**2. Platelet biology**

Platelets are blood cells formed during hematopoiesis. They are built from cytoplasmic fragments of the long extensions of megakaryocytes and are small, discoid, and anucleate cells [2]. These extensions are interwoven through bone marrow sinusoids and are fragmented by shear forces, thus forming new platelets in the blood [3]. Their circulating lifespan is 5-9 days and their major clearance mechanism is via Kuppfer cells and hepatocytes. This became known after discovering the lectin receptor on the cell surface [4]. It is known that the functional activity of platelets changes depending on their size and age, as younger and larger platelets demonstrate better hemostatic function unlike smaller and older cells [5,6,7]. Platelets measure from 1 to 4 µm in diameter and apart from being anucleated, they contain different organelles. They are discoid or ellipsoid in shape and have three distinguishable zones: peripheral or outer zone, organelle zone, and cytosol zone.

**Peripheral zone** – This is the outermost section and it contains antigens, glycoproteins and various enzymes. This zone connects the platelets with other cells and blood vessel linings. Large quantities of plasma proteins and coagulation factors are firmly attached to this surface. Inside the membrane there are proteins (mostly glycoproteins and a small amount of carbo‐ hydrates). This membrane contains a double-layer of phospholipids, cholesterol and glycoli‐ pids. Glycoproteins have a number of specific receptors for certain coagulation factors, such as GPIb (thrombin receptors) and the von Willerband factor. The GPIIbIIIa complex is formed from glycoproteins IIb and IIIa and it acts as a fibrinogen receptor. Platelet adhesion and aggregation is affected by those glycoproteins [8].

**Organelle zone** – It is built from a variety of structures: dense granules, alpha granules, Golgi apparatus, dense tubular system and open canalicular system, lysosomes and mitochondria.

The dense granules (or dense bodies) are dense structures containing 65% of the total platelet adenosine-diphosphate and adenosintriphosphate. Serotonin, pyrophosphate, antiplasmin and large amounts of calcium, necessary for platelet aggregation are also stored there [8-10].

Alpha granules contain various growth factors (platelet-derived growth factor— PDGF, transforming growth factor beta—TGF-b, etc.), and clotting factors. Many of the 30 bioactive proteins, playing a key role for hemostasis are contained in these granules; hemostasis is considered to be the first stage of wound healing [11,12].

Upon platelet activation, internal membrane systems including the dense tubular system (DTS) and the open canalicular system (OCS) interact. By means of cellular membrane invaginations, the open canalicular system helps the communication between the endoplasm and the external milieu. The mitochondria synthesize ATP, which is functionally very important. Acid phos‐ phatize enzymes, glucosamidase, and galactosidase are contained in the lysosomes. Many particles contain glycogen, thus storing energy for cells. The Golgi apparatus is part of the membrane system as well [8,13].

**Cytosol zone** - It contains microtubules and forms a circumferential zone around platelets. Microtubules are connected to microfilaments. This is how the platelet cytoskeleton is formed; it directs the cell's movement, eliminates secreted products, and retracts clots. Upon platelet activation microtubules could be seen in platelet pseudopods [8].
