Increased tartrate-resistant acid phosphatase (TRAP)-positive staining following treatment with oncostatin M (OSM) with either IL-1 or tumour necrosis factor alpha (TNF-α) in murine joints. Mice were injected intra-articularly with adenoviral vectors as described in Fig. 1, and the animals were sacrificed at day 7 following administration. (a) No significant TRAP-positive cells outside the bone marrow were seen in control joints. Some TRAP-positive staining cells were located at the synovium-bone interface in (b) OSM-treated joints, (c) IL-1-treated joints and (d) TNF-α-treated joints. In both the (e)–(h) OSM + IL-1 and (i)–(l) OSM + TNF-α combinations, significant TRAP-positive staining was located at the front of the synovium–bone and the pannus–subchondral bone junctions, which were interposed between the bone surface and the 'erosive front' of the synovium (e, i and j). At many sites of focal bone erosions (arrows), TRAP-positive multinucleated cells were seen at the erosion front within the synovium (e, h–j), and within erosion pits in the bone (h and k). Furthermore, TRAP-positive staining cells were also seen at the cartilage/bone junction (l). b, bone; bm, bone marrow; c, cartilage; f, fracture; m, muscle; mn, multinucleated cells; s, synovial cells. (a)–(d), (g)–(l) Bar = 50 μm; (e), (f) bar = 20 μm.