Osteoblast bone cell. Coloured scanning electron micrograph (SEM) of an osteoblast bone cell. Osteocytes are osteoblasts (bone-producing cells) that have become trapped within bone cavities (lacunae, depressions in bone). They produce and secrete the organic matrix of the bone (osteoid). As soon as osteoid is formed, calcium salts crystallise inside it to form hard, mineralised bone. Magnification: x4000, when printed 10 centimetres wide.