The Extracellular Matrix

In solid tissues and organs the cells are pushed right up against each other. In fact the epithelial cells have special junctions to attach them to their neighbors as tightly and closely as possible. For other tissues like the liver and brain, there is actually a small gap of somewhere around 20 nm between neighboring cells.

All the 'stuff' found in between the cells found in connective tissue is called the
Extracellular Matrix.
This 'stuff' is defined as consisting of (1)amorphous ground substance and (2)fibers.

Now, in contrast, the cells of connective tissues are separated by vast expanses of extracellular matrix. This extracellular matrix contains various types of ground substance of hydrated glycoproteins and proteoglycans. And it is the types, amounts and arrangements of these constituents that determine the mechanical properities of that connective tissue. As you well know, the properties and consistency of these connective tissues vary quite a bit; the dermis, cartilage, tendons, and bone.

In bone and cartilage the extracellular matrix must provide structural rigidity. In tendons, it must provide tensile strength. In skin, the extracellular matrix must provide elasticity.

The amorphous ground substance consists of highly hydrated polysaccharides, proteoglycans, and glycoproteins. These molecules as we will see are very large, and very branched. They carry small charges on their atoms and so attract charged (polar) water molecules. In this way, they are described as holding water in the connective tissue spaces. That's why you cannot push on your skin and move any fluid out from underneath your skin, even though the dermis is full of water. The water molecules are 'held' in the dermis by their attraction to the charged, branching polysaccharides, glycoproteins and proteoglycans.

The other component of the extracellular matrix are large, thick fibers. These thick fibers act like the steel bars that are put into concrete to reinforce it. The most prevalent and familar one of these protein fibers are the collagen fibers.

One thing to remind yourself of is to think about where all these molecules came from. The thick protein fibers like collagen and the highly branched polysaccharides, glycoproteins and proteoglycans are all synthesized by the cells found in the connective tissues and then exported (exocytosed) into the extracellular space, making it the extracellular matrix. We will see later that if the manufacturing, exporting or degredation of these molecules somehow becomes defective, certain diseases will result.

While we are still trying to see the big picture of all this, also keep in mind that these proteins, polysaccharides, glycoproteins, and proteoglycans found in the extracellular space are attached to the surface of the cells. We will mention later the importance of these special membrane proteins that act to attach all these extracellular molecules.

Amorphous Ground Substance:

Picture a pine tree (a Christmas tree, if you'd like) with it's trunk at the center,
it's branches, and then the pine needles sticking up and around from the branches.
The molecular equivalent is floating around in the extracellular matrix.
The entire molecule (pine tree) is called a proteoglycan.
The 'trunk' of the pine tree is a molecule of Hyaluronic acid;
the 'branches' are proteins called core proteins;
and the needles branching out everywhere are molecules called glycosaminoglycans (GAG's).

Let's look at each type of molecule, one at a time. First are the glycosaminoglycans (GAG's). Now don't panic. Let's dissect the name: 'glycos'-'amino'-'glycans'. This large molecule starts with a sugar, glucose. This glucose has an amino group (NH2) attached to it. So that's where the '--aminoglycan' part of the name comes from. Now also linked to this amino containing glucose is another glucose-like sugar. So there is the final name of this molecule, 'glycosaminoglycan'. There, not so bad now is it. What you really need now is a picture to show you. Let's take a look.

This specific glycosaminoglycan (GAG) is Keratan Sulfate. I know, it looks way to complicated to ever understand. But we can. Look at the right part of this molecule first. There is the framework of our good friend, 6-carbon glucose. You'll also see attached to carbon #2 a nitrogen group. There's our '--aminoglycan' part of the molecule. Now on the left of this molecule you'll also see another 6-carbon glucose framework attached to the first one on the right. So you see you can recognize this glycosaminoglycan.

There are some variations on this framework of two 6-carbon glucoses attached to each other. These are the different types of GAG's. Take a look:


Dermatan Sulfate


Chondroitin Sulfate


Heparin Sulfate (Heparin)

Now take one of these disaccharides as a unit and start linking them together. And I mean linking them together. These chains of repeating disaccharide units can extend for 10,000 or more. That's a long chain of GAG's. Although it is a bit hard to read, on the diagrams of the GAG's you can see some charges. These free charges can attract charged water molecules, 'holding water'.

These very, very long sugar chains are the pine needles of our Chrismas tree example. The 'pine tree branch' that hundreds of these individual sugar chain 'needles' are attached to is a long protein molecule called the 'core protein'.

