the extra cellular matrix of connective tissue

8/3/2019 The Extra Cellular Matrix of Connective Tissue

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THE EXTRACELLULAR MATRIX

OF CONNECTIVE TISSUESAA


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Introduction

Tissues are not made up solely of cells

A substantial part of their volume is extracellularspace

which is largely filled by an intricate network ofmacromolecules constituting theextracellular matrix

The extracellular matrix in connective tissue is

frequently more plentiful than the cells itsurrounds, and it determines the tissue'sphysical properties
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.glossary.4754http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.glossary.4754


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COMPOSITION

This matrix is composed of a variety of proteins andpolysaccharides that are secreted locally and assembled into anorganized meshwork in close association with the surface of the cellthat produced them.

The proteoglycan molecules in connective tissue form a highlyhydrated, gel-like ground substance in which the fibrous proteinsare embedded.

The polysaccharide gel resists compressive forces on the matrixwhile permitting the rapid diffusion of nutrients, metabolites, andhormones between the blood and the tissue cells.

The collagen fibers both strengthen and help organize the matrix,and rubberlike elastin fibers give it resilience.

Finally, many matrix proteins help cells attach in the appropriatelocations

Consist of ground substance, fiber and cell


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Two main macromolecule

Two main classes of extracellular

macromolecules make up the matrix:

(1) polysaccharide chains of the class called

glycosaminoglycans (GAGs), which are usuallyfound covalently linked to protein in the form of

proteoglycans,

(2) fibrous proteins, including collagen, elastin,

fibronectin, and laminin, which have both

structural and adhesive functions.


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The Extracellular Matrix of Animals

Glucosaminoglycan (GAG)-usually linked to protein

in the form of proteoglycans, resists compressiveforce Fibrous proteins

- Structural: collagen and elastin, tensile strength- Adhesive: fibronectin and laminin


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http://upload.wikimedia.org/wikipedia/en/f/f5/Extracellular_Matrix.png


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http://upload.wikimedia.org/wikipedia/en/f/f5/Extracellular_Matrix.png


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Figure 19-37. The relative dimensions and volumes occupied by

various macromolecules. Several proteins, a glycogen granule, and a

single hydrated molecule of hyaluronan are shown.

2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith

Roberts, and Peter Walter.


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Figure 19-59. The comparative shapes and sizes of some of the major

extracellular matrix macromolecules. Protein is shown in green, and

glycosaminoglycan in red.


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Function

Connective tissues form the framework of thevertebrate body

Attachment

Strengthen Organization

Resist compressive force

Resilience

Permitting rapid diffusion

Tissue repair


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Variation

Variations in the relative amounts of the

different types of matrix macromolecules

and the way in which they are organized in

the extracellular matrix give rise to anamazing diversity of forms, each adapted

to the functional requirements of the

particular tissue


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ECM is made by cell within It

Fibroblast

Chondroblasts

Osteoblast


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Scanning electron micrograph of fibroblasts

in connective tissue


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GAG Chains Occupy Large amount

of space

Unbranched polyscharide chain

Repeating dissaccharide unit

Consist of ; Hyaluronan

Chondroitin sulfate and dermatan sulfate

Heparan sulfate

Keratan sulfate


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Glycosaminoglycan

hyaluronan, 25,000Nonsulfated discccharide units ;Space filler; wound repair; lubricant;

chondroitin sulfate and dermatan sulfate,

heparan sulfate and heparin, keratan sulfate

Negative Charge Na+ H2O withstand compressive forces

Less than 10% by weight; Porous hydrated gels

Classified by sugar residues, type of linkage, the numberand location of sulfate


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Schematic structures of GAGs. Heparin/HS and hyaluronic acid (HA) are glycosaminoglycans; chondroitin-4-sulfate (C4S),

chondroitin-6-sulfate (C6S) and dermatan sulfate (DS) are galactosaminoglycans; keratan sulfate (KS) is a sulfatedpolylactosamine. Since heparin/HS structures are highly heterogeneous, only their most abundant disaccharide unit IdoA(2-

OSO3)-GlcNSO3(6-OSO3) is shown here.


