Collagen Explained: Types, Functions, and Role in Bone Healing

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Approximately 20% of body proteins are made of collagen. There are approximately 20 types of collagen. Collagen is a tough but flexible framework that mechanically supports proteoglycans and cells. Collagen is made by fibroblasts.

Type I Collagen
The bone is made of type I collagen, and so are structures attached to the bone, such as ligaments, tendons, and menisci. If cartilage is injured, it receives support from the bone and heals by forming fibrocartilage, which is predominantly type I collagen. All these structures primarily consist of type I collagen. The annulus fibrosus of the intervertebral discs is also composed of type I collagen because it is attached to the bone. A defect in type I collagen may lead to osteogenesis imperfecta, characterized by fragile bones and blue sclera. Ehlers-Danlos syndrome, presenting as loose joints and stretchy skin, may also be associated. The majority of collagen in the body is type I, accounting for approximately 90%.

Type II Collagen
Type II collagen is found in the nucleus pulposus of the discs and the hyaline cartilage of the joints. Type II collagen is highly stable, with a very long half-life of approximately 25 years. Cartilage cells do not produce new hyaline cartilage. The hyaline cartilage components and framework are designed to last a long time. A genetic defect in type II collagen results in spondyloepiphyseal dysplasia (SED).

Type III Collagen
Type III collagen is found in blood vessels and skin. The bone develops first, followed by cartilage, and then the skin, which consists of type III collagen. Type III collagen is linked to the development of Dupuytren's contracture. During tendon healing, fibroblasts produce type III collagen in the first 1-7 days. The tissue subsequently remodels, replacing type III collagen with type I collagen over the next 18 months.

Type X Collagen
Type X collagen is produced by hypertrophic chondrocytes during endochondral ossification and is present in:

Fracture callus

Growth plates

Heterotopic ossification

Type X collagen is associated with cartilage calcification and is found near calcified cartilage. The presence of hard, calcified cartilage corresponds with type X collagen. A defect in type X collagen leads to Schmid metaphyseal chondrodysplasia.

Type XI Collagen
Type XI collagen acts as an adhesive that stabilizes the collagen lattice.

Bone Healing and Collagen
If a fracture is fixed with a compression plate, it undergoes primary bone healing, meaning there is no cartilage involvement. This type of healing relies on type I collagen, as bone is formed directly.

If a fracture is treated with a rod, cast, brace, external fixator, or bridge plate instead of a compression plate, secondary bone healing occurs. This process involves cartilage formation and an abundance of callus. Initially, cartilage formation introduces type II collagen. As the cartilage undergoes calcification or mineralization, type X collagen appears. Finally, the soft callus transitions into a hard callus, ultimately leading to the predominance of type I collagen, which signifies complete bone healing.