[Iron deposits, cell populations and proliferative activity in pigmented villonodular synovitis of the knee joint].

Pigmented villonodular synovitis (PVNS) of the knee is a tumor-like process of uncertain nature. A chronic inflammation as well as a neoplastic process have been proposed in the literature. The aim of our study was to characterise the prevalent inflammatory cells, the proliferating cell populations, and the iron deposit distribution in PVNS in order to get insights into pathogenetically relevant processes of this condition. Thirteen cases of PVNS of the knee as well as 8 normal controls were analysed histochemically for iron deposits and immunohistochemically for the distribution of vascular structures and inflammatory cell populations. Collagen type I expressing fibroblastic cells were identified by in situ hybridization. The proliferative cell compartment was characterized using MIB-1 staining. Our analysis showed no correlation between intra- or extracellular iron deposits and proliferation, giant cell formation, vascularity, number of CD 68-positive cells, and foam cell formation. Instead, iron deposits were associated with collagen matrix formation. All PVNS specimens showed a significant increase of chronic inflammatory infiltrates compared to all normal synovial membrane specimens investigated. The identification of the proliferative cell compartments showed that besides fibroblastic cells many of the mononuclear, partly CD 68 positive cells were Ki-67 positive. Foam cells, iron-loaded cells, and giant cells were, however, negative for the Ki-67 antigen. PVNS appears to originate from the interplay of proliferating, partly CD 68 positive mononuclear cells and fibroblasts, both activated by an excessive iron load. Giant cells probably develop by fusion of CD 68-positive histiocytic cells. Foam cells are most likely secondary to fatty tissue destruction.