PHOSPHATIDYLSERINE PLASMA-MEMBRANE ASYMMETRY IN-VIVO - A PANCELLULAR PHENOMENON WHICH ALTERS DURING APOPTOSIS

The distribution of phospholipids across the two leaflets of the plasm a membrane is important for many cellular processes including phagocyt osis and hemostasis. In the present study we investigated the in vivo plasma membrane distribution of the aminophospholipid phosphatidylseri ne in mouse embryos with a novel technique employing Annexin V, a Ca2 dependent phosphatidylserine binding protein, conjugated to fluoresce in isothiocyanate and biotin. Annexin V directly applied to cryostat s ections labeled the plasma membrane of all cells at the interface, In contrast, Annexin V injected intracardially into viable mouse embryos labeled almost exclusively apoptotic cells, These apoptotic cells were visible in all tissues and derived from all germ layers. Our experime nts demonstrate that phosphatidylserine is asymmetrically distributed between the two leaflets of the plasma membrane in virtually all cell types in vivo and that this asymmetry is lost early during apoptosis.