Scanning electron microscopy of murine macrophages. Surface characteristics during maturation, activation, and phagocytosis.

The present report describes the surface architecture of critical point dried mouse peritoneal macrophages, after attachment and spreading on glass, and during maturation and active phagocytosis of rabbit erythrocytes and latex spheres. This study also compares the appearance of unstimulated cells with that of thioglycollate and endotoxin-stimulated cells. Activated cells, particularly thioglycollate-stimulated macrophages, showed more rapid and extensive spreading, a larger surface area, and more prominent ruffled membranes and filopodia. Many fine cytoplasmic pits were also evident and these may represent the sites of formation of numerous pinocytotic vesicles. The sequence of events during the various stages of phagocytosis was well visualized with the scanning electron microscope. Multiple large, round, and hemispherical craters were observed in these cells, and particles were engulfed through these structures and subsequently ingested. The findings are discussed in the light of current knowledge of the macrophage plasma membrane.