Characterization of the ultrastructural features of EP by electron microscopy. Mature MKs incubated with bone marrow cells for 12 hours were fixed and processed for transmission EM. Forty-five EP events were observed from 5 experiments. (Ai-Cii) Transmission EM images of EP. Scale bars, 2 µm or (for magnification of Aiii) 500 nm. (Ai) Large, round emperisome with a smooth vacuolar membrane surrounding a neutrophil (polymorphonuclear leukocyte, PMN). (Aii) The emperisome extends membrane protrusions toward the engulfed neutrophil. (Aiii) The emperisome tightly wraps around the engulfed neutrophil. Magnification shows close membrane approximation between neutrophil and emperisome membranes (arrowheads). (Bi) Internalized neutrophil partly covered by the emperisome and partly exposed to the DMS of the MK. (Bii) Neutrophil residing within the cavities of the DMS. (Ci) Internalized neutrophil partly covered by the emperisome (Cia) and partly exposed to organelles of the MK cytoplasm (Cib). (Cii) Two neutrophils fully reside inside the MK cytoplasm. Only the neutrophil membranes remain visible (arrowheads). (D) Frequency of the previously described EP stages (n = 45).