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Figure 19-40. Examples of a small (decorin) and a large (aggrecan)

proteoglycan found in the extracellular matrix. These two proteoglycans arecompared with a typical secreted glycoprotein molecule, pancreatic ribonuclease B.

All three are drawn to scale. The core proteins of both aggrecan and decorin

contain oligosaccharide chains as well as the GAG chains, but these are not

shown. Aggrecan typically consists of about 100 chondroitin sulfate chains and

about 30 keratan sulfate chains linked to a serine-rich core protein of almost 3000

amino acids. Decorin decorates the surface of collagen fibrils, hence its name.

2002 by Bruce Alberts, Alexander Johnson, Julian Lewis,

Martin Raff, Keith Roberts, and Peter Walter.


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An aggrecan aggregate from fetal bovine cartilage


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The repeating disaccharide sequence of a

dermatan sulfate glycosaminoglycan (GAG) chain

Dwarf: prematurely aged appearance, generalized

defects in skin , joints, muscles, and bones.


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The repeating disaccharide sequence in

hyaluronan, a relatively simple GAG


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Hyaluronan acts as a space filler

and cell facilitator during repair

Lubricant

Need hialuronidase enzyme

Not from inside the cell


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Proteoglycans

Can regulate the activities of secreted

proteins

Composed of GAG chains covalently

linked to core protein

Acts as co-receptors


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The linkage between a GAG chain and its

core protein in a proteoglycan molecule

Glycosul transferases

Sulfation and epimerization reaction

95 % carbohydrate by weight


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Proteoglycans Can Regulate the Activities of

Secreted Signaling Molecules

Fibroblast growth factor (FGF) binds to heparan sulfate

chains of proteoglycans which is a required step for FGF

to activate its cell-surface receptor

TGF- binds to the core proteins of several matrixproteoglycans, ex. Decorin; binding to decorin inhibits the

activity


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Cell-surface proteoglycans act as co-receptors

Syndecans

- extracellular domain:

carries chondroitin sufate and heparan sulfate GAG

chains- intracellular domain

interact with actin cytoskeleton

- Serve along with integrins as receptors for collagen,

fibronectin, and also binds to fibroblast growth factor

and present it to FGF receptor


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Collagens

Major proteins of ECM Fibrous protein, triple stranded-helical structure,

alfa chain (Gly-X-Y) Rich proline and glycine Human genome contain 2 distinct gene coding

for different collagen alfa chain Less than 40 types of collagen molecules have

been found

Divide into collagen fibrils, fibrils associatedcollagen, network-forming collagen, anchoringfibrils


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Type Collagen

Note that types I, IV, V, IX, and XI are

each composed of two or three types of

chains, whereas types II, III, VII, XII, XVII,

and XVIII are composed of only one typeof chain each. Only 11 types of collagen

are shown, but about 20 types of collagen

and about 25 types of chains have beenidentified so far.


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Electron micrograph of fibroblasts surrounded by collagen

fibrils in the connective tissue of embryonic chick skin


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Figure 19-43. The structure of a typical collagen molecule. (A) A model of part of a

single collagen chain in which each amino acid is represented by a sphere. The chain

is about 1000 amino acids long. It is arranged as a left-handed helix, with three amino

acids per turn and with glycine as every third amino acid. Therefore, an chain iscomposed of a series of triplet Gly-X-Y sequences, in which X and Y can be any amino

acid (although X is commonly proline and Y is commonly hydroxyproline). (B) A model

of part of a collagen molecule in which three chains, each shown in a different color,

are wrapped around one another to form a triple-stranded helical rod. Glycine is the only

amino acid small enough to occupy the crowded interior of the triple helix. Only a short

length of the molecule is shown; the entire molecule is 300 nm long.(by B.L. Trus.)