Keep your imagination open. The previously described 'branch', as three dimensionally big as it is, is still only one branch of hundreds attached to the 'pine tree trunk'. The 'tree trunk' consists of a molecule called 'Hyaluronic Acid'.


Hyaluronic Acid

Now don't let this confuse you, but hyaluronic acid is a GAG, a long chain of repeated disaccharides. There's our pine tree shaped molecule. The entire molecule is called a 'proteoglycan'. And getting back to the point, these different types of proteoglycans are found in the connective tissues, in between the cells. At the beginning of this web page I kept referring to 'highly hydrated polysaccharides, proteoglycans, and glycoproteins'. These are the proteoglycans we just defined. These huge, charged, branching molecules along with the attracted water is the 'amorphous ground substance'.

Glycoproteins:
It should be clear as to how the cell manufactures proteins. Once the protein is produced by translations, it is usually modified by folding it a specific way, cleaving off parts of it, and sometimes combining it with another protein. All of this is usually referred to as post-translational modification. In some cases the protein will have a sugar or sugars (quite long branching sugar groups commonly) added to it. This is what is called a glycoprotein. The construction of these branching chains of sugars that are later added to a protein is quite involved and very interesting. However, we will not be going into how this is done.

Fibers:


The other major component of the extracellular matrix are the large fibers. These fibers are:
A) Collagen (structural)


A Collagen 'Fibril', a triple helix of 3 tropocollagens.

The collagens account for about 25% of the total protein in an adult. That's a lot. It's highest concentrations are in tendons, bones, cartilage, and the dermis. You notice that I called them the collagens. There are close to 20 slightly different molecular forms of 'collagen', hence the family of collagens. They all consist of three polypeptide chains that form a characteristic triple helix (like a piece of string that has three fibers wound around each other).

Interesting Facts About The Collagens:

The amino acid 'glycine' may make-up more than 33% of all the amino acids.
  And the amino acid 'proline' makes-up a large percentage also.

Collagen contains the amino acids '3-hydroxyproline, 4-hydroxyproline, and 5-hydroxylysine.
  These three amino acids do not have transfer RNA's. They are not represented in the genetic code.
  They are synthesized posttranslationally from prolines and lysines already inserted into the collagen polypeptide chain.
  This is why some people argue that there are in fact 23, not 20 amino acids in total.

You can find a 'Glu-Gal' disaccharide attached to the hydroxy group of hydroxylysine.

Since the essential amino acids (those amino acids that you must obtain from your diet, your body cannot make them)
  such as isoleucine, phynylalanine, and tyrosine are in very low amounts in collagens, the collagens are not a very good dietary source of protein. Jell-O (gelatin) is denatured collagen and so is not a good source of protein.

The three intertwined polypeptides of collagen is called Tropocollagen. By having several different polypeptide
  chains, all from the collagen family, the different combinations account for the many different types of collagens.

Every third amino acid in these polypeptide chains is the amino acid 'glycine'.
  Since the helical curve to the tropocollagen triple helix has 3 amino acids per turn, all these glycines are on the same side.

The enzyme 'collagenase' will cut tropocollagen which then unravels and is now susceptible to other peptidases such as pepsin and trypsin.

The tropocollagen triple helix fibers are secreted out of the cell and eventually crossed linked together outside the cell to form the collagen fibril.

Each individual collagen protein is first synthesized on a ribosome with 'extra' amino acids on both ends and is called preprocollagen. These extra amino acids are cut off at two separate times. The first cleavage removes the first few extra amino acids. A later cleavage removes the rest. The two small fragments cut off from both ends, at two different times are called 'propeptides'. After the first cleavage, 'preprocollagen' is converted into 'procollagen'. After the second cleavage, 'procollagen' is converted into 'collagen' or what we call 'tropocollagen'.

The three strands of preprocollagen are assembled into a triple helix in the endoplasmic reticulum prior to the cleaving off of the first set of propeptides. It is these outermost extra amino acids that help assemble the three free preprocollagen molecules into the triple helix.

Once the triple helical preprocollagen is formed the first set of propeptides are cleaved off, converting the preprocollagen molecule into the 'procollagen' molecule. It is the procollagen molecule that is secreted out of the cell. If it is improperly coiled, it cannot be secreted and is degraded in the cell.

The two functions of the propeptides is to (a)initiate the formation of the triple helix in the ER and (b)they prevent premature fibril cross linking while still in the ER.

Once outside the cell in the extracellular matrix, enzymes there cleave off the second set of additional amino acids (the second set of propeptides). Now the tropocollagen triple helices can cross link to form fibrils.

The amount of collagen increases with age, and the amount of cross linking also increases over time. That's why the meat of young animals is soft and tender while that of older animals is tough and hard to eat.

