Collagen

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Type I collagen fibers under a electron microscope at three different magnifications (collagen fiber (left), collagen fibrils (center)). Individual fibrils show a characteristic banding pattern which results from the overlap of the tropocollagen constituents. The periodicity of this pattern is typically 64 nanometers.

Collagen is a protein that forms the main structural component of connective tissues in animals. It is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content.

Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendons, ligaments and skin. It is also abundant in corneas_, cartilage, bones, blood vessels, the gut_, `intervertebral discs`_ and the dentin in teeth. In muscle tissue, it serves as a major component of the endomysium.

Contents

1   Etymology

The name "collagen" comes from the Greek kolla meaning "glue" and suffix -gen denoting "producing".

2   Function

Tough bundles of collagen called collagen fibers are a major component of the extracellular matrix that supports most tissues and gives cells structure from the outside, but collagen is also found inside certain cells. Collagen has great tensile strength, and is the main component of fascia, cartilage, ligaments, tendons, bone and skin

Along with soft keratin, it is responsible for skin strength and elasticity, and its degradation leads to wrinkles that accompany aging. It strengthens blood vessels and plays a role in tissue development.

3   Applications

Collagen is a raw material for major industries in leather, glue, cosmetics_, `food processing`_ (gelatin), etc. [1]

4   Substance

4.1   Collagen fibers

Collagen is visible in meat without a microscope as sheets or cables.

When collagen fibers form sheets or cables we can see them in meat without a microscope, and we may detect them as gristle if they do not gelatinize when meat is cooked. [1]

Collagen fibers consist of numerous collage fibrils bound together. Collagen fibers are visible with a light microscope.

Collagen fibres are extracellular (outside the cells forming them). Although collagen fibres are located outside the cell, the initial stages of collagen fibril assembly may be within the cell, with fibril morphology being regulated by a special site on the fibroblast membrane.

Under a light microscope, collagen fibres in the connective tissue framework of meat range in diameter from 1 to 12 micrometres (0.001 millimetre = 1 micrometre).

Collagen fibres do not often branch and, when branches are found, they usually diverge at an acute angle.

Collagen fibres from fresh meat are white, but usually they are stained in histological sections for examination under a microscope. The most common stain for light microscopy is eosin, which stains collagen fibres pink. Unstained collagen fibres may be seen by polarized light since they are birefringent (with two refractive indices like A bands). By rotating the plane of polarized light, collagen fibres appear bright against an otherwise dark background (when two Polaroid lenses are perpendicular they block most of the light, but collagen fibres can rotate the light so they appear bright). The birefringence of collagen fibres in meat is lost during cooking at the point gelatinization occurs.

Collagen fibres have a wavy or crimped appearance which disappears when they are placed under tension.

Collagen fibres fluoresce with a blue-white light when excited with UV light enabling the amount of connective tissue on a cut meat surface to be measured very rapidly. Peak excitation is around 370 nm so that the prominent 365 nm peak emission of a mercury arc lamp may be used. Some indication of collagen fibre diameter may be obtained by spectrofluorometry (measuring the wavelengths of fluorescence) because the fluorescence is quenched (fades) rapidly. Thus, large collagen fibres retain a central core with a pre-quenching emission spectrum for longer than small fibres. Fat only fluoresces weakly, to about the same extent as areas of muscle with a low connective tissue content. Collagen fluorescence increases with animal age.

Collagen fibres are formed by cells called fibroblasts.

Collagen fibres shrink when they are placed in hot water, and ultimately they may be converted to gelatin_. Around 65ºC, the triple helix is disrupted and the alpha chains fall into a random arrangement. The importance of this change? It tenderizes meat with a high connective tissue content.

4.2   Collagen fibrils

Collagen fibrils are fibrils are small but strong. Collagen fibrils consist of tropocollagen.

Electron microscopy shows collagen fibrils with diameters ranging from 20 to 100 nm (0.001 micrometre = 1 nanometre). Collagen fibrils typically have diameters which are multiples of 8 nm showing the manner in which they grow radially.

Collagen fibrils are formed from long tropocollagen molecules which are staggered in arrangement but tightly bound laterally by covalent chemical bonds.

For electron microscopy, when negatively stained with heavy metals, the stain spreads into the spaces between the ends of molecules, and collagen fibrils appear to be transversely striated. The periodicity of these striations is 67 nm but often shrinks to 64 nm as samples are processed for examination.

Collagen microfibrils (even smaller structures that make up fibrils) may appear to have a tubular structure with an electron-lucent lumen (appearing empty under the electron microscope).

4.3   Tropocollagen

Tropocollagen is the most abundant protein in the animal body. Large amounts of tropocollagen are found in animal skin. In pig skin, for example, collagen fibres are tightly woven from two directions to form a tight meshwork. [1]

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Tropocollagen is a high molecular weight protein (300,000 Daltons) formed from three polypeptide strands twisted into a triple helix. Each strand is a left-handed helix twisted on itself, but the three strands are twisted into a larger right-handed triple helix. The triple helix is responsible for the stability of the molecule and for the property of self-assembly of molecules into microfibrils.

The flexible parts of each strand projecting beyond the triple helix (telopeptides) are responsible for the bonding between adjacent molecules. In other words, the cross links binding tropocollagen molecules together laterally are made between the helical shaft of one molecule and the non-helical extension of an adjacent molecule.

Tropocollagen molecules from older animals are more resistant to heat disruption than those from younger animals.

