Interior of a bone. Coloured scanning electron micrograph (SEM) of a long bone fractured to show its interior. The outer layer (pink) of the bone comprises proliferating cartilage. In the centre is spongy, or cancellous bone, which contains bone marrow (red) the site of blood cell production. Spongy bone is characterised by a honeycomb arrangement, comprising a network of trabeculae (cream, centre). The perimeter of the bone is made from thicker bone, called compact, or cortical bone.