Your bones are alive and always being rebuilt. Osteoblasts are the builder cells that lay down fresh new bone.
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Osteoblasts line up in a row on the surface of a bone, like bricklayers on a wall. They're plump and roughly box-shaped.
They work hard while you grow, making your bones longer and thicker. They also fix and rebuild bone all through life.
Some osteoblasts get walled in by the bone they made. Then they become osteocytes, cells that live inside the bone and keep watch.
First, osteoblasts make a soft layer full of collagen. Then minerals like calcium settle into it and make it hard.
That's why foods with calcium, like milk, yogurt, cheese, and leafy greens, are good for your bones. Running and jumping help too, because they tell your bones to stay strong.
Scroll up to the 3D model and press Build bone to watch a new layer appear.