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Figure 19-50. The shaping of the extracellular matrix by cells. This

micrograph shows a region between two pieces of embryonic chick heart

(rich in fibroblasts as well as heart muscle cells) that were cultured on a

collagen gel for 4 days. A dense tract of aligned collagen fibers has formedbetween the explants, presumably as a result of the fibroblasts in the

explants tugging on the collagen. (From D. Stopak and A.K. Harris, Dev. Biol.

90:383398, 1982. Academic Press.)


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Figure 19-46. Cross-links formed between modified lysine side chains

within a collagen fibril. Covalent intramolecular and intermolecular cross-

links are formed in several steps. First, certain lysines and hydroxylysines are

deaminated by the extracellular enzyme lysyl oxidase to yield highly reactive

aldehyde groups. The aldehydes then react spontaneously to form covalent

bonds with each other or with other lysines or hydroxylysines. Most of thecross-links form between the short nonhelical segments at each end of the

collagen molecules.


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Series of post translational

modification

Pro alfa chain

Propeptide

Hydroxylation and glycosylated procollagen


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Figure 19-47. The intracellular and extracellular events in the formation of acollagen fibril. (A) Note that collagen fibrils are shown assembling in the extracellular

space contained within a large infolding in the plasma membrane. As one example of

how collagen fibrils can form ordered arrays in the extracellular space, they are shown

further assembling into large collagen fibers, which are visible in the light microscope.

The covalent cross-links that stabilize the extracellular assemblies are not shown. (B)

Electron micrograph of a negatively stained collagen fibril reveals its typical striatedappearance. (B, courtesy of Robert


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Elastin

The main component of elastic fiber and coveredby micofibril (fibrillin)

To recoil after transient stretch (lung, skin andblood vessel)

Five times more extensible than rubber band

Interwoven collagen and elastic : to preventtissue from tearing

Not rich proline and glycine, somehydroxyproline

tropoelastin


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Figure 19-49. Type IX collagen. (A) Type IX collagen molecules binding in a

periodic pattern to the surface of a fibril containing type II collagen. (B) Electron

micrograph of a rotary-shadowed type-II-collagen-containing fibril in cartilage,

sheathed in type IX collagen molecules. (C) An individual type IX collagen

molecule. (B and C, from L. Vaughan et al., J. Cell Biol. 106:991997, 1988.

The Rockefeller Unive


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Figure 19-52. Stretching a network of elastin molecules. The molecules

are joined together by covalent bonds (red) to generate a cross-linked

network. In this model, each elastin molecule in the network can expand and

contract as a random coil, so that the entire assembly can stretch and recoillike a rubber band


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Figure 19-51. Elastic fibers. These scanning electron micrographs show (A) a

low-power view of a segment of a dog's aorta and (B) a high-power view of the

dense network of longitudinally oriented elastic fibers in the outer layer of the

same blood vessel. All the other components have been digested away with

enzymes and formic acid. (From K.S. Haas, S.J. Phillips, A.J. Comerota, and

J.W. White,Anat. Rec. 230:8696, 1991. Wiley-Liss, Inc.)


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Fibronectin

Non collagen protein having multiple domain asreceptor or binding site

Organizing the matrix and helping cell attach

As repellent that keep cells out of forbidden area Guide cell movement by serving as track along

which cells can migrate

Glycoprotein for many cell-matrix interaction

Composed by dimer of two large subunit joinedby disulfide bond

Type III fibronectin


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Fibronectin

Can exist both in a soluble form,

circulating in the blood and insoluble

fibronectin fibrils

Binds to integrin through an RGD Motif, as

type III repeat

Other RGD sequence : blood clotting, anti-

clotting drug

Disintegrins from snake


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Figure 19-53. The structure of a fibronectin dimer. (A) Electron micrographs of individual fibronectin dimer

molecules shadowed with platinum; red arrows mark the C-termini. (B) The two polypeptide chains are similar

but generally not identical (being made from the same gene but from differently spliced mRNAs). They are

joined by two disulfide bonds near the C-termini. Each chain is almost 2500 amino acids long and is folded into

