As discussed in the previous post on the Bone Physiology – Bone Composition blog series, there are three primary components of bone composition: cells, organic bone matrix, and inorganic bone matrix. In Part 1, we talked about bone cells and how they play a crucial role in bone formation and remodeling. In this blog, we will focus on bone matrix and its organic and inorganic components.
Bone Matrix Composition
Bone matrix is a musculoskeletal tissue that consists of a complex composition of organic and inorganic components. Although bone matrix composition can vary amongst individuals, it typically has the following composition (Bostrom, 2000*):
- Inorganic component: bone mineral – 60-70%
- Organic component: proteins (e.g., collagen & growth factors), lipids, and other macromolecules – 22-35%
- Water – 5-8%
Due to this multi-component composition, bone matrix serves a variety of biological functions. As the main component of the skeletal system, the collagen and mineral components of bone matrix provide biomechanical support for the entire body and protect vital organs. Bone mineral additionally serves as a reservoir for calcium and phosphorus, key elements used throughout the body. Bone matrix also contains non-collagenous proteins such as growth factors that are actively involved in bone formation, repair, and remodeling.
Organic Bone Matrix – Proteins, Lipids, and Other Macromolecules
Collagen is the main protein component in bone accounting for approximately 90% of the organic bone matrix composition. By convention, the collagen that is found in bone is referred to as Type I collagen. However, there are more than 20 types of collagen found in various tissues throughout the body. They all have common properties, such as being high in certain amino acids (e.g., glycine and hydroxyproline), and are formed into a triple helix of approximately 300 amino acids for each chain, and 300nm long. After secretion by cells, these large triple helices aggregate together into collagen fibrils. The collagen fibrils are held together by inter-molecular and inter-fibrillar bonds, or cross-links.