3. Wound healing process

platelet-rich plasma production and characterization. This lack of consensus also prevents a

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

robust cytoskeletal structure, and granules (alpha, delta, and lambda) [2–5].

dothelial system after approximately 10 days in the circulation [9–11].

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

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

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

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

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

. Platelets are removed by macrophages in the reticuloen-

standard approach in the PRP [1–9].

88 Hot Topics in Burn Injuries

mediators they contain [6, 7].

during activation [11, 12].

about 140,000–400,000 platelets/mm<sup>3</sup>

There are three overlapping stages to wound healing: inflammatory, proliferative, and remodeling. Inflammation is the first response to tissue damage. The goal is to provide rapid hemostasis and initiate a series of reactions leading to tissue regeneration. When blood exits from damaged vessels, a hematoma that fills the tissue space occurs, and platelets have crucial roles in this process. Cytokines and growth factors released from activated platelets and other cells result in several events, including cell migration, proliferation, differentiation, and matrix synthesis [16–19]. The fibrin mesh in the hematoma serves as a transient matrix to continue regenerative space and ensure a scaffold for migration and proliferation of cells [18, 20].

Neutrophils, inflammatory cells which first infiltrate the wound area and have lifetimes limited to hours and days, provide rapid defense against infections and removal of tissue debris. Then a flow of monocytes and T lymphocytes occurs to wound area [16, 17, 19, 21].

After monocytes reach the wound area, they differentiate into macrophages, and macrophages become predominant cell types in this region. The macrophages, which have lifetimes limited to days to months, support neutrophils in their functions and increase secretion of factors from neutrophils [16–18, 21]. The role of T lymphocytes in a successful wound healing process is still not clearly understood [19]. The mesenchymal stem cells migrate to the wound site to form an unstable cell line that will serve as a skeleton for or formation of the bone, cartilage, fibrous tissue, blood vessels, and other tissues [17]. Fibroblasts migrate to the wound site and begin to proliferate to produce extracellular matrix [17, 22]. Blood vessel endothelium close to the injury area proliferates to create new capillaries, and then these new vessels extend to the damaged site. These activities are regarded as the first steps of angiogenesis [16, 17].

During the proliferative phase, which is the second stage of wound healing, damaged and necrotic tissue is removed from the surrounding and replaced by living tissue that is in accordance with the original tissue structure of that region (e.g., bone, cartilage, fibrous tissue). Mesenchymal stem cells differentiate into fibroblasts, osteoblasts, chondrocytes, and other cell types which are required to produce the appropriate tissue type [17].

The third phase, the remodeling phase, is the final stage of wound healing. During this phase, the newly generated tissue reshapes and reorganizes to more closely resemble the original tissue [17].
