They are a living and growing part of any animal. All bones have blood vessels and nerves supplying them nutrients and information. Bone tissue contains three different types of specialised generative cells called 'Osteoblasts', 'Osteocytes' and 'Osteoclasts'. Osteoblasts are the commonest, they form the bone matrix around themselves by laying down collagen fibres and depositing the crystals of mineral material. They are the real bone builders.
Osteocytes are branched cells.
They lie embedded in the bone matrix where they monitor and help control the mineral balance of the body. Osteoclasts break down the matrix during normal bone renewal and growth, they are also important in the healing of fractures. Mammal bone material may appear spongy 'Cancellous bone' or hard and solid 'Compact Bone' depending on how much empty space there is within the matrix. As well as the main minerals of calcium and phosphorous, bones also contain sodium, magnesium, carbonate and citrate. Mammal compact bone has several layers to it.