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


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.

Resting Phase


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

Resorption Phase


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.

Formation Phase

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

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 3rd and 4th decade bone formation equals bone resorption, while from the 4th decade on bone formation is less than bone resorption.