Extracellular Matrix in Animal Tissues

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Questions and Answers

What is the primary function of the extracellular matrix in animal tissues?

To regulate the behavior of cells (correct)

To act as a storage unit for nutrients

To protect cells from pathogens

To solely provide mechanical support

Which type of cells are primarily responsible for producing the macromolecules in the extracellular matrix?

Epithelial cells

Fibroblasts (correct)

Osteoblasts

Chondrocytes

How does the orientation of the cytoskeleton inside a cell affect the extracellular matrix?

It controls the orientation of the matrix produced outside. (correct)

It has no effect on the matrix.

It influences the chemical composition of the matrix.

It determines the type of cell that will be formed.

What can the extracellular matrix become when it undergoes calcification?

Bone or teeth Signup and view all the answers

What role do cells such as chondrocytes and osteoblasts play in relation to the extracellular matrix?

They help organize and secrete matrix macromolecules. Signup and view all the answers

Which characteristic does NOT describe the extracellular matrix?

It is a passive structure providing support. Signup and view all the answers

What type of structure does the extracellular matrix help form in jellyfish?

Jelly-like substance Signup and view all the answers

What is one way the extracellular matrix contributes to the tensile strength of tendons?

By adopting a rope-like organization Signup and view all the answers

What distinguishes hyaluronan from other glycosaminoglycans (GAGs)?

All its disaccharide units are identical and it is not covalently linked to proteins. Signup and view all the answers

Which of the following statements about the synthesis of GAGs is accurate?

Hyaluronan is spun out directly from the cell surface. Signup and view all the answers

What role does hyaluronan play during embryonic development?

It acts as a space filler and promotes cell movement. Signup and view all the answers

In what way do proteoglycans differ from glycoproteins?

Proteoglycans have much longer and unbranched sugar chains. Signup and view all the answers

What enzymes are responsible for the degradation of excess hyaluronan?

Hyaluronidase Signup and view all the answers

What is the primary component that makes the proteoglycan aggrecan significant in cartilage?

It has a mass of about 3 × 10^6 daltons with over 100 GAG chains. Signup and view all the answers

What role do sulfation and epimerization play in the synthesis of GAGs?

They modify the negative charge and the structure of individual sugars. Signup and view all the answers

Which characteristic of proteoglycans contributes to their potential for heterogeneity?

Variations in core protein sequences. Signup and view all the answers

What is the typical length of GAG chains in proteoglycans?

Around 80 sugars long. Signup and view all the answers

How do GAGs and proteoglycans form larger polymeric complexes?

By noncovalent associations with fibrous matrix proteins. Signup and view all the answers

What modification do GAGs undergo while being synthesized in the Golgi apparatus?

Sulfation and epimerization of sugars. Signup and view all the answers

Why do mice that cannot produce decorin have fragile skin?

Decorin is vital for regulating fibril assembly. Signup and view all the answers

What unique property does the massive aggrecan-hyaluronan complex display?

It occupies a volume similar to that of a bacterium. Signup and view all the answers

What process is used to fix and stain tissue while preventing GAG chain collapse?

Freeze substitution Signup and view all the answers

What are syndecans primarily characterized by in their structure?

A membrane-spanning core protein Signup and view all the answers

What amino acid is present as every third amino acid in collagen α chains?

Glycine Signup and view all the answers

Which type of collagen is the principal collagen of skin and bone?

Type I Signup and view all the answers

What is the primary role of type IV collagen?

Forming the basal lamina Signup and view all the answers

What defines the structure of a typical collagen molecule?

A triplet Gly-X-Y sequence Signup and view all the answers

Which collagens are known as fibril-associated collagens?

Types IX and XII Signup and view all the answers

What is the role of glycosylphosphatidylinositol (GPI) anchors in plasma membrane proteoglycans?

Attaching core proteins to lipid bilayers Signup and view all the answers

How do collagen fibrils typically aggregate in mature tissues?

