Extracellular vesicles (EV) are spherical membrane structures released by different types of normal and also cancer cells (1). The best known example are the vesicles derived from blood platelets (2). These membrane structures were detected, among others, in urine, blood, amniotic fluid, saliva, or semen. Under physiological conditions they are released from the cell membranes during maturation and aging of cells. Cell activation, induced by various factors (complement proteins, cytokines, stress factors), results in increased secretion of these structures. Recently involvement of vesicles in many diseases, including cardiovascular, autoimmune, diabetes, infectious diseases and cancer metastasis was studied (3). Determining the origin and the number of circulating microparticles facilitates explanation of their participation in the pathogenesis of various disorders. A few publications have revealed an increased release of these structures during long-term storage of blood for transfusions (4-8).

First report concerning the vesicles was published in 1967 and has been described by Wolf. The author termed the structures “platelet dust”, the presence of which he found in human blood plasma. Structures secreted by activated blood platelets showed prothrombotic properties, owing to the presence of tissue factor – thrombospondin (9). The vesicles were isolated by ultracentrifugation, and observed by the electron microscopy. Discovery of the “platelet dust” marked the beginning of a new era regarding microvesicles formation and their role in the organism. Afterwards first publications on microvesicles released from erythrocytes (10), monocytes (11) and endothelial cells were published (12).

The membrane vesicles are a heterogeneous group of different sizes, origin, biological and physical properties, mechanism and source of formation. Due to the size and origin, the vesicles were divided into: smaller exosomes, with a diameter of 30-100 nm, and bigger, ectosomes, measuring from 0.1 to 1 μm (13). Previous nomenclature of these structures was based on the origin of the cells, from which they are formed. In recent years terminology was based on the mechanism of their formation. Consensus on terminology and classification of vesicles was obtained in 2013 (14). Among the vesicles, the exosomes generated within the multivesicular bodies (MVBs) were separated. During the fusion of MVBs with the intraluminal membrane, vesicles are secreted from cells. After that, their size ranges from 40 to 150 nm. Major markers are tetraspanins (CD9, CD63, CD81, CD82), and markers of lipid rafts: flotillins 1 and 2. Other markers, such as heat shock proteins (HSP), MHC molecules, various components of endosomal sorting complex required for transport (ESCRT) and member of the Ras superfamily of monomeric G proteins (Rab) are used to identify these vesicles (15, 16).

The exosomes are formed and maturate in the intracellular (endosomal) vesicles. Afterwards they are secreted out of the cell by the fusion of vesicle membrane with the cell membrane, what involves soluble N-ethylmaleimide – sensitive factor attachment receptor transmembrane proteins (SNARE). In the process of exosome formation, Ca²⁺ dependent, vesicle associated membrane proteins (VAMP) are included. It was shown that in leukemia cells, the VAMP-7 protein plays an important role in the formation of exosomes during their fusion with the cell membrane. VAMP-1,-2,-3 proteins were identified in the exosomes secreted from tumor cells (13). Recent studies have shown that there are 2 types of exosomes: immunologically active exosomes that are involved in antigen presentation and costimulation and those containing RNA and mediating the genetic communication between cells (17).

Ectosomes are formed extracellularly, directly from the cell membrane by blebbing. The vesicle detachment occurs due to the contraction of cell, and then peeling it in specific sites. On the cell surface, the buds are formed, which mechanism involves active participation of cytoskeleton proteins e.g. actin. In the cell membrane proteins and lipids many changes take place. Similarly to the apoptosis, disruption of phospholipid asymmetry appears. In the cell membrane movement of phospholipids: phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine and sphingomyelin takes place with the participation of flippase, floppase and scramblase. Ca²⁺ ions play an important role during the formation of microvesicles. As a result, the increased concentration of Ca²⁺ activates enzymes, such as gelsolin, aminophospholipid translocase, floppase, scramblase and calpain, resulting in the change of cell membrane asymmetry (18). Exposure of phosphatidylserine on the outside layer of the plasma membrane seems to be one of the main microvesicle properties, although there are also microvesicles without exposed phosphatidylserine. RhoA protein, belonging to the family of small guanosine-5’-triphosphates (GTPases), Rho-activated protein kinases (ROCK) and actin-binding (LIM) kinases are regulators of ectosome secretion. Other regulator, is calpain – Ca²⁺ dependent enzyme, which participates in the regulation of cytoskeleton proteins, where it plays a significant role in the ectosome formation. Furthermore it was also concluded that the ADP-ribosylation factor 6 (ARF6) protein is essential both in these structure formation and secretion (13).