Rib fracture. Coloured scanning electron micrograph (SEM) showing the internal structure of a rib fracture. Numerous bone cells (purple) in bone lacunae lie in a compact bone area of the rib. Periosteum and muscle surround the bone. A bone-producing cell is called an osteoblast. During bone formation osteoblasts become progressively trapped in the bone matrix at sites called lacunae. The osteoblast produces osteoid, the organic bone matrix that is a mass of collagen fibres and glycoprotein cement, Magnification: x80 when printed at 15 centimetres wide.