Is our skeleton permanently under construction?

Our bones are exposed to enormous stresses every day. Tiny parts of our skeleton are continually breaking and must be repaired. Two types of cells share this responsibility: osteoclasts and osteoblasts. The former break down damaged bone tissue. After that the osteoblasts ensure new bone mass is built up. It takes six months for all the calcium in an adult’s bones to be totally replaced. If osteoclasts destroy more than osteoblasts can build up, osteoporosis can ensue, making the skeleton more prone to breakage.