What is bone made up of?

Bone is made up of an organic part and an inorganic part.

• Organic - this is also known as the osteoid. It consists of 90% collagen and other specialised proteins such as osteocalcin and osteonectin. It gives the bone a degree of flexibility.
• Inorganic - this is made up predominantly of a compound called hydroxyapatite, an inorganic molecule made up of calcium, phosphate and hydroxyl groups. It forms salts other ions such as bicarbonate, potassium and sodium ions. This gives the bone its strength and resistance to damage.

Bones also contain a number of cells including:

• Osteocytes - cells contained within the neurovascular channels through bone (Haversian canals)
• Osteoblasts - cells found on the outside (periosteum) and inside (endosteum) surfaces of bone. They secrete the organic part of bone in the process of bone remodelling (see 'What is bone remodelling?')
• Osteoclasts - cells found on the outside (periosteum) surface that act to resorb (digest) bone before it is laid down again in a more appropriate manner in the process of bone remodelling (see 'What is bone remodelling?')