Fig. 1. Thin section cut near the surface of a U937 macrophage fixed after five minutes of infection, with L. pneumophila infected at an MOI of 20. Within the phagosome is a L. pneumophila bacterium. Attached to the basal surface of the phagosomal membrane are a series of vesicles of the ER. The bracketed region in (a) is shown at higher magnification in (b). The endosomal membrane is 70 Å thick, whereas the attached ER membranes are only 60 Å thick.