five or six domains connected by flexible polypeptide segments. Individual domains are specialized for binding

to a particular molecule or to a cell, as indicated for five of the domains. For simplicity, not all of the known

binding sites are shown (there are other cell-binding sites, for example). (C) The three-dimensional structure of

two type III fibronectin repeats as determined by x-ray crystallography. The type III repeat is the main repeating

module in fibronectin. Both the Arg-Gly-Asp (RGD) and the synergy sequences shown in redform part of the

major cell-binding site (shown blue in B). (A, from J. Engel et al., J. Mol. Biol. 150:97120, 1981. Academic

Press; C, from Daniel J. Leahy,Annu. Rev. Cell Dev. Biol. 13:363393, 1997. Annual Reviews.)


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Figure 19-54. Coalignment of extracellular fibronectin fibrils and

intracellular actin filament bundles. (A) The fibronectin is revealed in two rat

fibroblasts in culture by the binding of rhodamine-coupled anti-fibronectin

antibodies. (B) The actin is revealed by the binding of fluorescein-coupled anti-

actin antibodies. (From R.O. Hynes and A.T. Destree, Cell15:875886, 1978.

Elsevier.)


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Basal Laminae Are Composed Mainly of Type

IV Collagen, Heparan Sulfate Proteoglycan,Laminin, and Entactin


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How type IVcollagen molecules

are thought toassemble into a

multilayerednetwork


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Three ways in which basal laminae (yellow

lines) are organized

A current model of the molecular


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A current model of the molecular

structure of a basal lamina


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Regeneration experiments indicating the special character of

the junctional basal lamina at a neuromuscular junction


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Laminin

Figure 19-57. The structure of laminin. (A) The subunits of a laminin-1 molecule. This multidomain

glycoprotein is composed of three polypeptides (, , and ) that are disulfide-bonded into an

asymmetric crosslike structure. Each of the polypeptide chains is more than 1500 amino acids long.

Five types of chains, three types of chains, and three types of chains are known; in principle,

they can assemble to form 45 (5 3 3) laminin isoforms. Several such isoforms have been found,

each with a characteristic tissue distribution. (B) Electron micrographs of laminin molecules

shadowed with platinum. (B, from J. Engel et al., J. Mol. Biol. 150:97120, 1981. Academic Press.)


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Matrix Degradation

Important for tissue repair

In remodelling so as to adapt to the stresses

Enable to divide and to travel

Need ability to cut through matrix becauseembedded and rounded, unable to extend

(impermeable)

To escape from confinement by basal laminaneed localized degradation

Like cancer cell can spread and proliferate

MD i l li d t th i i it f


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MD is localized to the vicinity of

cells

Need proteoses Matrix metalloproteases; bound Ca and Zn

Serine proteases

Degrade collagen, laminin, fibronectin Others : collagenases

Three basic mechanism : local activation

(plasminogen), confinement by receptorand secretion of inhibitor (TIMP andSerpin)


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How theextracellularmatrix could

propagate order

from cell to cellwithin a tissue


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Figure 19-63. The importance of proteases bound to cell-surface receptors. (A) Human prostate cancer cells make andsecrete the serine protease uPA, which binds to cell-surface uPA

receptor proteins. (B) The same cells have been transfected withDNA that encodes an excess of an inactive form of uPA, whichbinds to the uPA receptors but has no protease activity. Byoccupying most of the uPA receptors, the inactive uPA prevents theactive protease from binding to the cell surface. Both types of cellssecrete active uPA, grow rapidly, and produce tumors when injectedinto experimental animals. But the cells in (A) metastasize widely,

whereas the cells in (B) do not. To metastasize via the blood, tumorcells have to crawl through basal laminae and other extracellularmatrices on the way into and out of the bloodstream. Thisexperiment suggests that proteases must be cell-surface bound tomediate migration through the matrix.


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Penutup

Connective tissue contain matrix

extracellular

the extra cellular matrix of connective tissue

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