Into larger, cablelike bundles Signup and view all the answers

What term describes collagens that help attach the basal lamina to underlying connective tissue?

Anchoring fibrils Signup and view all the answers

What is a unique feature of the amino acid composition of collagens?

Rich in proline and glycine Signup and view all the answers

Which cell type is primarily responsible for producing collagen fibrils?

Fibroblasts Signup and view all the answers

What is significant about the role of glypicans in cell signaling?

They influence growth factor effects Signup and view all the answers

What are the three major classes of macromolecules that compose the extracellular matrix?

Glycosaminoglycans, fibrous proteins, and noncollagen glycoproteins Signup and view all the answers

Which macromolecule forms a gel-like 'ground substance' in connective tissue?

Glycosaminoglycans (GAGs) Signup and view all the answers

What characteristic of glycosaminoglycans (GAGs) contributes to their ability to form hydrated gels?

Their high density of negative charges Signup and view all the answers

Which glycosaminoglycan is noted for being the most densely charged biological molecule?

Heparin Signup and view all the answers

What is the primary role of collagen fibers in the extracellular matrix?

Strengthening and organizing the matrix Signup and view all the answers

What structural feature of GAGs allows them to occupy large volumes relative to their mass?

Highly extended conformation Signup and view all the answers

Which of the following GAGs is particularly abundant in early embryos?

Hyaluronan Signup and view all the answers

What is the effect of the negative charges on GAG chains in connective tissue?

They attract cations and water, creating turgor Signup and view all the answers

Which of the following proteins is known for providing resilience in the extracellular matrix?

Elastin Signup and view all the answers

What distinguishes proteoglycans from other glycoproteins?

Covalent linkage with glycosaminoglycans Signup and view all the answers

What role does the polysaccharide gel play in the extracellular matrix?

It resists compressive forces and allows diffusion Signup and view all the answers

Which component modifies the behavior of the extracellular matrix through cross-linking or degradation?

Matrix-associated proteins and enzymes Signup and view all the answers

What is the consequence of defects in glycosaminoglycan (GAG) production?

Defects in multiple body systems Signup and view all the answers

In connective tissue, what is the relationship between collagen and GAGs?

Collagen provides a framework that GAGs fill Signup and view all the answers

What contributes to the stability of the collagen triple helix?

Hydrogen bonds formed by hydroxyproline and hydroxylysine Signup and view all the answers

Which type of collagen accounts for approximately 90% of the body's collagen?

Type I Signup and view all the answers

What is the consequence of vitamin C deficiency, specifically in relation to collagen production?

Failure to hydroxylate proline and lysine Signup and view all the answers

How are procollagen molecules converted to collagen molecules?

By proteolytic cleavage of propeptides Signup and view all the answers

What type of collagen deficiency is associated with dwarfism?

Type II Signup and view all the answers

Which feature distinguishes type IV collagen from fibril-forming types?

Formation of a sheetlike network Signup and view all the answers

What role do propeptides serve in collagen synthesis?

They prevent premature fibril formation Signup and view all the answers

What condition is primarily characterized by fragile skin and loose joints due to collagen defects?

Vascular Ehlers-Danlos syndrome Signup and view all the answers

What is an important function of the enzyme lysyl oxidase in collagen formation?

Deamination of certain lysines Signup and view all the answers

Which of the following types of collagen is associated with skin blistering?

Type VII Signup and view all the answers

What is the role of glycosylation in collagen synthesis?

To enhance collagen stability during synthesis Signup and view all the answers

What is the primary characteristic of fibril-associated collagens compared to fibril-forming types?

Fibril-associated collagens do not form fibrils Signup and view all the answers

Which collagen types have multiple α chains or different combinations?

Type II and Type XI Signup and view all the answers

How are collagen fibrils organized in mammalian skin?

