When you see a skeleton in a museum, it’s easy to think of bone as dead and unchanging - like rock. The bone inside your body is the opposite. It’s a living tissue, full of cells and blood vessels, that’s constantly being broken down and rebuilt your entire life.
Two types of cells run the show. Osteoclasts are the demolition workers - they chew away at old, damaged, or unneeded bone. Osteoblasts are the builders - they lay down fresh new bone in its place. Every year, about 10% of your skeleton gets replaced this way. By age 30 or so, every adult has gone through this process so many times that almost every bone in their body has been rebuilt from scratch many times over.
Why bother? A few reasons. Bones get tiny stress fractures from daily use, and the rebuilding fixes them. Bones also act as the body’s calcium bank - when your blood needs calcium, osteoclasts release some from your bones. And exercise actually makes your bones thicker; the more you stress them (within reason), the more osteoblasts build them up. That’s why activities like running and weight training keep bones healthy.
