PS exposure

Phosphatidyl serine (PS),  normally on the inside of the membrane is exposed in apoptosis or programmed cell death and provides an early signal for cell removal. Whereas PS exposure of platelets is essential in hemostasis PS exposure on RBC will contribute to thrombosis when PS exposing cells are not rapidly removed. The figure shows RBC in which phospholipid asymmetry was lost in a subpopulation and  exposed to fluorescently labeled di-annexin which identifies cells that expose PS. 

Phospholipid asymmetry in the membrane lipid bilayer is maintained by the ATP driven flipase which transports PS to the inner monolayer. ROS stress and an increase in cytosolic calcium with reduce flipase activity and activate scrambling, leading to PS exposure. 

The flowcytometry histogram shows cells labeled with di-annexin, and allows calculation of the percentage of cells that expose PS. These cells can be visualized in image flowcytometry.