In a wickerwork pattern to resist tensile stress in multiple directions. Signup and view all the answers

What is the main function of fibril-associated collagens?

To mediate interactions of collagen fibrils with one another. Signup and view all the answers

Which component is primarily responsible for the elastic properties of tissues?

Elastin. Signup and view all the answers

What governs the size and arrangement of collagen fibrils in connective tissue?

The behavior of the connective-tissue cells. Signup and view all the answers

What type of collagen does type IX associate with in cartilage?

Type II collagen. Signup and view all the answers

How does elastin contribute to the mechanical properties of tissues?

By allowing tissues to stretch and recoil after deformation. Signup and view all the answers

What mechanism helps form elastic fibers from tropoelastin?

Covalent cross-links between lysines in elastin. Signup and view all the answers

What is a key distinction between fibrillar collagens and fibril-associated collagens?

Fibrillar collagens form stable fibrils in the extracellular matrix. Signup and view all the answers

What role does fibronectin play in collagen formation?

It guides the organization of collagen fibrils. Signup and view all the answers

What feature do collagen fibrils in tendons exhibit?

Parallel bundles aligned along the major axis of tension. Signup and view all the answers

What is a characteristic of the elastic fibers found in tissues such as blood vessels?

They are at least five times more extensible than rubber bands. Signup and view all the answers

Which type of collagen does type XII associate with in tendons?

Type I collagen. Signup and view all the answers

What is the polymer chain conformation of elastin molecules when in a relaxed state?

Loose random-coil. Signup and view all the answers

Which tissue organization feature is found in mature bone?

Orderly plywoodlike layers with fibrils aligned nearly at right angles. Signup and view all the answers

What is the primary role of elastin in the extracellular matrix of arteries?

To enable elasticity and stretch Signup and view all the answers

What occurs as a result of mutations in the elastin gene?

Narrowing of the aorta and artery proliferation Signup and view all the answers

What is the relationship between microfibrils and elastin?

Microfibrils serve as a scaffolding for elastin deposition Signup and view all the answers

What is one characteristic of fibrillin in the context of elastic fibers?

It binds to elastin and is essential for fiber integrity Signup and view all the answers

What happens to the extracellular matrix when fibroblasts interact with the collagen they have secreted?

The matrix contracts and becomes denser Signup and view all the answers

What type of connective tissue is primarily formed by the actions of fibroblasts?

Tendons and ligaments Signup and view all the answers

What impact does the composition and density of the extracellular matrix have on fibroblasts?

It regulates proliferation and migration Signup and view all the answers

How do glycoproteins in the extracellular matrix assist migrating cells?

They act as tracks or repellents for cells Signup and view all the answers

What role does fibronectin play in the extracellular matrix?

It binds other matrix macromolecules and receptors Signup and view all the answers

What is a characteristic of tissues with a denser extracellular matrix?

They can be more resistant to deformation Signup and view all the answers

What can excess matrix density indicate in the context of health?

A risk factor for certain fibrotic diseases Signup and view all the answers

What happens to collagen orientation when fibroblasts are placed in a culture with randomly oriented collagen fibrils?

Fibroblasts cause the collagen to align in response to their movement Signup and view all the answers

What is one main consequence of mutations in the fibrillin gene?

Risk of aortic rupture and skeletal issues Signup and view all the answers

What is the major repeat domain found in fibronectin?

Type III fibronectin repeat Signup and view all the answers

What is the consequence of mutant mice being unable to produce fibronectin?

Their endothelial cells fail to form proper blood vessels. Signup and view all the answers

Which binding site does the type III repeat of fibronectin contain?

Binding site for integrins Signup and view all the answers

How do fibronectin isoforms arise?

Through differential RNA splicing. Signup and view all the answers

What type of bonds join the two subunits of fibronectin?

Disulfide bonds Signup and view all the answers

What is the approximate length of each fibronectin subunit?

2500 amino acids Signup and view all the answers

Which sequence is essential for fibronectin's binding to integrins?

