Vacuolated macrophages in a joint fluid of a dog with villonodular synovitis

Reactive macrophages

There are increased numbers of leukocytes in this joint fluid from the hock of a dog with villonodular synovitis. The leukocytes are predominantly macrophages, most of which have distended lightly vacuolated cytoplasm (“activated”). Small lymphocytes (arrows) are also seen in low numbers and the cells are lining up in rows (“windrowing”) in the smear, suggesting retention of viscosity. Almost all of the macrophages have this distended foamy appearance, which is unusual. Typically there are <10-20% of macrophages with this cytologic appearance in normal joint fluids, but higher proportions (although not this high) are seen in joints with degenerative disease.