Collagen is the main structural protein in the extracellular matrix found in the body’s connective tissues. 

It is the most abundant protein in mammals.

It makes up from 25% to 35% of the whole-body protein content. 

It consists of amino acids bound together to form a triple helix of an elongated fibril.

Collagen proteins are divided into two main classes – fibril-forming collagens and non-fibril-forming (non-fibrillar) collagens – which are further divided into many different types based on individual structures and functions that the protein specifically has in the body.

Fibrillar collagen, producing the three-dimensional frameworks in different tissues and organs.

Non-fibrillar collagen is the major supporting component of the extracellular matrix.

This collagen helix is mostly found in connective tissue such as cartilage, bones, tendons, ligaments, and skin.

Collagen tissues may be rigid as in bone, or compliant as in a tendon,or have a gradient from rigid to compliant, as in cartilage depending on the degree

of mineralization.

It is abundant in corneas, blood vessels, the gut, intervertebral discs, and the dentin in teeth.

Collagen in muscle tissue, serves as a major component of the endomysium. 

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

The fibroblast cell is the most common cell that creates collagen. 

Gelatin, which is used in food and industry, is collagen that has been irreversibly hydrolyzed.

Over 90% of the collagen in the human body is type I collagen.

More than 28 types of collagen have been identified, described, and divided into several groups according to the structure they form.

All types of collagen contain at least one triple helix.

The five most common types of collagen are:

Type I: skin, tendon, vasculature, organs, bone 

Type II: cartilage 

Type III: reticulate, as reticular fibers, commonly found alongside type I

Type IV: basal lamina, the epithelium-secreted layer of the basement membrane

Type V: cell surfaces, hair, and placenta

The collagenous cardiac skeleton which includes the four heart valve rings, is bound to cardiac muscle. 

The cardiac skeleton also includes the separating septa of the heart chambers, the interventricular septum and the atrioventricular septum. 

Collagen adds to cardiac performance as a continuous torsional force opposed to the fluid mechanics of blood pressure emitted from the heart. 

The collagenous structure that divides the upper chambers of the heart from the lower chambers is an impermeable membrane that excludes both blood and electrical impulses.

Collagen prevents atrial fibrillation deterioration to ventricular fibrillation. 

Cardiac valvular leaflets are folded into shape by specialized collagen under variable pressure. 

The calcium deposition that occurs within collagen occurs as a natural function of aging. 

Calcified points within collagen provides  contrast in a moving display of blood and muscle, enabling cardiac imaging technology to arrive at ratios essentially in cardiac input and cardiac output

Collagen is used in cosmetic surgery, as a healing aid for burn patients for reconstruction of bone and a wide variety of dental, orthopedic, and surgical purposes. 

Human and bovine collagen is widely used as dermal fillers for treatment of wrinkles and skin aging.

With cosmetic use of collagen there is a chance of allergic reactions causing prolonged redness.

Patch testing prior to cosmetic use reduces such allergic reactions.

Collagen in bone grafting has a triple helical structure, so that it does not compromise the structural integrity of the skeleton, prevents it from being broken down by enzymes, enables adhesiveness of cells and it is important for the proper assembly of the extracellular matrix.

Collagen scaffolds are used in tissue regeneration: sponges, sheets, gels, or fibers: It  has favorable properties for tissue regeneration, such as pore structure, permeability, hydrophilicity, and stability in vivo.

Collagen scaffolds support deposition of cells, such as osteoblasts and fibroblasts, and facilitate growth to proceed normally.

Collagens are used in the construction of artificial skin substitutes for management of severe burns and wounds.

Such collagens may be derived from bovine, equine, porcine, or even human sources.

Collagen sometimes used in combination with silicones, glycosaminoglycans, fibroblasts, growth factors and other substances.

Collagen in the skin is damaged by ultraviolet rays, and is associated with increased elastin production, which leads to the increased production of metalloproteinases, enzymes that repair damaged collagen.

Metalloproteinase enzymes usually end up causing more harm to the collagen by incorrectly restoring the skin. 

