Model for calcium-stimulated PS translocation. (A) In the absence of stimulation, the plasma membrane of most healthy cells is asymmetric, with PS and PE restricted to the inner leaflet and PC largely restricted to the outer leaflet. (B) Following stimulation with a calcium ionophore, K⁺ and Cl^- ions leave the cell, water follows, and the cell shrinks.²¹  As a consequence of cell shrinkage, the plasma membrane buckles, prior to the shedding of microvesicles/blebs. At the apex of microvesicles/blebs the packing of phospholipids is relatively loose, reducing the energetic barrier to outward movement of PS and PE. A decrease in lipid packing in the outer leaflet allows the insertion of MC540. In the inner leaflet, PS and PE are tightly packed, increasing the energetic favorability of outward phospholipid “flop.” At the base of microvesicles/blebs, PC is tightly packed in the outer leaflet and PS/PE loosely packed in the inner leaflet, favoring inward phospholipid flip. (C) PS and PE flop out at the apex, and PC flips in at the base. Thus, averaged across the microvesicle/bleb, transport of phospholipids appears headgroup-nonspecific and bidirectional (“scrambling”). (D) As it is no longer tethered to cytoplasmic/cytoskeletal proteins, PS in the outer leaflet is relatively free to move laterally outside of the bleb (unfilled arrow). (E) Microvesicles/blebs are shed. Phospholipid balance is partially restored, with most PS/PE distributed in inner and outer leaflets. Consistent with the model above, both we³²  and others³³  have found that shed particles, but not cell remnants, exhibit high levels of exposed PS, though some movement of PS out of the bleb is likely and has been found on cell remnants in other systems.³⁴