
Cell-Cell and Cell-ECM Attachments
- Cells interact with each other through cell-cell junctions and with the ECM through cell-ECM attachments
- These attachments are crucial for tissue structure and function
ECM Molecules
- The ECM consists of three primary classes of molecules:
- Structural proteins (e.g., collagens and elastins) that provide strength and flexibility
- Protein-polysaccharide complexes (proteoglycans) that embed structural molecules
- Adhesive proteins (e.g., fibronectins and laminins) that link ECM components to cells
Adhesive Glycoproteins
- Fibronectins and lamins allow cells to attach to the extracellular matrix (ECM).
Collagen
- The major component of the ECM.
- The most abundant protein in the animal kingdom.
- It represents 25% of the total protein in the human body.
- Collagen is secreted by fibroblasts in tissues like skin and tendons, and by osteoblasts in bones.
- It provides strength and resilience to connective tissue.
Elastins
- Elastin fibers provide elasticity and flexibility to the ECM.
- They stretch when tension is applied and recoil to their compact form when the tension is released.
Proteoglycans
- Composed of glycosaminoglycans (GAGs) attached to core proteins.
- GAGs are large carbohydrates with repeating disaccharide units.
- Proteoglycans are linked to collagen fibers, contributing to the fiber/network structure of the ECM.
- They trap extracellular fluid (water), acting as a cushion for cells.
- They provide resistance to forces of compression.
- Their interaction with GAGs forms a highly hydrated gel-like “ground substance.”
Glycosaminoglycans (GAGs)
- Linear polymers of repeating disaccharide units.
- Highly negatively charged due to carboxyl and sulfate groups.
- Strongly hydrophilic, attracting and trapping extracellular fluid, which contributes to resistance against compression forces.
- Covalently linked to proteins.
- Interact with proteoglycans to form a gel-like structure that provides mechanical support for tissues.
Hyaluronate
- An exception among GAGs as it exists freely, not only as part of proteoglycans.
- Has lubricating properties.
- Found in abundance in areas where friction needs to be reduced, such as joints.
Basal Lamina
- A thin, specialized ECM layer attached to the basal surfaces of epithelial cells.
- It is approximately 50 nm thick.
- Functions as a structural support.
- Laminins, a family of proteins, are the major adhesive glycoproteins in the basal lamina.
- Cells can modify the basal lamina by secreting enzymes like matrix metalloproteinases (MMPs), which degrade the ECM locally.
Integrins
- Cell surface receptors that bind to ECM constituents, such as laminins.
- Function to integrate the cytoskeleton with the ECM.
ECM and Cells
- The ECM serves as a non-living framework that supports and surrounds cells.
- It forms a complex, chemically and physically crosslinked network.
- It separates different tissue spaces.
- The interactions between the ECM and cells are bidirectional and complex.
- Cells receive external signals from the ECM.
- Cells can remodel the ECM.
- The ECM promotes angiogenesis (formation of new blood vessels).
- The ECM influences cell shape, fate, and metabolism.
