Compartmentalized membrane exposure of phosphatidylserine in proplatelet-displaying MKs.

Live 13-day-old budding MKs were stained with FITC–annexin V (B) and counterstained with a R-PE anti-CD41 mAb (A) as well as Hoechst 3324 (C). Note that in the entire MK-forming platelet, the proplatelets (arrow) does not stain with annexin V, although the central part of the MK is positive for annexin V, indicating localized membrane phosphatidylserine exposure. The arrowhead points to a proplatelet (annexin negative) originating from another MK that is not shown in this field. In the other MKs, no staining with FITC–annexin V is observed. Scale bar equals 20 μm.