Os mecanismos regenerativos do plasma rico em plaquetas Uma revisão

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Cytokine 144 (2021) 155560
Available online 15 May 2021
1043-4666/© 2021 Elsevier Ltd. All rights reserved.
Review article 
The regenerative mechanisms of platelet-rich plasma: A review 
Rafael Gonzalez dos Santos a, Gabriel Silva Santos a,*, Natasha Alkass b, Tania Liana Chiesa c, 
Gabriel Ohana Azzini a, Lucas Furtado da Fonseca d, Antonio Fernando dos Santos e, 
Bruno Lima Rodrigues a, Tomas Mosaner f, José Fábio Lana a 
a IOC – Instituto do Osso e da Cartilagem/The Bone and Cartilage Institute, 1386 Presidente Kennedy Avenue – 2nd Floor, Room #29, Zip Code 13334-170, Indaiatuba, 
SP, Brazil 
b Queensland University of Technology, 2 George St, Zip Code 4000, Brisbane, Queensland, Australia 
c QML Pathology, 11 Riverview Place, Murarrie, Zip Code 4172, Brisbane, Queensland, Australia 
d Universidade Federal De São Paulo - Escola Paulista de Medicina, 715 Napoleão de Barros St, Vila Clementino, Zip Code 04024-002, São Paulo, SP, Brazil 
e FARMERP- Faculdade de Medicina de São José do Rio Preto, 5416 Brigadeiro Faria Lima Avenue, Vila Sao Pedro, Zip Code 15090-000, São José do Rio Preto, SP, 
Brazil 
f IOC – Instituto do Osso e da Cartilagem/The Bone and Cartilage Institute, 1386 Presidente Kennedy Avenue – 2nd Floor, Room #29, Zip code 13334-170, Indaiatuba, 
SP, Brazil 
A R T I C L E I N F O 
Keywords: 
Orthobiologics 
Platelet-rich plasma 
Regenerative medicine 
Tissue healing 
Inflammation 
A B S T R A C T 
Orthobiologics continue to gain popularity in many areas of medical science, especially in the field of regen-
erative medicine. Platelet-rich plasma derivatives are orthobiologic tools of particular interest. These biologic 
products can be obtained via centrifugation of a patient’s whole blood and the components can then be sub-
sequently isolated, concentrated and ultimately administered into injured tissues, particularly in areas where 
standard healing is disrupted. The elevated concentration of platelets above the basal value enables accelerated 
growth of various tissues with minimal side effects. The application of autologous orthobiologics is a relatively 
new biotechnology undergoing expansion which continues to reveal optimistic results in the stimulation and 
enhanced healing of various sorts of tissue injuries. The local release of growth factors and cytokines contained in 
platelet alpha granules accelerates and ameliorates tissue repair processes, mimicking and supporting standard 
wound healing. This effect is greatly enhanced upon combination with the fibrinolytic system, which are 
essential for complete regeneration. Fibrinolytic reactions can dictate proper cellular recruitment of certain cell 
populations such as mesenchymal stem cells and other immunomodulatory agents. Additionally, these reactions 
also control proteolytic activity in areas of wound healing and regenerative processes of mesodermal tissues 
including bone, cartilage, and muscle, which makes it particularly valuable for musculoskeletal health, for 
instance. Although many investigations have demonstrated significant results with platelet-rich plasma de-
rivatives, further studies are still warranted. 
1. Introduction 
The use of autologous hematological components has become a 
highly attractive therapeutic tool in many areas of medical science for 
the treatment of various debilitations. Platelet-rich plasma (PRP), for 
example, is a popular treatment modality and has raised a significant 
level of attention amongst medical professionals over the last two de-
cades. This is mainly attributed to the promising potential of PRP in 
enhancing regenerative processes, which has long been appreciated by 
oral and maxillofacial surgeons, veterinarians, dermatologists and sports 
medicine experts [1,2]. 
Platelet-rich plasma can be generated by the centrifugation of an 
individual’s whole blood, carrying a variety of cells with a primary focus 
on concentrated platelets above baseline [3,4], as Fig. 1 illustrates. It 
may be obtained via the use of commercial kits or in-house techniques, 
for example. Although there are many different protocols in the 
* Corresponding author at: IOC – Instituto do Osso e da Cartilagem/The Bone and Cartilage Institute, 1386 Presidente Kennedy Avenue – 2nd floor, Room #29, 
Indaiatuba, São Paulo, Brazil. 
