Distinct joint pathology in an inducible Jun mouse model of psoriasis. Microscopic images of a mouse toe from (a) a wildtype littermate control and (b)–(d) JunB/Jun double-mutant mice. Tartrate-resistant acid phosphatise-stained paraffin sections demonstrate (b) a proliferative periostitis affecting both the underlying bone as well as (c) the overlying nail base and dermis with numerous infiltrating neutrophilic granulocytes (antineutrophil NEU47 staining). In advanced stages, (b) an almost complete destruction of the distal phalanx and (d) bone erosions with osteoclasts invading the bone tissue (arrows) can be observed. In contrast, in transgenic mice expressing human TNFα, no destruction of the distal phalanx and no erosive arthritis of the distal interaphalangeal joints are found: (e) wildtype control and (f) tartrate-resistant acid phosphatase staining. (f) Pannus formation and osteoclast-mediated subchondral bone destruction, similar to human rheumatoid arthritis, is consistently observed (arrow). Magnification: (a), (b), (e) and (f), 50×; (c) and (d), 200×.