Arg-Gly-Asp (RGD) Signup and view all the answers

What is indicated about the fibronectin gene in the human genome?

It consists of about 50 exons of similar size. Signup and view all the answers

What role do the flexible polypeptide chains play in fibronectin?

They allow for protein folding and flexibility. Signup and view all the answers

In addition to fibronectin, which type of proteins also commonly contains RGD sequences?

Structurally diverse components of the extracellular matrix. Signup and view all the answers

How does fibronectin contribute to cell attachment?

By presenting integrin-binding domains. Signup and view all the answers

What type of assembly do fibronectin proteins undergo to increase structural diversity?

Assembly into oligomeric forms. Signup and view all the answers

What is a common feature of extracellular proteins like thrombospondin and tenascin?

They can form oligomeric structures. Signup and view all the answers

What characteristic best describes the repeated domains in matrix proteins?

They are often involved in binding functions. Signup and view all the answers

Which glycosaminoglycan is characterized as a nonsulfated type with a repeating disaccharide sequence?

Hyaluronan Signup and view all the answers

What is the primary function of elastin in connective tissues?

Form extensible fibers Signup and view all the answers

Which type of collagen is predominantly found in cartilage?

Type II Signup and view all the answers

What defines a proteoglycan?

A protein linked to one or more glycosaminoglycan chains Signup and view all the answers

Which sequence is recognized as a binding site for integrins within fibronectin?

RGD sequence Signup and view all the answers

What role do matrix metalloproteases play in the extracellular matrix?

Degrade matrix proteins Signup and view all the answers

Which component is central to the formation of the basal lamina?

Type IV collagen Signup and view all the answers

What is the significance of the type III fibronectin repeat in extracellular matrix protein structure?

It serves as an adhesion site Signup and view all the answers

What distinguishes fibrillar collagen from other types of collagen?

It has long rope-like structures Signup and view all the answers

Which feature is shared by integrins in binding to fibronectin?

They interact with the RGD sequence Signup and view all the answers

What is the primary role of laminin in the basal lamina?

To organize the sheet structure Signup and view all the answers

Which component is critical for assembly and organization of the basal lamina by interacting with laminin?

Integrin receptors Signup and view all the answers

How does type IV collagen differ from fibrillar collagens?

Its helical structure is interrupted in multiple regions Signup and view all the answers

What is a consequence of lacking the laminin γ1 chain in mice?

Death during embryogenesis Signup and view all the answers

Which laminin isoform is specifically found in skin?

Laminin-332 Signup and view all the answers

What role does the basal lamina play in tissue regeneration after injury?

It acts as a scaffold for regenerating cells Signup and view all the answers

Which of the following molecules is NOT a direct component of the basal lamina?

Fibrillin Signup and view all the answers

How do specialized cells like macrophages interact with the basal lamina?

By cutting through it using proteases Signup and view all the answers

What characterized the assembly of the laminin network?

Cellular receptors binding at the 'heads' of laminins Signup and view all the answers

In humans, a defect in which laminin component can lead to junctional epidermolysis bullosa?

Laminin-332 Signup and view all the answers

Which statement accurately describes the structure of laminin?

It is composed of three disulfide-bonded polypeptide chains Signup and view all the answers

What type of bonds hold the polypeptide chains in laminin together?

Disulfide bonds Signup and view all the answers

How do fibronectin fibrils primarily assemble in the extracellular matrix?

In the presence of fibronectin-binding proteins on cell surfaces Signup and view all the answers

What is the primary function of type IV collagen in the basal lamina?

To give tensile strength and support Signup and view all the answers

What is the role of integrins in the assembly of fibronectin fibrils?

To provide a linkage between fibronectin and the actin cytoskeleton Signup and view all the answers

What characteristic of fibronectin molecules allows them to bind to each other?

