Light micrograph of a section through the jawbone of a foetal cat showing bone (blue) formation (ossification). This is intramembranous ossification, which occurs in flat bones. Osteoblast cells (purple) 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. Magnification: x1,200 when printed at 15cm wide.