Bone Biology

Bone is a living tissue and the entire skeleton is replaced about every 7 years.

The process of bone turnover is called remodelling. The phases of remodelling are:

1. Activation phase
2. Resorption phase
3. Formation phase

Activation

The first phase of bone remodelling is undertaken by osteoclast activation. The osteoclasts (or bone resorptive cells) are attracted to the surface of the bone.

Resorption

Osteoclasts adhere to and digest the bone matrix leaving holes or cavities in focal areas of the bone.

Formation

Osteoclasts are removed and osteoblasts (bone forming cells) are attracted to the newly formed cavities. New layers of a protein matrix (collagen) are laid down within these cavities. Finally, the mineralisation process occurs whereby calcium and phosphate are embedded within this matrix until the cavity is filled.

This process of bone resorption followed by bone formation (old bone replaced by new bone) is called coupling.

Electron micrograph slide of osteoclast leaving hole in bone

During the first two decades of life bone formation is greater than bone resorption. During the 3^rd and 4^th decade bone formation equals bone resorption, while from the 4^th decade on bone formation is less than bone resorption.