Exposure of cryptic binding sites upon stretching Signup and view all the answers

What is the thickness range of the basal lamina?

40–120 nm Signup and view all the answers

Which of the following is NOT a role of the basal lamina?

Catalyzing enzymatic reactions Signup and view all the answers

Which two components are major parts of the basal lamina?

Laminin and type IV collagen Signup and view all the answers

How does the basal lamina influence cell behavior?

By organizing proteins in adjacent membranes and influencing cell polarity Signup and view all the answers

Where does the basal lamina typically lie?

Beneath epithelial cells and surrounding individual cells Signup and view all the answers

What ensures that fibronectin fibrils assemble only where they are mechanically needed?

Tension exerted by cells on their surfaces Signup and view all the answers

What does the basal lamina act as in the kidney glomerulus?

A selective filter between cell sheets Signup and view all the answers

In addition to laminin and type IV collagen, which other protein is commonly found in the basal lamina?

Perlecan Signup and view all the answers

What happens to type III fibronectin repeats when they are stretched?

They unfold and expose cryptic binding sites Signup and view all the answers

What is the structural role of the basal lamina with respect to the cells it surrounds?

To provide mechanical connection and support Signup and view all the answers

What ensures proper assembly of fibronectin fibrils on cell surfaces rather than in the bloodstream?

Cell surface interactions and mechanical tension Signup and view all the answers

What is the role of the basal lamina at the neuromuscular junction following nerve or muscle injury?

It remains intact and helps reconstruct the synapse at the correct location. Signup and view all the answers

Which component of the basal lamina is responsible for regulating the assembly of acetylcholine receptors in muscle cells?

Agrin Signup and view all the answers

What is a consequence of defects in the components of the basal lamina at the neuromuscular junction?

Some forms of muscular dystrophy Signup and view all the answers

Why is the degradation of the extracellular matrix important for tissue repair?

It allows for the remodeling and adaptation of tissues. Signup and view all the answers

Which type of proteases is primarily involved in degrading matrix proteins such as collagen and laminin?

Matrix metalloproteases Signup and view all the answers

How do cells typically localize protease activity when degrading the extracellular matrix?

Through specific anchoring proteins and membrane-associated activators. Signup and view all the answers

What is a significant consequence of inadequate matrix degradation within tissues?

Inhibition of cell division and migration. Signup and view all the answers

What role does localized degradation of matrix components play during white blood cell migration?

It allows their escape through the basal lamina of blood vessels. Signup and view all the answers

What happens to the junctional basal lamina when a frog muscle and its motor nerve are destroyed?

It remains intact, preserving the synaptic sites. Signup and view all the answers

What type of metalloproteases cleave collagen at specific sites to maintain structural integrity?

Collagenases Signup and view all the answers

Which of the following is a critical ability for cells regarding the extracellular matrix?

To degrade and resynthesize matrix components. Signup and view all the answers

What can result from the proteolytic cleavage of matrix proteins?

It generates fragments with specific biological activities. Signup and view all the answers

Which factor is significant for the balance of matrix protease activity?

Specific mechanisms that ensure precise timing and location of activity. Signup and view all the answers

What is a key function of the junctional basal lamina with respect to synaptic components?

To control the localization of synaptic components on both sides of the lamina. Signup and view all the answers

How do proteoglycans contribute to the action of growth factors in the extracellular matrix?

By generating large local reservoirs that limit diffusion. Signup and view all the answers

What is a primary role of the basal lamina in relation to epithelial cells?

To provide a barrier keeping cells in their compartments. Signup and view all the answers

In Drosophila, how do membrane-associated proteoglycans like Dally function during development?

They concentrate and regulate the localization of signal proteins. Signup and view all the answers

What effect do glycosaminoglycans (GAGs) have on the extracellular matrix?

They attract water and increase the volume of extracellular space. Signup and view all the answers

Which of the following explains the interaction between fibroblast growth factors (FGFs) and proteoglycans?

