Osteoblastic osteosarcoma, light micrograph. Osteosarcoma is a malignant neoplasm in which the tumour cells produce immature bone or osteoid. Based on the morphology, the most common types of osteosarcomas are osteoblastic, chondroblastic, and fibroblastic. This is an osteoblastic osteosarcoma. There are many classification systems for grading osteosarcoma. One of the widely used system is the 4-tiered (Broder's) system based on the percentage of anaplasia in the tumour. The majority of the conventional central osteosarcomas fall in the grade 3 or grade 4 categories. The tumours assigned grade 1 are low-grade central osteosarcomas and the usual parosteal osteosarcomas. Osteosarcomas of the jaw often tend to show grade 2 histology. This image shows a focus of a grade 4 osteoblastic osteosarcoma with highly anaplastic tumour cells. Haematoxylin and eosin stain.