**2.1 How are microparticles produced?**

Microparticles are thought to be produced by an active process of vesiculation or shedding from the cell surface and utilizing ATP in the process. Various enzymes involved in the production of MP have been studied. The balance of several enzymes regulating membrane homeostasis is believed to be key in the production of MP. An inward aminophospholipid enzyme 'translocase' or 'flippase'and an outward enzyme 'floppase' have been postulated to maintain the dynamic symmetrical state of the phophoslipid bilayer membrane (Diaz and Schroit 1996, Montoro-Garcia*, et al*, Morel*, et al*). In a resting membrane the flippase enzyme is more active thereby ensuring that phosphotidyl serine (PS) is at the inner membrane.The activation of phospholipid nonspecific enzyme known as 'scramblase' is said to be responsible for disruption of membrane asymmetry and several mechanisms participating in the regulation of the transmembrane migration of phosphatidylserine (PS) in activated cells lead to microparticle shedding (Diaz and Schroit 1996, Enjeti*, et al* 2008, Morel*, et al* 2006). After stimulation, calcium is released from intracellular stores. Calcium depletion induces the activation of store-operated calcium entry (SOCE) through channels in the plasma membrane and this process is thought to be regulated by transient receptor potential channel (TRPC) proteins (Diaz and Schroit 1996, Montoro-Garcia*, et al*). The transverse redistribution of PS is under the control of SOCE. Several other process such as Raft integrity, cytoskeleton organization and MAP kinase pathway (Ras-ERK) are also involved in membrane remodelling (Diaz and Schroit 1996, Montoro-Garcia*, et al*, Morel*, et al* 2006). Microparticles typically have phosphotidyl serine on the outer surface (although PS negative MP have also been recently described) and ABCA1, a member of the ATP-binding cassette family of transporters, is a potential candidate for the transport of PS to the surface (Diaz and Schroit 1996, Morel*, et al* 2006).

#### **2.2 Role of MP in coagulation and haemostasis**

Normal coagulation is a complex process triggered by endothelial damage and exposure of tissue factor and collagen which initiates a platelet plug formation at the site of injury. This leads to activation of a cascade of enzymes, which forms a fibrin clot. Microparticles of different cell origin could play a role in fibrin clot formation, enhance platelet leukocyte interactions and influence other plasma proteins such as von Willebrand's factor. Given that platelet MP constitute the majority of the circulating MP, they are considered an important effector of the haemostatic process (Morel*, et al* 2006). Some MP have also been described to carry molecules with anticoagulant function on their surface (Freyssinet 2003). The balance of pro and anticoagulant bearing MP in the endovascular milieu is likely to influence the propensity to bleed or clot in a particular patient.


Table 2. The possible pro and anticoagulant markers on the surface of microparticles (Enjeti*, et al* 2007, Morel*, et al* 2006).