Proteoglycans enhance FGF activity through oligomerization. Signup and view all the answers

What is a characteristic role of fibronectin within the extracellular matrix?

To promote the activities of growth factors like VEGF and HGF. Signup and view all the answers

What happens to the signaling protein Dpp when the gene encoding Dally is mutated in Drosophila?

Dpp activity is reduced, leading to abnormal oocyte development. Signup and view all the answers

How does the structure of type IV collagen contribute to its function in the basal lamina?

It aggregates into a sheet-like mesh that supports the basal lamina. Signup and view all the answers

What potential consequences arise from the inactivation of specific proteoglycans?

They may lead to severe developmental defects. Signup and view all the answers

Which of these components primarily provides elasticity to the extracellular matrix?

Elastin molecules. Signup and view all the answers

What role do matrix glycoproteins typically play in cell communication?

They provide direct binding sites for specific cell-surface receptors. Signup and view all the answers

In what way do proteoglycans affect morphogen gradients in an embryo?

They concentrate morphogens in a confined area to promote differentiation. Signup and view all the answers

Which property of extracellular matrix proteins enhances their ability to interact with signal proteins?

The extensive arrays of binding domains they contain. Signup and view all the answers

How do extracellular matrix components like laminin contribute to cell behavior?

By generating signals that modify cellular response. Signup and view all the answers

Study Notes

Extracellular Matrix

The extracellular matrix (ECM) is a complex network of macromolecules present in animal tissues, influencing cell behavior.
It is composed of glycosaminoglycans (GAGs), fibrous proteins (mainly collagens), and glycoproteins.
Variation in the relative amounts and organization of these components results in a diverse range of materials, from hard bone to jelly-like substances.

Components of the Extracellular Matrix

Glycosaminoglycans (GAGs) are highly charged polysaccharides that usually bind to proteins forming proteoglycans.
GAGs attract cations, absorb water, and create a hydrated gel that resists compression forces.
Hyaluronan, a simple GAG, is a space filler during morphogenesis and wound healing.
Proteoglycans are composed of GAG chains covalently linked to a core protein, synthesized and modified in the endoplasmic reticulum and Golgi apparatus.
They come in various sizes and can form large aggregates with other matrix molecules.
Collagens are fibrous proteins with a triple-stranded helical structure, rich in proline and glycine.
Collagen fibrils assemble into higher-order structures, providing tensile strength.
Fibril-associated collagens link fibrils to each other and other matrix components.
Network-forming collagens create structures like the basal lamina and anchoring fibrils.

Extracellular Matrix Function

The ECM provides physical support, regulates cell behavior, and facilitates diffusion of nutrients and metabolites.
Variation in ECM composition and organization contributes to the unique characteristics of different tissues.
It interacts with cells through cell-matrix junctions, influencing their survival, development, and function.
It plays a vital role in tissue morphogenesis, wound healing, and tissue regeneration.

Cell-Matrix Interactions

Cells secrete ECM components and organize their spatial arrangement.
Fibroblasts are the primary producers of ECM components in connective tissues.
Specialized cells like chondrocytes and osteoblasts produce ECM components in cartilage and bone.
Cells interact with the ECM through transmembrane proteins called integrins, influencing cell signaling and behavior.

Collagen Types

Collagen is a fibrous protein found in connective tissues, bones, cartilage, and skin.
Collagen types are classified based on their structure, function, and tissue distribution.
Collagen type XVII is a transmembrane protein found in hemidesmosomes.
Collagen type XVIII is a core protein of a proteoglycan in the basal lamina.

Collagen Synthesis

Collagen genes are large and contain multiple exons.
Pro-α chains are synthesized on ribosomes and injected into the endoplasmic reticulum (ER).
Propeptides are located at the N- and C-terminal ends of pro-α chains.
Hydroxylation and glycosylation occur in the ER lumen.
Proline hydroxylation requires ascorbic acid (vitamin C).
Scurvy, due to vitamin C deficiency, disrupts collagen synthesis leading to fragile blood vessels, loose teeth, and impaired wound healing.

