Platelets and Platelet-rich Plasma
The blood platelet is a tiny, anucleate cell responsible for the initiation of formation of a thrombus (Figure 1). Platelets are formed from a fragment of megakaryocyte membrane that is pre-packaged with a myriad of molecules and complexes necessary for its primary function, which is to sense signs of trauma within the vasculature and aggregate together to stem the loss of blood. One of the primary steps in thrombus formation is platelet activation, which is driven by 'outside-in' signalling, initiated through a vast repertoire of G-protein coupled receptors, integrins and glycoprotein channels on the surface of the platelet (Li et al., 2010). The activation of platelets can occur through numerous mechanisms by a seemingly endless number of agonists, including but not limited to, thrombin, collagen, adenosine diphosphate (ADP), thromboxanes, serotonin, oxidised LDL and extracellular divalent cations (Lopez-Vilchez et al., 2009; Li et al., 2010; Wraith et al., 2013; Shen et al., 2017).
Granule release in activated platelets. Platelets express numerous glycoprotein, integrin and G- protein-coupled receptors that bind to a myriad of soluble and matrix proteins and molecules, resulting in tightly orchestrated intracellular signalling. This intracellular signalling significantly increases cytoplasmic calcium levels and causes drastic changes in the platelet cytoskeleton, resulting in ashape change in the platelet to an 'echinocytic' formation. During this process, granular storage compartments migrate inwards to the centre of the platelet and fuse with the plasma membrane and release their contents into the extracellular milleiu. PAR1/4, protease-activated receptors 1/4; GPVI, glycoprotein VI; TXA2, thromboxane A2; TP, thromboxane protstanoid receptor; 5-HT, 5-hydroxytryptomine; P2Y, purinergic receptor 2Y; vWF, von Willebrand Factor; IL-8, interleukin-8; CCL5, chemokine ligand 5; SDF-1a, stromal cell-derived factor 1 alpha; FGF, fibroblast growth factor; EGF, endothelial growth factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; TGFb1, transforming growth factor beta 1.
A core platelet response to activation is the release of the contents of intracytoplasmic granules. Platelets contain two main granule stores, the alpha and dense granules, both of which replete with factors critical for an effective platelet response to vascular damage (Figure 1). Where alpha and dense granules are lacking, the conditions grey platelet syndrome and delta storage pool deficiency can arise. Both of these conditions are associated with an increased bleeding tendency (Bolton-Maggs et al., 2006). It is also of note that more recently, platelet secretory behaviour has been shown to extend beyond the realm of granular stores and also involves activation-dependent synthesis and release of cytokines and other bioactive molecules (Heijnen and van der Sluijs, 2015). It is, therefore, clear that the contents of platelet intracytoplasmic granules and de novo synthesis of agents are essential for the haemostatic response, and the descriptions on the functions of platelet releasate have historically focussed on its role in haemostasis (Rendu and Brohard-Bohn, 2001). However, the catalogue of bioactive proteins and molecules released by activated platelets can have multiple physiological effects which include increased angiogenesis, cell proliferation, cell differentiation and regulation or attenuation of apoptosis (Bir et al., 2011; Au et al., 2014; Golebiewska and Poole, 2015). The therapeutic role of the platelet releasate in driving tissue regeneration is of growing interest throughout modern medicine.
PRP is a term used to describe a fraction of the blood after processing. It is typically isolated from autologous whole blood retrieved by phlebotomy into a citrate-based anticoagulant. This is then subjected to differential centrifugation, resulting in the removal of red blood and immune cells, leaving behind a high concentration of platelets within plasma. Commercial sources of PRP are available, which can provide a predetermined concentration of platelets. However, in many cases, PRP is derived 'in-house', produced according to many subtle protocol variations. It is not uncommon for resulting PRP to retain varying concentrations of RBCs and WBCs; such contamination and absence of standardisation may result in conflicting findings regarding the effects of PRP in different applications.
In recent years, there has been significant interest in exploiting PRP in regenerative medicine. Particular attention has been paid to musculoskeletal (Scully et al., 2019, 2020), oral-maxillofacial (Xu et al., 2020) and osteoarthritis (Evans et al., 2020) applications to name but a few. For a more comprehensive account, the reader is referred to a review (Scully et al., 2018).
Hum Reprod. 2021;36(7):1737-1750. © 2021 Oxford University Press