Cytosol
- The region of the cytoplasm between and surrounding organelles
- A gel-like substance
- Contains 20-30% proteins
Cytoskeleton
- A complex network of interconnected filaments and tubules throughout the cytosol
- Extends from the nucleus to the inner surface of the plasma membrane
- Gives cells their shape and allows for cellular processes
- Dynamic and changeable
Functions of the Cytoskeleton
- Maintains cell shape
- Enables cell movement
- Facilitates cell division
- Positions and actively moves membrane-bounded organelles
- Provides a matrix for enzyme attachment
- Plays a role in cell signaling
- Involved in cell-cell adhesion
Major Structural Elements
- Microtubules
- Microfilaments
- Intermediate filaments
Properties of Structural Elements
- Each element has a distinct size, structure, and intracellular distribution
- Each element is formed by polymerization of a specific protein subunit
Properties of Structural Elements (Table)
- Microtubules: 25 nm diameter, hollow cylinders, α- and β-tubulin subunits, rigid, dynamic
- Microfilaments: 7 nm diameter, two intertwined actin chains , flexible, dynamic
- Intermediate filaments: 8-12nm diameter, various proteins like keratin, vimentin, and lamin, strong and stable
Types of Microtubules
- Axonemal Microtubules: Highly organized, associated with cellular movement, found in cilia and flagella
- Cytosolic Microtubules: Loosely organized, dynamic network, involved in mitotic and meiotic spindles, maintaining cell shape, placement and movement of vesicles, found in all animal and plant cells
Microtubule Structure
- Straight and hollow cylinders
- Varying lengths
- Consists of longitudinal arrays known as protofilaments
- Protofilaments are built from α- and β-tubulin subunits
- All dimers in the protofilaments are oriented the same way, with plus (+) and minus (-) ends
- Can be singlets, doublets, or triplets
Microtubule Assembly
- Lag phase: MT formation is initially slow due to slow nucleation of tubulin dimers into clusters (oligomers)
- Elongation phase: MT grows rapidly by adding tubulin dimers at either end
Plateau Phase
- Microtubule (MT) assembly is balanced by disassembly when free tubulin concentration becomes limiting.
- Mass of MTs increase during this phase.
Treadmilling of Microtubules
- MT nucleation and assembly occur at both ends of the microtubules, but growth is much faster from one end than another.
- The faster growing end is called the plus end, and the slower growing end is called the minus end.
- Minus ends are often anchored at the centrosome.
- MT dynamics are confined to plus ends.
- The different growth rates of the plus and minus ends of microtubules are due to different critical concentrations required for assembly at each end.
- If free tubulin concentration is higher than the critical concentration for the plus end but lower than the critical concentration for the minus end, assembly will occur at the plus end while disassembly occurs at the minus end.
- This simultaneous assembly and disassembly is called treadmilling.
- A tubulin molecule incorporated at the plus end is displaced along the MT until it is eventually lost by depolymerization at the minus end.
Microtubule Polarity in Animal Cells
- Microtubule-organizing centers (MTOCs) determine the distribution of microtubules in cells.
- The orientation of microtubules within a cell depends upon its function.
Microtubules in Cell Division
- Microtubules in a dividing cell have their minus ends anchored in the centrosome and their plus ends pointing away from the centrosome.
- Cell division is preceded by division of the centrosome.
Axonemal Microtubules
- Microtubules are structural components of flagella and cilia.
- Cilia and flagella are MT-based organelles that function as antennae and propellers in eukaryotic cells.
- Flagella are whip-like appendages that undulate to move cells, and they are longer than cilia.
- Cilia are hair-like structures that cause movement in the unicellular paramecium.
- Cilia are also found in specialized linings in eukaryotes.
Microfilaments
- Microfilaments (MFs) have a diameter of about 7 nm.
- MFs are involved in the development and maintenance of cell shape, cell migration, and a variety of cell movements.
- MFs are involved in muscle contraction by interacting with myosin.
- Actin is the protein building block of microfilaments.
- Actin is an extremely abundant protein in virtually all eukaryotic cells, including those of plants, algae, and fungi.
- Actin folds into a roughly U-shaped globular molecule with a central cavity that binds ATP or ADP.
- Actins can be divided into muscle specific actins and non-muscle actins.
Polymerization of Actins
- Muscle-specific actins are known as alpha-actins.
- Non-muscle actins are known as beta- and gamma-actins.
- Actin monomers are oriented in the same direction within a microfilament, resulting in inherent polarity.
- MFs are less rigid than microtubules.
- The plus (+) end of the microfilament is the fast-growing end.
- The minus (-) end of the microfilament is the slow-growing end.
- Monomers polymerize into a helical chain.
The Architecture of MFs
- MFs allow cells to adopt different shapes and perform different functions.
- Stress fibers help cells exert strong forces on their surroundings.
- The cell cortex is a loosely organized meshwork of MFs.
- Lamellipodia and filopodia at the leading edge of the cell allow it to move along a surface.
Polymerization of Actins
- Monomeric actin binds to ATP.
- The polymerization mechanism of MFs is similar to that of MTs.
