Photomicrographs of osteochondroma. (A) Hematoxylin and eosin staining showing proliferation of chondroid tissue deeply into marrow spaces, producing trabecular ossification. A2 is a higher magnification of panel A1. (B) Immunostaining for BMP-4, which is diffusely positive in the chondrocytes and surrounding matrix (arrows). (C) Immunostaining for BMP-2, which is slightly positive in the trabecular bone (arrows). (D) Immunostaining for PCNA, which is positive in a small number of tumor cells.