Fusion between monocytes and macrophages. Macrophages were membrane labeled with the fluorescent dye PKH-2 (green). Monocyte membranes were not labeled, nuclei of both cell types were stained with the HOECHST dye (blue). (A) and (B) Freshly isolated monocytes stimulated for 3 days; several MGC have formed. (C) and (D) 1-week cultured macrophages stimulated with conditioned medium; no fusions have occurred. (E) and (F ) Freshly isolated monocytes added to 1-week cultured macrophages without stimulation; no spontaneous MGC formation. (G) and (H) Stimulation of monocytes and macrophages; two MGC can be seen, one derived from monocytes only, the other containing at least one macrophage. Original magnification × 100 (bar = 50 μm).