Distinct morphology of fast and slow EP within a single MK. Mature MKs were stained with anti-CD41 AF488 (green) and coincubated with fresh bone marrow cells stained with anti-Ly6G AF594 (red). DNA was stained with Hoechst 33342 (blue). (A-B) Images were obtained using a Perkin Elmer Ultraview Vox Spinning Disk Confocal attached to a Nikon Ti inverted microscope with a 60x (1.4NA) objective. Scale bars, 10 µm. (A) An MK showing fast and slow EP simultaneously to illustrate morphological differences of both forms. Two neutrophils undergoing slow EP (*) assume a sessile state. A third neutrophil (arrowhead) enters the MK after 31 minutes and extends dynamic membrane protrusions to propel itself through the MK cytoplasm, exiting within few minutes. (B) Representative image of a slow EP neutrophil residing near the MK nucleus. (C) The passage of neutrophils undergoing fast EP (<10 minutes) and slow EP (>60 minutes) through MKs was tracked using the ImageJ plugin TrackMate to determine the mean speed of migration.