As the skin is exposed to UVA rays on a daily basis, this process keeps repeating, resulting in wrinkles and leathery skin.

Collagen is a component of skin tissue that benefits all stages of wound healing.

The addition of collagen to the wound bed allows closure to occur.

Collagen is used as a natural wound dressing and has properties that artificial wound dressings do not have, including resistance  against bacteria, which is of vital importance in a wound dressing. 

Collagen helps to keep the wound sterile, because of its natural ability to fight infection. 

Collagen burn dressings allow healthy granulation tissue to form very quickly over the burn, helping it to heal rapidly.

Collagen performs the following functions in the 4 phases of wound healing:

Guiding function: Collagen fibers serve to guide fibroblasts to migrate along a connective tissue matrix.

Chemotactic properties: The surface on collagen fibers can attract fibrogenic cells which help in healing.

Nucleation: In the presence of certain neutral salt molecules can act as a nucleating agent causing formation of fibrillar structures: orienting new collagen deposition and capillary growth.

Hemostatic properties: Blood platelets interact with the collagen to make a hemostatic plug.

The collagen protein is composed of a triple helix, which generally consists of two identical chains (α1) and an additional chain that differs slightly in its chemical composition (α2).

The amino acid composition of collagen is atypical for proteins:  high hydroxyproline content. 

Procollagen is  modified by the addition of hydroxyl groups to the amino acids proline and lysine. 

Its later glycosylation results in the formation of the triple helix structure of collagen. 

The hydroxylase enzymes that perform these reactions require vitamin C as a cofactor, a long-term deficiency in this vitamin results in impaired collagen synthesis and scurvy.

Synthesis of collagen occurs inside and outside of the cell. 

There are about 44 genes are associated with collagen formation, each coding for a specific mRNA sequence, resulting in preprocollagen formation.

Procollagen is a transferee  vesicle to the Golgi apparatus.

In the Golgi apparatus, the procollagen goes through one last post-translational modification.

The procollagen is packaged into a secretory vesicle destined for the extracellular space, and collagen peptidases alter it to tropocollagen. 

Defects in this step of procollagen to tropocollagen produces one of the 

 collagenopathies known as Ehlers-Danlos syndrome: this step is absent when synthesizing type III, a type of fibrilar collagen.

Lysyl oxidase, an extracellular copper-dependent enzyme, produces the final step in the collagen synthesis pathway. forming the collage fibril.

Lysyl oxidase acts on lysines and hydroxylysines producing aldehyde groups, which will eventually undergo covalent bonding between tropocollagen molecules: polymers of tropocollogen is known as a collagen fibril.

Glycine is found at almost every third residue of collagen.

Proline makes up about 17% of collagen.

Collagen contains two uncommon derivative amino acids modified post-translationally by different enzymes, both of which require vitamin C as a cofactor.

Cortisol stimulates degradation of skin collagen into amino acids.

Collagen can be attach to cell membranes via several types of protein, including fibronectin, laminin, fibulin and integrin.

((Vitamin C)) deficiency causes ((scurvy)), in which defective collagen prevents the formation of strong connective tissue. 

With scurvy gums deteriorate and bleed, with loss of teeth; skin discolors, and wounds do not heal. 

Autoimmune diseases such as lupus erythematosus or rheumatoid arthritis may attack healthy collagen fibers.

Bacterial and viruses secrete virulence factors, such as the enzyme collagenase, which destroys collagen or interferes with its production.

A single collagen molecule, tropocollagen, is used to make up larger collagen aggregates, such as fibrils. 

Tropocollagen is approximately 300 nm long and 1.5 nm in diameter, and it is made up of three polypeptide strands, each of which has the conformation of a left-handed helix.

Three polypeptides coil to form tropocollagen. 

Many tropocollagens then bind together to form a fibril, and many of these then form a fiber.

Collagen has a regular arrangement of amino acids in each of the three chains of its collagen subunits. 

Collagen-related diseases most commonly arise from genetic defects or nutritional deficiencies: affecting  the biosynthesis, assembly, postranslational modification, secretion, or other processes involved in normal collagen production.

