2. Platelets: origin, structure, distribution, and their roles in hemostasis

Platelets, discovered in the nineteenth century, are small, nucleus-free cytoplasmic cellular structures which are round or oval-shaped and have about 2 μm diameter and derived from megakaryocytes (a type of white blood cell) in the bone marrow [1]. These cellular structures were initially believed to be involved only in the hemostasis and pathological thrombus formation. Although, platelets do not have nucleus, many organelles are found in their cytoplasm including abundant mitochondria, several loops of microtubular coils giving them a robust cytoskeletal structure, and granules (alpha, delta, and lambda) [2–5].

The platelets organize the migration of cells associated with wound healing (neutrophils, macrophages, stem cells, etc.) as well as the formation of the initial clot by means of the inflammatory mediators they contain [6, 7].

Alpha granules are formed during megakaryocyte maturation, and each platelet contains approximately 50–80 alpha granules, each bound by a unit membrane [8, 9]. Alpha granules are about 200–500 nm in diameter and contain more than 30 bioactive mediators each playing a fundamental role in hemostasis and/or tissue healing. Platelets reside intravascularly and are concentrated in the spleen. The normal mean concentration of platelets in normal blood is about 140,000–400,000 platelets/mm<sup>3</sup> . Platelets are removed by macrophages in the reticuloendothelial system after approximately 10 days in the circulation [9–11].

After tissue damage, the platelets become exposed to the damaged vessel, and these damaged vessels are places where the platelets directly contact with collagen, the basement membranes of capillaries, and subendothelial microfibrils [10]. This interaction causes the platelets to aggregate at the damaged site and change from a rounded shape to one that includes large, sticky protuberances or pseudopodia. This course is called "activation." The alpha-granules fuse with the platelet plasma membrane and release their protein contents to the surroundings during activation [11, 12].

Blood clotting begins via one of two pathways called intrinsic and extrinsic pathways [10]. The intrinsic one is started by damage or alteration to the blood, itself, whereas the extrinsic pathway is started via the contact of blood and factors that are extraneous to the blood (e.g., damaged tissue). Both cascades are associated with a series of reactions in which the inactive factors are activated. These series of reactions facilitate the formation of other mediators from precursors that go on to catalyze subsequent reactions, leading to the formation of a final clot. Although both pathways are initiated in different ways, they overlap and share common steps in the later stages of clot formation [9, 11]. The platelets participate in many levels of the reaction sequence that produces fibrin thread and are component of the final clot structure, which comprise a fibrin mesh, with the activated platelet aggregate and red and white blood cell complex within. Since calcium ions are necessary for blood clotting, an effective agent capable of binding calcium ions or removing it from the environment prevents the progress of the coagulation process. Citrate, which binds to calcium ions and forms the calcium citrate molecule, is a soluble but unionizable substance. Classical blood preservatives include citrate dextrose and citrate phosphate dextrose as well as other substances to maintain cellular viability [9, 13–15].
