Neutrophil transit time through MKs is bimodally distributed: fast and slow EP. 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 DRAQ5 (blue). (A) Histogram depicting the duration of neutrophil transit through MKs of 28 EP events reveals a bimodal distribution with peaks between 0 and 10 minutes (fast EP) and >60 minutes (slow EP). Results pooled from 5 independent experiments. Bimodality was confirmed by Hartigan’s dip test (D = 0.16, P = 7.89 × 10−7). (B-C) Images were obtained using a W1 Yokogawa Spinning Disk Confocal attached to a Nikon Ti inverted microscope with a Plan Fluor 40x/1.3 Oil DIC H/N2 objective. Scale bars, 10 µm. (B) Representative image sequence of fast EP. The neutrophil (*) enters the MK on the right side, migrates through the MK cytoplasm, and egresses on the opposite side within 10 minutes. (C) Representative image sequence of slow EP. The neutrophil (*) is already inside the MK at the beginning of the image acquisition and remains inside for at least 60 minutes, showing no migration inside the MK.