Close-up of an osteosarcoma, or osteogenic sarcoma, of the femur (thigh bone). This is an aggressive bone cancer that arises from cells of mesenchymal origin that exhibits osteoblastic differentiation. Tumour cells are very pleomorphic (anaplastic), resembling bone cells or fibroblast cells, frequently showing multinucleated osteoclast-like giant cells. Osteosarcoma is the most common histological form of primary bone sarcoma. Osteosarcomas usually start in a long bone, often near the knee.