**3. Functions of the platelets**

where tissue regenerates ex vivo. Here, the major idea is regeneration of fully functional tissue on the spot, while damaged tissue is provided for by cellular elements and the process is managed by local factors. PRP appears to be the main source of autologous products in regenerative medicine and a true precursor and foundation of the healing process along with scaffold and stem cells [1]. PRP is used in regenerative medicine, because it provides two of the three components (growth factors and scaffolds), necessary for complete tissue regenera‐ tion. This review discusses the state-of the-art-studies on PRP action mechanisms in surgical and non-surgical implementation (treatment of tendon injuries, cartilage damage, muscle

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

**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

**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

trauma, cartilage and bone pathologies, and wound healing).

**2. Platelet biology**

174 Immunopathology and Immunomodulation

zone, organelle zone, and cytosol zone.

aggregation is affected by those glycoproteins [8].

considered to be the first stage of wound healing [11,12].

Besides participation in hemostasis, platelets also have other functions. To obtain hemostasis, the interaction of three main mechanisms is necessary: vascular response, platelet activity, and clot formation. When platelets are not activated by various stimuli, they are present in blood circulation in a state of quiescence in disc-shape form. The variety of stimuli can comprise physical or chemical ones, or a combination of both. The sub-endothelial collagen forming in the wound as a result of trauma, along with the von Willebrand factor(vWF), are the main factors that activate blood platelets in vivo, as well as thrombin, adenosine diphosphate, or a combination of them.

In experimental conditions the major activators for the study of platelets are collagen, thrombin and adenosine diphosphate, as well as their synthetic substitutes and calcium ions. Integrin A2b1, glycoproteins complex Ib-V-IX (GPIb-V-IX), and glycoprotein VI (GPVI) are the main collagen receptors. The involvement of collagen by these receptors is done as follows: after the process of binding of von Willebrand factor to glycoprotein Ib (GPIb), the exposed collagen binds to glycoprotein V (GPV) in the same complex, thus slowing down the platelets for a time, sufficient to allow for the further binding of the integrin a2b1 and glycoprotein VI (GPVI) with collagen [7,14].

According to Mehta et al., tissue damage leads to vascular exposure. Thus activating platelets and forming platelet plugs and blood clots. This is how natural hemostasis is provided. This naturally occurring hematoms consists of 95% red blood cells, 4% platelets, and 1% white blood cells. An analysis of a platelet-rich clot shows significant differences in composition as compared to naturally occurring clot, with 95% platelets, 4% red blood cells and about 1% white blood cells [15].

Platelets also have non-hemostatic functions. In addition to their primary role in hemostasis, platelets participate in many non-hemostatic processes. Their secretion contains many different substances [16]. On their external layer, a number of surface receptors are arranged, including adhesion proteins, cytokines, and lipopolysaccharide [17]. Interesting is the fact that platelets also release various substances depending on the stimuli, by which they are activated [16,18]. Alpha granules contain many substances with directly opposing activities. This implies the existence of a mechanism for specific release of only a specific content of the granule, which is possible, but this is still not well studied [19,20]. Inflammation, immunity, and tissue recovery are some of the most characteristic features of platelets [7].
