Bone resorption by macrophage polykaryons of giant cell tumour of tendon sheath.

The antigenic phenotype, ultrastructure and bone resorbing ability of mononuclear and multinucleated giant cells of four giant cell tumour of tendon sheath (GCTTS) lesions was assessed. Both the giant cells and the mononuclear cells exhibited the antigenic phenotype of cells of the monocyte/macrophage lineage. The giant cells, unlike osteoclasts, did not respond morphologically to calcitonin and showed ultrastructural and immunophenotypic features of macrophage polykaryons. However, like osteoclasts, the giant cells showed direct evidence of resorption pit formation on bone slices. This indicates that the GCTTS is composed of cells of histiocytic differentiation with the giant and mononuclear cell components expressing a similar antigenic phenotype. Bone resorption by macrophage polykaryons shows that this is not a unique defining characteristic of osteoclasts. Qualitative differences in the degree and pattern of bone resorption by macrophage polykaryons distinguish it from that of osteoclasts and may underlie the clinical behaviour of osteolytic lesions.