- Actin ATPase activity cleaves ATP to ADP upon polymerization.
- ATP hydrolysis acts as a molecular “clock”.
- Older actin filaments with ADP are unstable and disassemble from MFs.
Actin Binding Proteins
- Actin binding proteins regulate the polymerization, length, and organization of microfilaments.
- Actin-binding proteins are responsible for converting actin filaments from one form to another.
- Some proteins affect monomer availability and monomer addition.
- Capping proteins affect severing or growth of existing filaments.
- Crosslinking/bundling proteins affect filament organization.
“Pushing” Force
- Localized polymerization of actins at the leading edge of the cell drives forward protrusion of the plasma membrane.
- Intracellular movement and cell-to-cell spreading of pathogens is driven by actin polymerization.
- An example of this is the movement of Listeria monocytogenes.
- Listeria monocytogenes is a pathogenic bacterium that colonizes the epithelial lining of the human gut and is found in contaminated dairy products.
- Infection with Listeria monocytogenes can be lethal to newborns and immunocompromised individuals.
Myosins are Actin-Based Motor Proteins
- Myosins convert ATP hydrolysis into movement along actin filaments.
- There are many different classes of myosins (over 30 classes in humans).
- Some myosins move cargoes, while others slide actin.
- Myosin I can carry organelles or slide actin filaments along the membrane.
- Actomyosin, a complex of actin filaments and myosin motor proteins, is responsible for force generation during muscle contraction.
Intermediate Filaments
- Intermediate filaments (IFs) have a diameter of about 8–12 nm.
- IFs are the most stable and the least soluble constituents of the cytoskeleton.
- A common intermediate filament protein is keratin.
- Keratin is an important component of structures that grow from the skin of animals, including hair, appendages, and the outermost layer of the skin.
- IFs differ markedly in amino acid composition from tissue to tissue.
IF Assembly
- IF assembly starts with a pair of IF polypeptides.
- The central domains of the two polypeptides twist around each other with their N- and C-terminal ends aligned.
- Two dimers align laterally to form a tetrameric protofilament.
- Protofilaments assemble into larger filaments by end-to-end and side-to-side alignment.
- A fully assembled IF is thought to be eight protofilaments thick.
Mechanical Strength
- IFs often occur in areas of the cell that are subject to mechanical stress.
- IFs play a tension-bearing role.
- IFs are less susceptible to chemical attack than microtubules and microfilaments.
- In humans, naturally occurring mutations of keratins give rise to a blistering skin disease called epidermolysis bullosa simplex.
Cytoskeleton
- The cytoskeleton provides an intracellular scaffolding that organizes structures and shapes of cells.
- Microtubules are generally thought to resist bending when a cell is compressed, while microfilaments serve as contractile elements that generate tension.
- Intermediate filaments are elastic and can withstand tensile forces.
Motility
- Motility refers to the ability to move spontaneously and independently.
- This term can be applied to tissue, cellular, and subcellular levels.
Chemical Agents Used to Perturb the Cytoskeleton
- There are a variety of chemical agents used to perturb the cytoskeleton.
Extracellular Structures
- Cell-cell attachments and extracellular structures are important in building multicellular organisms.
Introduction
- Cell-cell adhesions and extracellular structures must be considered to understand how multicellular organisms are constructed.
- Animal cells have an extracellular matrix (ECM) that plays important roles in cellular processes including division, motility, differentiation, and adhesion.
- The extracellular structures themselves consist mainly of macromolecules that are secreted by the cell.
Cell-Cell Junctions
- Multicellular organisms have specific ways of joining cells in long-term associations to form tissues and organs.
- The specialized structures where two cells come together are called cell-cell junctions.
- The most common cell-cell junctions in animals are adhesive junctions, tight junctions, and gap junctions.
- Plant cells have specialized structures called plasmodesmata.
Extracellular Matrix (ECM)
- Tissues are not simply composed of cells, as cells need to interact with extracellular materials that are crucial for tissue structure and function.
- The ECM is a complex network of specific proteins and carbohydrates that fills the spaces between cells.
