Coloured scanning electron micrograph (SEM) of a section through the jawbone of a foetal mouse showing osteoblast cells (brown). Osteoblasts produce and secrete osteoid, a matrix of collagen fibrils and structural proteins. As soon as osteoid is formed, calcium salts crystallise inside it to form hard, mineralised bone. Some of the osteoblasts become trapped in the bone where they become osteocytes, cells that maintain the bony matrix. The picture was created as part of a cooperation with Quintessenz Publishing. Magnification: x5,000 when printed at 15cm wide.