**Section 1**

**Some Aspects of Pathogenesis of Thrombosis** 

**1** 

*Australia* 

**Microparticles: Role in Haemostasis and** 

*1,2Calvary Mater and John Hunter Hospitals, University of Newcastle,* 

Microparticles ( MP) are small membrane bound vesicles which have been described in circulation. They are derived from a variety of cells by an active process of shedding. They are bound by plasma membrane, are anucleate but may contain DNA or RNA and may be virtually derived from any cell (Ahn 2005, Mause*, et al*, Porto*, et al*). The majority of the microparticles in blood are derived from platelets. Previously considered as cell debris they are now regarded as vectors for transfer of biological information. The MP production is thought to reflect a balance between cell stimulation, proliferation and death. Based on their potential function and pathophysiologic effect, MP are thought to be physiological or patholological. MP play a role in normal haemostasis and abnormal amplification of MP production leading to a pathological state (Meziani*, et al*). For example, excessive MP from platelets may contribute to thrombosis (Siljander*, et al* 1996). Their role in vascular biology is being uncovered with increasing evidence for their role in venous thromboembolism. This chapter will explore the role of these MP in the physiology of haemostasis as well as pathology of thromboembolism. The final section will discuss the current state of art in the

Microparticles are submicron (<1.0µm) membrane bound circulating vesicles. Although anucleate, usually express cell surface antigen specific to the cell of origin, they may contain DNA or RNA and be virtually derived from any cell (Freyssinet 2003). The ISTH (International Society of Thrombosis and Haemostasis) vascular biology subcommittee defined these particles as being between 0.1-1.0 µm (SSCMembers Aug 2005). However, several other nanoscale techniques have demonstrated that particles <0.1 µm may also need to be considered as MP (Yuana*, et al*). Indeed, size range of MP is contentious with larger MP likely overlapping with small platelets and the smallest MP with exosomes (Gyorgy*, et al*, Jy*, et al*, Lawrie*, et al* 2009). Several factors may cause the production of MP from cells such as activation, complement mediated lysis, shearing stress, oxidative injury and active vesiculation (Horstman*, et al* 2004). The MP bear at least some surface characteristics of the parent cell and they differ from exosomes (0.03-0.1µm), which originate through the exocytosis of endocytic multivesicular bodies and play a role in antigen presentation

(Freyssinet and Dignat-George 2005, Horstman*, et al* 2004, Horstman*, et al* 2007).

**1. Introduction** 

methods used to detect and measure MP.

**1.1 Definition of a microparticle** 

**Venous Thromboembolism** 

Anoop K. Enjeti1 and Michael Seldon2

*2Hunter Area Pathology Service,* 