Collagen Assembly

Three pro-α chains assemble into a triple-stranded helix called procollagen.
The propeptides guide the formation of the triple helix.
After secretion, propeptides are removed by proteolytic enzymes, converting procollagen to collagen.
Collagen molecules assemble into fibrils.
Covalent cross-links form between lysine residues, increasing the tensile strength of fibrils.

Fibril-associated Collagens

Fibril-associated collagens, such as types IX and XII, bind to fibrillar collagens and affect their organization.
They have non-helical domains that make them flexible.
Type IX collagen binds to type II collagen fibrils in cartilage, cornea, and vitreous humor.
Type XII collagen binds to type I collagen fibrils in tendons.

Elastin

Elastin provides elasticity to tissues like skin, blood vessels, and lungs.
It is rich in proline and glycine but not glycosylated.
Tropoelastin is the precursor of elastin.
Cross-linking of tropoelastin forms elastic fibers.
Elastin fibers are interwoven with collagen fibrils to prevent tearing.
Mutations in the elastin gene cause problems with blood vessel elasticity.

Fibrillin

Fibrillin is a glycoprotein that forms microfibrils.
Microfibrils provide scaffolding for elastin deposition.
Mutations in the fibrillin gene lead to Marfan syndrome.

Extracellular Matrix Organization

Cells influence the organization of the extracellular matrix.
Fibroblasts play a role in collagen fibril alignment.
The density and composition of the extracellular matrix vary in different tissues.
Abnormal matrix density is associated with fibrotic diseases and cancer.

Multidomain Glycoproteins

Multidomain glycoproteins, like fibronectin, help organize and stabilize the extracellular matrix.
Fibronectin binds to cell surface integrins, collagen, and fibrin.
These glycoproteins can influence the function of growth factors.
They provide tracks for cell migration and prevent cells from entering forbidden areas.

Fibronectin

Fibronectin is a dimeric protein
Fibronectin plays a critical role in blood vessel formation (angiogenesis)
Fibronectin subunits are linked by disulfide bonds at their C-terminal ends
Each fibronectin subunit contains repeated domains
Fibronectin undergoes alternative splicing to produce multiple isoforms
Fibronectin binds to collagen, proteoglycans, and integrins
Fibronectin binds to integrins via the Arg-Gly-Asp (RGD) tripeptide sequence
Fibronectin is essential for cell adhesion and migration.

Fibronectin Assembly and Tension

Fibronectin can exist in soluble and insoluble forms
Fibronectin fibrils form on cell surfaces through interactions with integrins
Integrins link fibronectin to the actin cytoskeleton
Tension generated by the cytoskeleton stretches fibronectin, exposing cryptic binding sites
Tension-dependent assembly ensures fibril formation only where needed.

The Basal Lamina

The basal lamina is a specialized extracellular matrix
The basal lamina is essential for epithelial cell function
Major components of the basal lamina include laminin, type IV collagen, nidogen, and perlecan
The basal lamina controls cell polarity, metabolism, and survival
The basal lamina acts as a selective barrier and filter.

Laminin and Type IV Collagen

Laminin is the primary organizer of the basal lamina
Laminins are heterotrimers composed of α, β, and γ chains
Laminin interacts with cell receptors, particularly integrins and dystroglycan
Type IV collagen forms a network that provides tensile strength to the basal lamina.

Basal Lamina Assembly

Laminin and type IV collagen interact with each other and other basal lamina components
Cell-surface receptors organize the assembly of the basal lamina
Defects in laminin-332 result in junctional epidermolysis bullosa, a blistering disease.

Basal Lamina Functions

The basal lamina functions as a barrier and filter
It influences cell migration
It plays a critical role in tissue regeneration
It directs the formation of synapses in the neuromuscular junction.

