Canine osteosarcoma (OS) cell lines are tumorigenic in mice and yield tumours with histological descriptors characteristic of OS. (A,D,G) Primary tumour growth patterns of canine OS cells in Beige-SCID mice. Each line represents growth of an individual mouse. H&E staining (B,E,H) of primary tumour resulting from orthotopic injection of KOS-001, KOS-003 and KOS-004 cell lines showed streams of closely packed spindloid tumour cells within a fibrous stroma and small islands of osteoid (see arrow heads) are scattered between the tumour cells. (C,F,I) Primary tumours of KOS-001, KOS-003 and KOS-004 cell lines showed positive ALP staining. Scale bar = 100 μm.