Mechanism of Platelet Rich Plasma (PRP) Therapy to Promote Tissue Healing
The concept known today as PRP first appeared in the field of hematology in the 1970s. Hematologists coined the term PRP decades ago in an attempt to describe plasma obtained from platelet counts above basal values in peripheral blood. More than a decade later, PRP was used in maxillofacial surgery as a form of platelet-rich fibrin (PRF). The fibrin content in this PRP derivative is of great value for its adhesive and homeostatic properties, while PRP has persistent anti-inflammatory properties and stimulates cell proliferation. Finally, around the 1990s, PRP became popular, and eventually, the technology was transferred to other medical fields. Since then, this positive biology has been extensively studied and applied to treat various musculoskeletal injuries in professional athletes, further contributing to its widespread media attention. In addition to being effective in orthopaedics and sports medicine, PRP is used in ophthalmology, gynecology, urology and cardiology, pediatrics and plastic surgery. In recent years, PRP has also been praised by dermatologists for its potential to treat skin ulcers, scar revision, tissue regeneration, skin rejuvenation and even hair loss.
Considering the fact that PRP is known to directly manipulate healing and inflammatory processes, the healing cascade must be introduced as a reference. The healing process is divided into the following four stages: hemostasis; inflammation; cellular and matrix proliferation, and finally wound remodeling.
1. Tissue Healing
A tissue-healing cascade is activated, a process that leads to platelet aggregation, clot formation, and development of a temporary extracellular matrix (ECM. Platelets then adhere to exposed collagen and ECM proteins, triggering the presence of α-granules in the Release of Bioactive Molecules. Platelets contain a variety of bioactive molecules, including growth factors, chemokines, and cytokines, as well as pro-inflammatory mediators such as prostaglandins, prostatic cyclin, histamine, thromboxane, serotonin, and bradykinin.
The final stage of the healing process depends on the remodeling of the wound. Tissue remodeling is tightly regulated to establish a balance between anabolic and catabolic responses. During this phase, platelet-derived growth factor (PDGF), transforming growth factor (TGF-β) and fibronectin stimulate the proliferation and migration of fibroblasts, as well as the synthesis of ECM components. However, the timing of wound maturation is largely dependent on the severity of the wound, individual characteristics, and the specific healing capacity of the injured tissue, and certain pathophysiological and metabolic factors can affect the healing process, such as tissue ischemia, hypoxia, infection, Growth factor imbalances, and even metabolic syndrome-related diseases.
A pro-inflammatory microenvironment that interferes with the healing process. To complicate matters, there is also high protease activity that inhibits the natural action of growth factor (GF). In addition to having mitogenic, angiogenic, and chemotactic properties, PRP is also a rich source of many growth factors, biomolecules that may counteract deleterious effects in inflamed tissues by controlling exacerbated inflammation and establishing anabolic stimuli. Given these properties, Researchers may find great potential in treating a variety of complex injuries.
2. Cytokine
Cytokines in PRP play key roles in manipulating tissue repair processes and regulating inflammatory damage. Anti-inflammatory cytokines are a broad spectrum of biochemical molecules that mediate pro-inflammatory cytokine responses, mainly induced by activated macrophages. Anti-inflammatory cytokines interact with specific cytokine inhibitors and soluble cytokine receptors to modulate inflammation. Interleukin (IL)-1 receptor antagonists, IL-4, IL-10, IL-11 and IL-13 are classified as the main anti-inflammatory cytokines. Depending on the type of wound, some cytokines , such as interferon, leukemia inhibitory factor, TGF-β and IL-6, can exhibit pro- or anti-inflammatory effects. TNF-α, IL1 and IL-18 have certain cytokine receptors that may inhibit the pro-inflammatory effects of other proteins [37]. IL-10 is one of the most potent anti-inflammatory cytokines, it can down-regulate pro-inflammatory cytokines such as IL-1, IL-6 and TNF-α, and up-regulate anti-inflammatory cytokines. These counter-regulatory mechanisms play critical roles in the production and function of pro-inflammatory cytokines. In addition, certain cytokines may trigger specific signaling responses that stimulate fibroblasts, which are critical for tissue repair. The inflammatory cytokines TGFβ1, IL-1β, IL-6, IL-13, and IL-33 stimulate fibroblasts to differentiate into myofibroblasts and improve the ECM [38]. In turn, fibroblasts secrete cytokines TGF-β, IL-1β, IL-33, CXC, and CC chemokines, which promote pro-inflammatory responses by activating and recruiting immune cells such as macrophages. These inflammatory cells have multiple roles at the wound site, primarily by promoting wound clearance – as well as the biosynthesis of chemokines, metabolites and growth factors, which are essential for the remodeling of new tissue. Thus, cytokines present in PRP play an important role in stimulating cell type-mediated immune responses, driving the resolution of the inflammatory phase. In fact, some researchers have named this process “regenerative inflammation,” suggesting that the inflammatory phase, despite patient disquiet, is a critical step necessary for the tissue repair process to reach a successful conclusion, given the epigenetic mechanisms by which inflammatory signals promote cellular plasticity.
