Immunohistochemical staining of canine and feline osteosarcomas. (a) Canine osteosarcoma, erythropoietin (EPO); (b) feline osteosarcoma, EPO; tumour cells and stromal cells showed cytoplasmic and nuclear staining. (c) Canine osteosarcoma, erythropoietin receptor (EPO-R); (d) feline osteosarcoma, EPO-R; EPO-R was expressed in tumour cells and giant cells. (e) Canine osteosarcoma, platelet-derived growth factor BB (PDGF-BB); (f) feline osteosarcoma, PDGF-BB; osteosarcoma cells showed a cytoplasmic granular staining pattern. In some samples a perivascular distribution was observed. Scale bar = 100 μm.