Chondroblastoma is an osteoid-forming, but not cartilage-forming neoplasm.

Chondroblastoma is defined as a 'benign tumour, characterized by highly cellular and relatively undifferentiated tissue composed of rounded or polygonal chondroblast-like cells' and the 'presence of cartilaginous intercellular matrix' (WHO). An extensive analysis of the extracellular matrix composition and gene expression pattern of a large series of chondroblastoma cases shows, however, that type II collagen, which is the main component of any cartilage matrix, is not expressed by the neoplastic cells of this tumour entity and is not deposited into the extracellular tumour matrix. Instead, osteoid and fibrous matrix is formed, with its typical biochemical composition. The multifocal expression of aggrecan proteoglycan in most chondroblastomas explains the bluish, pseudo-chondroid appearance of some of the matrix-rich areas of chondroblastomas. This study did not show chondroid matrix formation or chondroblastic cell differentiation in chondroblastomas, suggesting that chondroblastoma should be classified as a specific bone-forming, rather than cartilage-forming neoplasm.