Platelets are the key particles which drive haemostasis and coagulation in the body. Thus they are very sensitive to physiological insults, which creates problems upon their collection for transfusion to thrombocytopenic patients, as it is more advantageous to prevent their activation, or the platelet storage lesion, for as long as possible. The longer they are stored for, the more advanced the lesion progresses, and so suitable tests to assess viability for transfusion are performed. However it is more important that novel techniques to hold the platelet storage lesion at bay are developed, to retain stores of platelets and to prevent shortages of these life-saving bodies.
The separating tissue between the outer environment and a person’s peripheral blood vessel may only be a few millimetres, or less, at certain locations in the body. Should this barrier be breached, the person could potentially haemorrhage to the point of fatality, if the coagulation system was not present, and the blood platelet is the key to this system. However, in certain circumstances, such as in thrombocytopenia, the system is compromised by a lack of functional platelets (PLTs), and in such cases, platelet transfusion may be the only life-saving treatment available. This dissertation aims to look at the physiological importance of platelets, how they are prepared for transfusion, and also the difficulties experienced in this preparation and how they are, and may potentially be in the future, overcome.
Platelets are anucleated cells/fragments, produced by megakaryocytes in the bone marrow, naturally having a discoid morphology with dimensions 3ï¿½m x 0.5 ï¿½m, expressing ABO antigens, human leukocyte antigen-I and human platelet antigens.[1,2] The normal range for platelets in the blood is 150×109-400×109/L, and each platelet survives in the blood for approximately 9.5 days.[3,4] The loss of endothelial integrity in a blood vessel exposes the collagen in the subendothelium, which expresses a ligand called the von Willebrand factor (vWF). A receptor complex for vWF is present on platelet membranes, and is made from glycoprotein-Ib (GPIb), GPV and GPIX, anchoring the platelet to the subendothelium. This primary step in the haemostatic response is secured by further adhesion molecules, e.g. GPVI binds fibrillar collagen and ?6ï¿½1 integrin binds laminin, and signalling occurs inside the platelet.[3,5]
Src kinases and phosphatidylinositol 3-kinase, PI(3)K, transduce the signal, activating phospholipase C-? to produce inositol trisphosphate, IP3. This complexes with its receptor, IP3R, on the dense tubular system (DTS) inducing Ca2+ release. Following this, many Ca2+ dependent activities occur, such as thromboxane A2, TXA2, (produced from prostaglandin H2) and ADP production and release.
These act in paracrine and autocrine fashions on platelets, increasing adhesion molecule affinities for subendothelial matrix proteins, thus further securing the adhesion. Also TXA2, ADP, thrombin and the other released factors typically act through G protein coupled receptors (GPCRs). For TXA2 and ADP, the G? subunit activates PLCï¿½ to increase intracellular Ca2+ from the DTS even more, using IP3, and also activates PI(3)K to activate the Rap1b GTPase, respectively. Therefore TXA2 augments the exocytotic release of granules further, and ADP activates talin, via Rap1b, to induce actin modification. This cortical remodelling cause pseudopod formation, and increased adhesion.[6,7] Artificially coloured scanning electron micrograph of a whole blood clot. 
Intracellular Ca2+ also causes the expression of CD62 (P-selectin) to form aggregates of platelets. These aggregates, again by Ca2+ signalling, express outer membrane phosphatidylserine, a classical apoptotic signal. In this circumstance it is also a coagulation signal, as it creates an unusually strong negative outer membranous charge, attracting coagulation factors such as factors Va and Xa to form thrombin from prothrombin. This thrombin formation through the aggregate further increases the activation already discussed, but also cleaves fibrinogen to form fibrin. This stabilises the aggregate to the status of a clot, which forms a plug once it retracts. During the course of this, microparticles (40nm x 1ï¿½m) are also released as exocytotic buds from the platelets. These are part of the apoptotic process, but are capable of the functions outlined above, and thus contribute to the role of platelets in haemostasis.[3,6,7]
There are a few methods by which these crucial particles are obtained from donors to be prepared for transfusion into cancer or thrombocytopenic patients. The standard whole blood derived platelet rich plasma (WBD-PRP) comes from normal donations. A closed system donation of the 450ml unit is received, and is soft spun producing a platelet concentrate in the supernatant. This supernatant is then hard spun to produce a platelet sediment, and approximately 4-8 of these are combined to give 1 platelet unit. This is a comparatively cheaper method to use, but exposes the recipient to an increased number of donors, thus presenting more potential for transfusion reactions (TRs) or infections.
A Buffy coat can be used, which uses the same method as WBD-PRP but exchanges the spin order i.e. hard spin first, soft spin second, and finally apheresis can be used. Apheresis is the most expensive procedure, and reduces the amount of blood components received, this also reduces the number of donors as well as the risk of TRs or infections.[1,8] It is important to have leukoreduced units of platelets as nearly 90% of TRs are cytokine induced, and 10% by antibodies. Thus, the TRAP Study Group strongly encourage the treatment of units with UVB light, or amotosalen, to reduce the number of active leukocytes present.[9,10]