Osteoblasts - cells which differentiate from pericytes and secrete both the matrix and mineral of bone.

Osteocalcin - a calcium binding protein, synthesised by the osteoblast and secreted into the matrix at the time of bone mineralisation. Mice bread without the osteocalcin gene10 develop heavy bones suggesting that osteocalcin is a negative regulator of bone formation

Osteoclast - a multi-nucleate cellcapable of removing both the organic and mineral component of bone. Osteoclasts activity is controlled by nearby osteoblasts.

Osteoid - the extracellular matrix inwhich bone forms. It is high in collagen and other bone proteins but lacks any crystal formation.

Osteonectin - a bone glycoprotein which has the property of binding to both collagen fibres and the hydroxyapatite crystals, and thus may be important in initiating bone mineralisation by acting as a template for nucleation. Osteonectin is also produced by endothelial cells and plateletes and is able to bindfibrinogen.

Osteopontin - an adhesive glycoprotein related to sialoproteins, which is secreted by osteoclasts to assist in their adhesion to the bone surface. After bone resorption it may then act as a signal to stimulate osteoblast activity

Osteoporosis - a reduction in bone mass which occurs commonly in post menopausal females, but also in older men. It is due to a reduction in the activity of the ovaries and a decreased secretion of oestrogen. Bone formation and bone healing are not affected but more bone is resorbed by osteoclasts than is replaced. Lack of exercise is also a factor in bone loss.

Oxytalin fibres - are related to elastic fibres, though they have a smaller core of elastin. They are found in the periodontal ligament and in the epidermis of thin skin, but not in the oral mucosa.