Iron carbonyl, a tracer for phagocytosis in scanning electron microscopy

Human peripheral blood mononuclear cells and polymorphonuclear (PMN) leukocytes, separated by Ficoll-Hypaque have been incubated with a preparation containing iron carbonyl, and studied by scanning electron microscopy (SEM) in the backscattered electron imaging (BEI) mode. Iron carbonyl particles were visible only on the cell surface when studied in the conventional secondary electron (SE) mode of the scanning electron microscope. However, ingested iron carbonyl particles were observed in high contrast when the backscattered electrons were selectively collected. The BEI mode of the scanning electron microscope appeared very useful in identifying phagocytic activity in mixed cell populations and in establishing direct correlations, at the level of each individual cell, between surface morphology and phagocytic activity.