Collagen synthesis is stimulated by wound healing and inflammation. Scar tissue is rich in collagen, as is wall of an abcess.

Diseases Related to Collagen
Scurvy
Osteogenesis Imperfecta
Ehlers-Danlos Syndrome
Spondyloepiphyseal Dysplasia
Epidermolysis Bullosa dystrophica

As you can see from this diagram, if the tropocollagen triple helix uses different collagen protein chains, a mutation in the gene for only one chain will allow this defective chain to intertwine with other normal, non-mutated chains in different combinations and different proportions, however, effecting every tropocollagen it becomes a part of.

B)Elastin (structural)
Elastin is a structural protein that gives elasticity to our tissues and organs. Elastin is found predominantly in the walls of our arteries, in our lungs, intestines, and skin, as well as in other elastic tissues.
It functions in connective tissue in partnership with collagen. Whereas collagen provides rigidity, elastin is the protein which allows the connective tissues in our blood vessels and heart tissues, for example, to stretch and then recoil to their original positions.

Imagine elastin within the body's connective tissue to act like a bunch of rubber bands that are tied together at a number of places. When the elastic bands are pulled, they will stretch, and when there is no longer a pull, they will return to their original relaxed state. You can't pull the elastin chain too far because the companion stiff collagen fibers in the connective tissue limit the stretching of the elastin fibers in the tissue.

Elastin is a very tough and relatively stable protein because it has many internal linkages. Those linkages make elastin resistant to the normal breakdown characteristic of most proteins.

Since elastin is relatively stable, do we need to make elastin throughout our lives? No! Normally the body stops making elastin once the body reaches maturity soon after puberty. A geneticist would say the same thing by stating that "the gene for elastin is turned off just after puberty." In other words, once the body has made its elastin, it will not make that protein any more. What is the consequence of not being able to make any more elastin after we mature? In two words, aging begins.

Elastin polypeptide chains are cross-linked together to form rubberlike, elastic fibers. Each elastin molecule uncoils into a more extended conformation when the fiber is stretched and will recoil spontaneously as soon as the stretching force is relaxed.

There rubber-band like fibers consist of the protein elastin. The elastin fibers are assmebled into 'microfibrils'. A glycoprotein associated with connecting these elastin fibers into microfibrils is fibrillin. In Marfan Syndrome the fibrillin is defective, resulting in patients who are tall, with long, spidery fingers (arachnodactyly), displaced lenses in the eyes (ectopia lentis), and the tunica media of the large arteries is abnormally weak. Many such patients die suddenly after a rupture of the aorta.

C)Laminins (Cell Adhesion)


You will notice how each laminin molecule with it's very characteristic
curved shape interlocks with other laminin molecules.

Laminins are large, complex, extra-cellular matrix glycoproteins that reside principally in the basement membrane (basal lamina). The laminin family of glycoproteins are major components of basement membranes. The laminins can self-assemble and bind to other matrix macromolecules. Through their binding to other molecules, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue functions, and promotion of tissue survival. Recent data from genetically manipulated mice and human inherited diseases have shown that the laminins are crucial for normal development and physiology. For example, in a type of congenital muscular dystrophy, laminin-2 is missing resulting in muscle degeneration. Also in epidermolysis bullosa, a severe skin blistering disease, defects in laminin-5 have been identified. To date 12 different forms of the laminin protein have been identified.

D)Fibronectin (Cell Adhesion)

The fibronectin dimer (FN) is involved in many cellular processes, including tissue repair, embryogenesis, blood clotting, and cell migration/adhesion. You can see it structurally has different regions (called 'domains') that will bind to collagen, or the cell membrane, or to heparin. In this way, it acts as a bridging molecule.

Fibronectin is a glycoprotein that serves as a linker in the ECM (extracellular matrix), or as a protein found in the plasma (plasma FN). The plasma membrane form is synthesized by hepatocytes, and the ECM form is made by fibroblasts, chondrocytes, endothelial cells, macrophages, as well as certain epithelial cells. Fibronectin sometimes serves as a general cell adhesion molecule by anchoring cells to collagen or proteoglycan substrates. FN also can serve to organize cellular interaction with the ECM by binding to different components of the extracellular matrix and to membrane-bound FN receptors on cell surfaces. The importance of fibronectin in cell migration events during embryogenesis has been well studied. Fibronectin's structure is rod-like, composed of three different types of homologous, repeating modules, Types I, II, and III. These modules, though all part of the same amino acid chain, can be envisioned as "beads on a string," each one joined to its neighbors by short linkers.