- The ECM plays a role in determining the shape and mechanical properties of organs and tissues.
Examples of ECM-Enriched Tissues
- Bone consists mainly of a rigid extracellular matrix that contains a small number of interspersed cells.
- Cartilage is another tissue constructed almost entirely of matrix materials, but the matrix is more flexible than in bone.
- Connective tissue surrounding glands and blood vessels has a relatively gelatinous extracellular matrix containing numerous interspersed fibroblast cells.
Cell-Cell and Cell-ECM Attachments
- Cells require mechanisms to bind to other cells.
- Cells also require mechanisms to bind to extracellular components.
ECM Molecules
- Despite diversity of function, ECM consists of three classes of molecules:
- Structural proteins provide the matrix its strength and flexibility.
- Examples are collagens and elastins.
- Protein-polysaccharide complexes, i.e., proteoglycans, provide the matrix in which structural molecules are embedded.
- The third class of ECM molecules are specialized proteins that interact with the structural and proteoglycan components, and they mediate cell-cell adhesion or cell-matrix adhesion.
Extracellular Matrix - ECM
- Adhesive glycoproteins, such as fibronectins and lamins, allow cells to attach to the matrix.
- The ECM acts as a scaffold for cells, providing physical support and organization for tissues.
- Collagen is a major component of ECM.
- 25% of the total protein in the human body is collagen.
- Collagen is secreted by fibroblasts in tissues like skin and tendons, and by osteoblasts in bone.
- Collagen provides strength and resilience to connective tissues.
- Elastins contribute to the ECM's elasticity and flexibility.
- When tension is exerted on elastin fibers, they stretch to their extended form and recoil to their compact form when tension is released.
- Proteoglycans are formed of glycosaminoglycans (GAGs) covalently attached to core proteins.
- GAGs are large carbohydrates with repeating disaccharide units.
- Proteoglycans link directly to collagen fibers, contributing to the fiber/network structure of the ECM.
- Proteoglycans trap extracellular fluid, serving as a cushion for cells and providing resistance to compression forces.
- Proteoglycans and GAGs interact to form a highly hydrated gel-like “ground substance” in the ECM.
- Glycosaminoglycans (GAGs) are linear polymers of repeating disaccharide units.
- GAGs are highly negatively charged due to carboxyl and sulfate groups.
- GAGs are strongly hydrophilic and trap extracellular fluid, providing resistance to compression forces.
- GAGs are covalently linked to proteins.
- GAGs interact with proteoglycans to form gel-like structures, providing mechanical support for tissues.
- Hyaluronate, a GAG, is an exception as it is found in the extracellular matrix as a free GAG.
- Hyaluronate has lubricating properties and is most abundant in areas requiring friction reduction, like joints.
Basal Lamina
- The basal lamina is a thin, specialized ECM layer attached to the basal surfaces of epithelial cells, measuring approximately 50nm thick.
- The basal lamina provides structural support for epithelial cells.
- Lamins are a family of proteins that are major adhesive glycoproteins found in the basal lamina.
- Cells can alter the properties of the basal lamina by secreting enzymes that catalyze changes in the lamina, such as matrix metalloproteinases (MMPs) that degrade ECM locally.
Integrins
- Integrins are cell surface receptors that bind to ECM constituents, like laminins.
- Integrins play a critical role in integrating the cytoskeleton with the extracellular matrix.
ECM and Cells: A Dynamic Relationship
- ECM serves as a non-living framework that surrounds and supports cells.
- ECM forms a complex network of chemically and physically crosslinked molecules.
- ECM separates one tissue space from another.
- Bidirectional interactions occur between cells and the ECM.
- Cells receive external information from the ECM.
- Cells frequently remodel the ECM.
- ECM promotes the formation of capillary-like structures (angiogenesis).
- ECM influences cell shape, fate and metabolism.
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This quiz explores key concepts related to the cytosol and cytoskeleton in cell biology. Topics include the structure and functions of cytoplasm, the complex network of the cytoskeleton, and its role in maintaining cell shape and facilitating cellular processes. Test your understanding of the major structural elements and their properties.
