Microvesicles

Microvesicles are fragments of plasma membrane ranging from 0,1-1µm shed from almost all cell types during activation. They originate directly from the plasma membrane of the cell and reflect the antigenic content of the cells which they originate from. It has recently been found that microvesicles can deliver mRNA between cells, leading to speculation on their possible role in intercellular exchange of genetic material.

Mechanism of shedding of MV
Under physiologic condition, the plasma membrane of cells has an asymmetric distribution of their phospholipids. Aminophospholipids, phosphatidylserine and phosphatidilethanolammine, are specifically sequestered in the inner leaflet of the membrane. The transbilayer lipid distribution is under the control of 3 phospholipidic pumps: an inward-directed pump, a flippase, an outward-directed pump, or floppase and a lipid scramblase, responsible for a non specific redistribution of lipids across the membrane. After cell stimulation, including apoptosis, a subsequent cytosolic Ca2+ increase, promotes the lost of the phospholipids asymmetry of the plasma membrane, subsequent phosphatidylserine exposure and there is a transient phospholipidic imbalance between the external leaflet at the expense of the inner leaflet leading blebbing of the plasmamembrane and microvesicles release.