3. Fibrin
Platelets carry several factors related to the fibrinolytic system that may upregulate or downregulate the fibrinolytic response. The temporal relationship and relative contribution of hematological components and platelet function in clot degradation remains an issue worthy of extensive discussion in the community. The literature presents many studies focusing only on platelets, which are known for their ability to influence the healing process. Despite numerous outstanding studies, other hematological components, such as coagulation factors and the fibrinolytic system, have also been found to make important contributions to effective wound repair. By definition, fibrinolysis is a complex biological process that relies on the activation of certain enzymes to facilitate the degradation of fibrin. The fibrinolytic response has been suggested by other authors that fibrin degradation products (fdp) may actually be molecular agents responsible for stimulating tissue repair, a sequence of important biological events prior to fibrin deposition and removal from angiogenesis, which is necessary for wound healing . The formation of a clot after injury acts as a protective layer that protects the tissue from blood loss, invasion by microbial agents, and also provides a temporary matrix through which cells can migrate during repair. The clot is due to the cleavage of fibrinogen by serine proteases and platelets aggregate in the cross-linked fibrin fibrous network. This reaction initiates the polymerization of fibrin monomers, the main event in blood clot formation. Clots can also act as reservoirs for cytokines and growth factors, which are released upon degranulation of activated platelets. The fibrinolytic system is tightly regulated by plasmin and plays a key role in promoting cell migration, growth factor bioavailability, and regulation of other protease systems involved in tissue inflammation and regeneration. Key components in fibrinolysis, such as urokinase plasminogen activator receptor (uPAR) and plasminogen activator inhibitor-1 (PAI-1) are known to be expressed in mesenchymal stem cells (MSCs) , a specialized cell type necessary for successful wound healing.
4. Cell Migration
Activation of plasminogen through the uPA-uPAR association is a process that promotes inflammatory cell migration as it enhances extracellular proteolysis. Since uPAR lacks transmembrane and intracellular domains, the protein requires co-receptors such as integrins and vitreins to regulate cell migration. Further, uPA-uPAR binding resulted in increased affinity of uPAR for vitreous connexins and integrins, promoting cell adhesion. Plasminogen activator inhibitor-1 (PAI-1) in turn disengages cells, destroying upar-vitrein and integrin- when it binds to uPA of the uPA-upar-integrin complex on the cell surface Interaction of glass voxels.
In the context of regenerative medicine, mesenchymal stem cells are mobilized from the bone marrow in the context of severe organ damage and thus may be found in the circulation of patients with multiple fractures. However, in certain circumstances, such as end-stage renal failure, end-stage liver failure, or during the onset of rejection following heart transplantation, these cells may not be detectable in the blood [66]. Interestingly, these human bone marrow-derived mesenchymal (stromal) progenitor cells cannot be detected in the blood of healthy individuals [67]. A role for uPAR in bone marrow mesenchymal stem cell mobilization has also been previously proposed, similar to what occurs in hematopoietic stem cell (HSC) mobilization. Varabaneni et al. The results showed that the use of granulocyte colony-stimulating factor in uPAR-deficient mice caused the failure of MSCs, again reinforcing the supportive role of the fibrinolytic system in cell migration. Further studies have also shown that glycosylphosphatidylinositol-anchored uPA receptors regulate adhesion, migration, proliferation, and differentiation by activating certain intracellular signaling pathways, as follows: pro-survival phosphatidylinositol 4,5-bisphosphate 3-kinase/Akt and ERK1/2 signaling pathways, and adhesion kinase (FAK).
