Collagen is one of the most important, if not the most important, protein in the human body. To date, 29 different types of collagen have been identified. Of these 29 types, 23 types can be detected in eye tissue.
A third of the proteins in the human body are made up of collagen. It is found in almost all tissues of the body. Collagen fibrils are a quaternary structure of collagen and are formed by the stringing together of several tropocollagen.
In this post you will learn all about collagen fibril. We will inform you about what exactly collagen fibrils are and how they are formed. After reading this, you will have a good understanding of the topic of collagen fibrils.
the essentials in brief
- Collagen is the most common protein in the human body. It is the organic component of bones and teeth and the essential component of cartilage, tendons, ligaments and skin. 28 different types of collagen are currently known (types I to XXVIII).
- The collagens are divided into several subgroups. Fibrillar collagens are part of the collagen family. A characteristic banding pattern is formed in the collagen fibrils, which is repeated every 67 nm (234 amino acids) and is known as the so-called D-period.
- Types I, II, III, V, IX and are counted among the fibrillar collagens. Type I collagen makes up the majority of collagen in the body in mammals and is also found in the blood vessels.
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Glossary entry: The term collagen fibril explained in detail
Below we have summarized and explained the most important facts about collagen fibrils for you. You will receive detailed information about the functions and types.
What is a collagen fibril?
Neighboring collagen molecules are not aligned in the fibrils. They are offset from each other by 67 nm, about a fifth of their length. This results in transverse striations in electron micrographs of metal-contrasted collagen fibrils (1).
Collagen fibrils vary greatly in diameter in different tissues, ranging from 20 nm to about 500 nm. (Image Source: 6335159 / Pixabay)
A characteristic band pattern is formed which is repeated every 67 nm or 234 amino acids and is referred to as the so-called D-period. The α chains are thus divided into four homologous regions D1–D4. Bands occurring in a D unit are referred to as ae (2).
Collagen fibrils are ordered polymers that can grow many micrometers long in mature tissue. They are often grouped into larger, cable-like bundles called collagen fibers. Type I collagen fibrils have a diameter of 50-500 nm in tendons, 40-100 nm in the skin and 25 nm in the cornea.
How are collagen fibrils formed?
The prerequisite for fibril formation is triple helix formation, in which three pro-α chains always wrap around one another and together form a triple helix. During fibril formation, several tropocollagens are linked to form a bundle. And thus the prerequisite for the subsequent fiber formation is created.
The accumulation of several collagen molecules leads to the next higher organizational unit, the so-called collagen fibrils. When the fibrils are formed, the collagen molecules are not just bundled together, but offset from one another by around 67 nm (3).
In different tissues, collagen fibrils have very different diameters (from 20 nm to 500 nm). This range of variation allows the composition of the fibrils to be adapted to the needs of the respective tissue (4).
What types of collagen are fibrillary?
28 different collagen types (types I to XXVIII) and at least 10 other proteins with collagen-like domains are known. The number of types shows the diverse functionality of collagen. One of the characteristics of collagen molecules is that there are fibrous (fibrillary) and non-fibrous (non-fibrillary) types of collagen. Types I, II, III, V and IX are classified as fibrillar collagens (5).
This group can be divided into the large fibril-forming group and the small fibril-forming group. The collagens that form large fibrils include type I, II, and III collagens, while type V and XI collagens form small fibrillar collagens. Such small fibrillar collagens are incorporated into the fibrils of the large fibrillar collagens.
As the name suggests, non-fibrillar collagens are incapable of forming fibrils. Unlike fibrillar collagens, they have non-collagenous domains that interrupt or are connected to the triple helix structure of the collagen molecule.
However, they share one common feature, which is at least one break in their triple helix.
Over 90% of the collagen in the human body is collagen type I. Collagen type I belongs to the fibrillar collagens. It is found in all tissues, most commonly skin and bones, but also in tendons and the cornea (6).
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What are the functions of collagen fibrils?
Collagen is the most common protein in mammals. A good supply of collagen keeps the vessels elastic and thus prevents heart and vascular diseases.
The function of the individual types in the body is determined by their properties. Type I is particularly widespread and is therefore found in bones, tendons, skin and the dentin of teeth. Type I is a major component of the organic portion of bone. In contrast, type II is thinner and is found mainly in the cartilage, intervertebral discs and vitreous of the eye(7).
| Type | Description |
|---|---|
| Type I | Skin, tendons, vessels, organs, bones |
| Type II | Cartilage (main collagen component of cartilage) |
| Type III | Reticulated (main component of net fibers) |
| Type IV | Forms the basal lamina |
| Type V | cell surfaces, hair and placenta |
The finest collagen of this type is type III and is found in smooth muscle, lymphoid tissue and bone marrow. Type IV occurs in the basement membrane and the lamina externa and the lens capsule.
The main ones are collagen type I, II, III and V, IX.
Type V is found in the basal lamina of smooth and striated muscle cells(8).
Conclusion
The collagens can be divided into two groups on the basis of their structural and functional characteristics: the group of fibrillar collagens and the group of non-fibrillar collagens. Fibrillar collagens form the most important components of the skeleton and connective tissue of the human body.
This type of collagen forms fibrils of varying lengths and thicknesses. The characteristic feature of fibrillar collagens is their long central triple helix in each chain. Types I, II, III, V and IX are classified as fibrillar collagens.
References
- Erickson B, Fang M, Wallace JM, Orr BG, Les CM, Banaszak Holl MM. Nanoscale structure of type I collagen fibrils: quantitative measurement of D-spacing. BiotechnolJ 2013
- Erickson B, Fang M, Wallace JM, Orr BG, Les CM, Banaszak Holl MM. Nanoscale structure of type I collagen fibrils: quantitative measurement of D-spacing. BiotechnolJ 2013
- Holmes DF, Lu Y, Starborg T, Kadler KE. Collagen Fibril Assembly and Function. Curr Top Dev Biol.
- Grant CA, Brockwell DJ, Radford SE, Thomson NH. Tuning the elastic modulus of hydrated collagen fibrils. Biophys J
- Gordon MK, Hahn RA. collagens. Cell Tissue Res. 2010;339(1):247-257.