Type I genetic defects of collagen genes: 

The most abundant collagen of the human body. 

It is present in scar tissue.

It is found in tendons, skin, artery walls, cornea, the endomysium surrounding muscle fibers, fibrocartilage, and the organic part of bones and teeth. COL1A1, COL1A2 genes.

Osteogenesis imperfecta, Ehlers–Danlos syndrome, and infantile cortical hyperostosis are related disease processes.

Type II collagen, hyaline cartilage, makes up 50% of all cartilage protein. 

Vitreous humour of the eye:COL2A1 gene 

Type III is the collagen of granulation tissue and is produced quickly by young fibroblasts before the tougher type I collagen is synthesized. 

Reticular fiber. 

Artery walls, skin, intestines and the uterusCOL3A1

Ehlers–Danlos syndrome, Dupuytren’s contracture

Type IV

Basal lamina; eye lens. 

Part of the filtration system in capillaries and the glomeruli of nephron in the kidney: COL4A1, COL4A2, COL4A3, COL4A4, COL4A5, COL4A6

Alport syndrome, Goodpasture’s syndrome

Type V accounts for most interstitial tissue, associated with type I collagen, associated with placenta, COL5A1, COL5A2, COL5A3 Ehlers–Danlos syndrome.

Type VI Mostly in  interstitial tissue, assoc. with type I collagen, COL6A1, COL6A2, COL6A3, COL6A5 , 

Ulrich myopathy, Bethlem myopathy, atopic dermatitis associated.

Type VIIForms anchoring fibrils in dermoepidermal junctions

Gene COL7A1

Epidermolysis bullosa 

Type VIII Some endothelial cellsCOL8A1, COL8A2

Posterior polymorphous corneal dystrophy

Excessive deposition of collagen occurs in scleroderma.

One thousand mutations that can lead to various diseases at the tissue level, have been identified in 12 out of more than 20 types of collagen. 

((Osteogenesis imperfecta )) is caused by a mutation in type 1 collagen, dominant autosomal disorder, resulting  in weakened  bones and irregular connective tissue.

Mild osteogenesis imperfecta has lowered levels of collagen type 1 while severe cases have structural defects in collagen.

Ehlers-Danlos syndrome – Associated with deformities in connective tissue.

Some rarer types of Ehlers-Danlos disease can be lethal, leading to the rupture of arteries. 

Each Ehlers-Danlos syndrome  is caused by a different mutation. 

The  vascular type of this disorder is caused by a mutation in collagen type 3.

Alport syndrome –is genetically, usually as X-linked dominant, but also as both an autosomal dominant and autosomal recessive disorder, sufferers have problems with their kidneys and eyes, loss of hearing can also develop during the childhood or adolescent years.

Collagen is one of the long, fibrous structural proteins.

Collagen functions are different from those of globular proteins, such as enzymes. 

Bundles of collagen called collagen fibers are a major component of the extracellular matrix that supports most tissues and gives cells structure.

Collagen is also found inside certain cells. 

Collagen has great tensile strength.

Collagen is the main component of fascia, cartilage, ligaments, tendons, bone and skin.

Collagen with elastin and soft keratin are responsible for skin strength and elasticity.

Collagen degradation leads to wrinkles that accompany aging.

Collagen strengthens blood vessels

Collagen plays a role in tissue development. 

Collagen os present in the cornea.

The lens of the eye is in a crystalline form. 

Collagen is used in cosmetic surgery and burn surgery. 

If collagen is subject to denaturation by heating, the three tropocollagen strands separate partially or completely into globular domains, with the formation of gelatin, which is used in many foods, including flavored gelatin desserts. 

Collagen is used for production of glue.

Collagen I most abundant collagen molecule in the collagen family.

Collagen I represents a major stromal component of tumor growth.

Any amount of collagen deposition in the bone marrow detected by trichrome staining is pathologic.

Megakaryocyte growth stimulation results in the elaboration transforming growth factor-beta (TGF-beta) and other cytokines, which promotes symphysis of collagen by bone marrow fibroblasts.

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