Bone cancer. Coloured scanning electron micrograph (SEM) of a cancerous osteoclast cell on the surface of a bone.Osteoclasts are large, multi-nucleated cells that form from the fusion of several macrophage cells in bone and they move by extending cellular processes. Normally, osteoclasts break down worn out bone and work with bone-forming cells called osteoblasts to repair bone. However, osteoclasts may become cancerous causing an osteoclastoma or giant cell tumour, a type of bone cancer that usually affects the ends of the long bones. Treatment is by surgical removal and the use of liquid nitrogen to kill remaining cells. Magnification: x3000 when printed 10cm wide.