Outside the cell, tropocollagen molecules become aligned in parallel formations, and then they link up laterally to form fibrils. It is likely that tropocollagen monomers are partially assembled together in groups before they are added to an existing collagen fibril. Firstly, vacuoles containing procollagen fuse to form a fibril-containing compartment. Then the cytoplasmic extensions withdraw from between several fibril-forming compartments to create a bundle-forming compartment. [1]

Within an individual tropocollagen molecule, the three polypeptide strands are linked together by stable intramolecular bonds originating in the non-helical ends of the molecule.

5   Classification

5.1   Type I

Type I collagen forms striated fibres between 80 and 160 nm in diameter in blood vessel walls, tendon, bone, skin and meat. It may be synthesized by fibroblasts, smooth muscle cells (around blood vessels) and osteoblasts (bone-forming cells).

5.2   Type II

Type II collagen fibres are less than 80 nm in diameter and occur in hyaline cartilage (for example, in the soft blade of the scapula) and in intervertebral discs. It is synthesized by chondrocytes (cartilage-forming cells).

5.3   Type III

Type III collagen forms reticular fibres in tissues with some degree of elasticity, such as spleen, aorta and muscle. It is synthesized by fibroblasts and smooth muscle cells, contributes substantially to the endomysial connective tissues around individual myofibres, provides a small fraction of the collagen found in skin and occurs in the large collagen fibres dominated by Type I collagen. It may have some function in regulating collagen fiber growth. Unlike Type I collagen fibres, reticular Type III fibres are highly branched.

5.4   Type IV

Type IV collagen occurs in the basement membranes around many types of cells and may be produced by the cells themselves, rather than by fibroblasts. Although basement membranes were once regarded as amorphous (like glue), many of them now are thought to be composed of a network of irregular cords. The cords contain an axial filament of Type IV collagen, ribbons of heparin sulphate, proteoglycan, and fluffy material (laminin, entactin and fibronectin). Type IV collagen occurs in the endomysium around individual muscle fibers. Instead of being arranged in a staggered array, the molecules are linked at their ends to form a loose diagonal lattice.

6   Production (Collagen biosynthesis)

The synthesis of the different polypeptide strands combining to make different types of tropocollagen is genetically regulated by the production of messenger RNA. The synthesis of polypeptide strands occurs on membrane-bounded polysomes, but the hydroxylation of lysine and proline occurs after the strands are assembled. Ascorbic acid is required for the hydroxylation of lysine and proline. Polypeptide strands enter the cisternae of the endoplasmic reticulum (a membranous assembly labyrinth within the cell), the terminal extensions of the strands are aligned, and then the strands spiral around each other. Procollagen or immature tropocollagen has long terminal extensions protruding from each end of the newly formed triple helix. Procollagen moves to the golgi apparatus and is packaged into vesicles moved to the cell surface, probably by microtubules. Except for some Type III procollagen molecules, the long terminal extensions are then enzymatically reduced in length. [1]

7   Properties

7.1   Strength

The great strength of collagen fibres, originates mainly from the stable intermolecular covalent bonds between adjacent tropocollagen molecules. Stable disulphide bonds between cystine molecules in the triple helix also occur.

During the growth and development of meat animals, covalent cross links increase in number, and collagen fibres become progressively stronger. Meat from older animals, therefore, tends to be tougher than meat from the same region of carcasses from younger animals.

This relationship is complicated in young animals by the rapid synthesis of large amounts of new tropocollagen. New tropocollagen has fewer cross links so, if there is a high proportion of new tropocollagen, the mean degree of cross linking may be low, even though all existing molecules are developing new cross links. As the formation of new tropocollagen slows down, the mean degree of cross linking increases.

Another complication is many of the intermolecular cross links in young animals are reducible (the collagen is strong but is soluble). In older animals, reducible cross links are converted to non-reducible cross links (the collagen is strong but is far less soluble and more resistant to moist heat).

Pyridinoline is a non-reducible cross-link causing increased heat stability of connective tissues from older animals. It is formed without enzymes by glycosylation (a reaction between lysine and reducing sugars).

Differences in the degree of cross linking may occur between different muscles of the same carcass, and between the same muscle in different species.

Nutritional factors such as high-carbohydrate diet, fructose instead of glucose in the diet, low protein, and pre-slaughter feed restriction may reduce the proportion of stable cross links.

8   Production

The rate of collagen turnover in skeletal muscle may be about 10% per day and the turnover time for collagen may be inversely proportional to collagen fibril diameter.

9   Disease

Scurvy is a disease that prevents your body from making collagen so you fall apart from within—your teeth fall out, your connections all loosen, and you hemorrhage and die.

10   References

[1](1, 2, 3, 4, 5) Howard Swatland. 16: Fibrous connective tissues. http://www.aps.uoguelph.ca/~swatland/HTML10234/LEC16/LEC16.html

Tendons often extend into the belly of a muscle or along its surface before they merge with its connective tissue framework, and Types I and III collagen both may be extracted from meat. Even within tendons, there may be some Type III collagen forming the endotendineum or fine sheath around bundles of collagen fibrils. In fibres composed of collagen Types I and II, fibrils have a straight arrangement whereas, in fibres of Type III collagen, the fibrils have a helicoidal arrangement.

Small diameter Type III collagen fibres are called reticular fibers since, when stained with silver for light microscopy, they often appear as a network or reticulum of fine fibres. The larger diameter collagen fibres formed from Type I collagen are not blackened by silver.


Collagen constitutes one to two percent of muscle tissue, and accounts for 6% of the weight of strong, tendinous muscles.