Pulmonary macrophage phagocytosis: quantification by secondary and backscattered electron imaging

A systematic method for quantifying phagocytosis of carbonyl iron beads by pulmonary macrophages is described. These techniques utilize secondary and backscattered electron imaging. The carbonyl iron composition has a high atomic contrast number which makes it ideal for visualization in the backscatter electron image of the scanning electron microscope. As a result, interiorized iron beads can easily be distinguished from beads attached to the surface of the cell. The morphology and phagocytic capacities of individual macrophages were compared. The results showed that functional heterogeneity exists within the normal pulmonary macrophage population. Ruffled macrophages avidly phagocytized carbonyl iron beads, while smooth cells were functionally impaired. In addition, macrophages which were altered morphologically by exposures to acidic culture conditions or to asbestos demonstrated a decreased capacity to phagocytize carbonyl iron beads. Accordingly, the surface morphology of pulmonary macrophages correlated with their phagocytic capacities. We propose that results obtained from the analyses of individual cells can be used to study the phagocytic capacity of macrophage populations.