MSCs have demonstrated further importance in the context of wound healing. For example, plasminogen-deficient mice exhibited severe delays in wound-healing events, suggesting that plasmin is critically involved in this process. In humans, loss of plasmin can also lead to complications of wound healing. Disruption of blood flow can significantly inhibit tissue regeneration, which explains why these regenerative processes are more challenging in diabetic patients.
5. Monocytes and Regeneration Systems
According to the literature, there is a lot of discussion about the role of monocytes in wound healing. Macrophages are mainly derived from blood monocytes and play an important role in regenerative medicine [81]. Since neutrophils secrete IL-4, IL-1, IL-6 and TNF-[alpha], these cells typically penetrate the wound site approximately 24-48 hours after injury. Platelets release thrombin and platelet factor 4 (PF4), two chemokines that promote the recruitment of monocytes and their differentiation into macrophages and dendritic cells. A striking feature of macrophages is their plasticity, i.e., their ability to switch phenotypes and transdifferentiate into other cell types such as endothelial cells, which subsequently display different functions in response to different biochemical stimuli in the wound microenvironment. The inflammatory cells express two major phenotypes, M1 or M2, depending on the local molecular signal that is the source of the stimulus. M1 macrophages are induced by microbial agents and thus have more pro-inflammatory effects. In contrast, M2 macrophages are typically generated by a type 2 response and have anti-inflammatory properties, which are typically characterized by increases in IL-4, IL-5, IL-9, and IL-13. It is also involved in tissue repair through the production of growth factors. The transition from M1 to M2 isoforms is largely driven by the later stages of wound healing, where M1 macrophages trigger neutrophil apoptosis and initiate clearance of these cells). Phagocytosis by neutrophils activates a chain of events in which cytokine production is turned off, polarizing macrophages and releasing TGF-β1. This growth factor is a key regulator of myofibroblast differentiation and wound contraction, allowing resolution of inflammation and initiation of the proliferative phase in the healing cascade [57]. Another highly related protein involved in cellular processes is serine (SG). This hematopoietic cell-secreted granulan has been found to be necessary for the storage of secreted proteins in specific immune cells, such as mast cells, neutrophils, and cytotoxic T lymphocytes. While many non-hematopoietic cells also synthesize serotonin, all inflammatory cells produce large amounts of this protein and store it in granules for further interaction with other inflammatory mediators, including proteases, cytokines, chemokines, and growth factor. Negatively charged glycosaminoglycan (GAG) chains in the SG appear to be critical for secretory granule homeostasis, as they can bind to and facilitate the storage of substantially charged granule components in a cell-, protein-, and GAG chain-specific manner. Regarding their involvement in PRP, Woulfe and colleagues have previously shown that SG deficiency is strongly associated with altered platelet morphology; defects in platelet factor 4, beta-thromglobulin, and PDGF storage in platelets; poor platelet aggregation and secretion in vitro and thrombosis in vivo form defects. The researchers therefore concluded that this proteoglycan appears to be a master regulator of thrombosis.
Platelet-rich products can be obtained by collecting and centrifuging an individual’s whole blood, separating the mixture into different layers containing plasma, platelets, leukocytes, and leukocytes. When platelet concentrations are higher than basal values, bone and soft tissue growth can be accelerated with minimal side effects. The application of autologous PRP products is a relatively new biotechnology that continues to show promising results in the stimulation and enhanced healing of various tissue injuries. The efficacy of this alternative therapeutic approach may be attributed to the topical delivery of a wide range of growth factors and proteins, mimicking and supporting physiological wound healing and tissue repair processes. Furthermore, the fibrinolytic system clearly has an important impact on overall tissue repair. In addition to its ability to alter cellular recruitment of inflammatory cells and mesenchymal stem cells, it modulates proteolytic activity in wound healing areas and during regeneration of mesodermal tissues including bone, cartilage and muscle, and is therefore key in musculoskeletal medicine component.
Accelerating healing is a highly sought-after goal by many professionals in the medical field, and PRP represents a positive biological tool that continues to offer promising developments in the stimulation and well-coordinated tandem of regenerative events. However, as this therapeutic tool remains complex, especially since it releases a myriad of bioactive factors and their various interaction mechanisms and signaling effects, further studies are required.
（The contents of this article are reprinted, and we do not provide any express or implied guarantee for the accuracy, reliability or completeness of the contents contained in this article, and are not responsible for the opinions of this article, please understand.）