Further cytological observation on 'activation' by superimposed antigen of inflammation-mediated macrophages

: Compact cell aggregates were induced by an injection of a superimposed antigen, sheep erythrocytes (SRBC) into PSK (a protein-bound polysaccharide)-'prepared' mouse footpad. Major cell types in the cell aggregate were macrophages which rapidly digested superimposed SRBC but not PSK substances which were retained over seven days within their phagosomes. Macrophages without PSK like substances-containing phagosomes occurred invariably outside the cell aggregate. The macrophages in the cell aggregate were interlocked via their cytoplasmic projections. Ruthenium red, a specific dye for extracellular proteoglycans, clearly revealed their surface coats and also closely contacted zones of adjoining macrophages. The surface coats were not uniform in distribution and became sporadically thickened deposits at invaginations of the plasma membrane. Ladder-like structures among adjacent cytoplasmic projections also showed a strong affinity to the dye. It is suggested that a primary function of these structures is the maintenance of close contact of aggregating macrophages. Another observation was that binucleate macrophages occurred in the cell aggregates. Careful inspection on sections of well-preserved tissues concludes that such cells were not formed by a cell fusion between mononucleate macrophages in the cell aggregates. Formation of such a binucleate cell from the macrophage in the aggregates is also discussed.