The nature of the neoplastic cells in a case of adamantinoma of the tibia was studied with an immunocytochemical method. The antigens investigated were factor VIII-related antigen and keratin, as markers for endothelial cells and epithelial cells, respectively. The tumor cells of adamantinoma stained strongly for keratin but were completely negative for factor VIII-related antigen. These results strongly suggest that the cells of tibial adamantinomas are of epithelial rather than endothelial nature, thus confirming previous light-microscopic observations and electron-microscopic studies performed on this tumor. Although the negativity for factor VIII-related antigen does not rule out by itself the presence of an endothelial component, the fact that the tumor cells are positive for keratin makes this possibility highly unlikely. Additional cases of this entity should be studied with these cytoplasmic markers in order to confirm the findings here presented.
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