Matrix Degradation

Cells degrade matrix using extracellular proteolytic enzymes (proteases)
Matrix degradation is essential for cell division, migration, and tissue repair
Matrix degradation is critical for the spread and proliferation of cancer cells.

Extracellular Matrix Degradation

Matrix metalloproteases and serine proteases work together to degrade matrix proteins like collagen, laminin, and fibronectin.
Some metalloproteases, like collagenases, are highly specific and cleave particular proteins at specific sites, preserving the matrix’s structure and allowing cell movement through limited proteolysis.
Other metalloproteases are less specific but are anchored to the plasma membrane, limiting their activity to where it’s needed, aiding cell division within the matrix.
Protease activity is controlled by:
- Specific anchoring proteins
- Membrane-associated activators
- Specific protease inhibitors
Proteolytic cleavage of matrix proteins doesn’t always destroy them; it can generate fragments with specific biological activities.
- Cleavage of type IV collagen by metalloproteases releases fragments that inhibit blood vessel formation.

Proteoglycans and Glycoproteins Influence Secreted Protein Activity

The extracellular matrix influences cell signaling.
Signal molecules like growth factors and morphogens diffuse through the extracellular fluid, interacting with the matrix and potentially altering their function.
Heparan sulfate chains in proteoglycans interact with secreted signal molecules like FGFs and VEGF, affecting cell proliferation.
Proteoglycans create local reservoirs of growth factors, limiting their diffusion and focusing their actions on nearby cells.
They also contribute to creating morphogen gradients in embryos, important for tissue patterning during development.
Proteoglycans enhance FGF activity by oligomerizing FGF molecules and interacting with cell-surface FGF receptors, enabling the FGF to cross-link and activate its receptors more effectively.
In Drosophila, the membrane-associated proteoglycans Dally and Dally-like govern the function of several signal proteins during development.
- They concentrate signal proteins in specific locations and act as co-receptors.
Dally is involved in localizing and regulating the signaling protein Dpp in the Drosophila ovary, which prevents germ-line stem cell differentiation.
Other matrix molecules, like type IV collagen and fibronectin, also interact with signal proteins.
Matrix glycoproteins contain binding domains that influence the presentation of signal proteins to their target cells.
These glycoproteins also contain domains that bind directly to cell-surface receptors, generating signals influencing cell behavior.

Extracellular Matrix Overview

The extracellular matrix (ECM) supports cell survival, development, shape, polarity, and movement.
The ECM is a network of protein fibers and glycosaminoglycan (GAG) chains.
GAGs are negatively charged polysaccharide chains, forming proteoglycans when linked to protein (except for hyaluronan).
GAGs attract water, increasing the volume of extracellular space.
Proteoglycans are also found on cell surfaces, often acting as co-receptors to help cells react to secreted signal proteins.
Fiber-forming proteins provide strength and resilience to the ECM.
- Fibrillar collagens are rope-like structures that aggregate into long fibrils, providing tensile strength and anchors for cells.
Elasticity is provided by elastin molecules, forming a cross-linked network of fibers and sheets that stretch and recoil.
The basal lamina is a specialized form of ECM underlying epithelial cells or surrounding other cell types.
- It is organized around a framework of laminin molecules linked by side-arms and binds to integrins and other receptors in the basal plasma membrane.
- Type IV collagen, nidogen, and the proteoglycan perlecan form a sheet-like mesh within the basal lamina.
Basal laminae support epithelia, create an interface between epithelia and connective tissue, filter in the kidney, act as barriers to keep cells organized, and influence cell polarity and differentiation.

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Description

Explore the essential roles and characteristics of the extracellular matrix (ECM) in animal tissues through this quiz. Discover how ECM contributes to cellular functions and tissue structure, and learn about the cells responsible for its production. Test your knowledge on calcification, tensile strength, and more related to the ECM.