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Osteoblasts. Coloured scanning electron micrograph (SEM) of bone-making osteoblast cells (irregular ovals). They are surrounded by the bone matrix they have created. Osteoblasts produce the organic matrix of bone, a substance called osteoid. This consists of a mass of collagen fibres and a glycoprotein cement. As soon as osteoid is formed, calcium salts crystallize inside it to form hard, mineralised bone. Osteoblasts that become trapped in cavities in the bone become osteocytes. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)