E-mail addresses: rafaelgsantos01@gmail.com (R.G. dos Santos), gabriel1_silva@hotmail.com (G.S. Santos), Natasha.alkass@outlook.com (N. Alkass), 
taniachiesa@outlook.com (T.L. Chiesa), drgabriel.azzini@gmail.com (G.O. Azzini), ffonsecalu@gmail.com (L.F. da Fonseca), fernando.ortopedia@gmail.com 
(A.F. dos Santos), brunolr.ioc@gmail.com (B.L. Rodrigues), contato@clinicaviveresanus.com.br (T. Mosaner), josefabiolana@gmail.com (J.F. Lana). 
Contents lists available at ScienceDirect 
Cytokine 
journal homepage: www.elsevier.com/locate/cytokine 
https://doi.org/10.1016/j.cyto.2021.155560 
Received 22 November 2020; Received in revised form 17 April 2021; Accepted 28 April 2021 
mailto:rafaelgsantos01@gmail.com
mailto:gabriel1_silva@hotmail.com
mailto:Natasha.alkass@outlook.com
mailto:taniachiesa@outlook.com
mailto:drgabriel.azzini@gmail.com
mailto:ffonsecalu@gmail.com
mailto:fernando.ortopedia@gmail.com
mailto:brunolr.ioc@gmail.com
mailto:contato@clinicaviveresanus.com.br
mailto:josefabiolana@gmail.com
www.sciencedirect.com/science/journal/10434666
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Cytokine 144 (2021) 155560
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literature there is a lack of consensus regarding an idealistic preparation 
of PRP, which creates variance in content among PRP products and, 
therefore, different terminologies [5]. 
The platelets present in PRP carry granules with a large number of 
active biomolecules which, upon activation, are released and subse-
quently stimulate the natural healing cascade [6]. Perhaps the preciosity 
behind PRP may be attributed to its capacity to render enormous 
quantities of growth factors and proteins which cooperatively stimulate 
the healing cascade in injured tissues. Depending on the tissue type and 
localization, the standard healing process may pose a great challenge, 
demanding a prolonged amount of time due to finite blood supply and 
comparably slower cell turnover [7]. A treatment plan with PRP enables 
accelerated neovascularization, increasing blood supply and nourish-
ment of nearby cells. This is essential for cellular regeneration and the 
restoration of damaged tissue. Additionally, PRP can also ameliorate 
other biological events including recruitment, proliferation and differ-
entiation of cells, which collectively contribute towards adequate 
healing [8]. Since PRP is quite a complex biological product containing a 
myriad of active biomolecules, its full mechanisms have yet to be 
completely elucidated, and more studies are warranted for further 
validation in the clinical scenario. 
The objective of this review is to discuss the biological roles of PRP 
and its derivatives as well as their implication in inflammation and the 
healing cascade in the context of regenerative medicine. 
2. The evolution of PRP 
The concept of what is known today as PRP first emerged around the 
1970s in the field of hematology [9]. Hematologists coined the term PRP 
decades ago in attempts to describe the obtained plasma with a platelet 
count above that of the basal value from peripheral blood [6]. Over a 
decade later, PRP was then implemented in maxillofacial surgery as 
platelet-rich fibrin (PRF). The fibrin content in this PRP derivative 
proved to be of great value due to its adherence and homeostatic 
properties, while PRP sustained anti-inflammatory characteristics and 
stimulation of cell proliferation [10]. Finally, around the 1990s the 
popularity of PRP began to rise and, eventually, this technique migratedto other medical fields [11]. Since then, this orthobiologic has been 
widely studied and utilized in the treatment of various musculoskeletal 
injuries in professional athletes which further contributed to its wide-
spread attention in the media [12]. In addition to being effective in the 
field of orthopedics and sports medicine, PRP has also been used in 
ophthalmology, gynecology, urology and cardiac, pediatric and plastic 
surgery, as examples [6]. In recent years, PRP has also been praised by 
dermatologists for its potential in the treatment of cutaneous ulcers, scar 
revision, tissue regeneration, skin rejuvenation and even alopecia 
[2,6,10,13–16]. 
3. Obtaining PRP 
There are various protocols describing the preparation of platelet- 
rich plasma, though the vast majority demands two centrifugations 
with specific time and centrifugal force (g) parameters [5]. PRP is ob-
tained by means of different density gradients of cell components pre-
sent in the blood. After the very first centrifugation, erythrocytes, which 
are the densest particles, will be separated from the plasma and remain 
at the bottom layer of the suspension (Fig. 1). Directly above the 
erythrocytes, a delicate layer of leukocytes is formed (TGF-β1, which counters 
the effect on chemokine transactivation by TNF-α. HGF appears to play 
an indispensable role in the anti-inflammatory effect attributed to PRP. 
This potent anti-inflammatory cytokine inhibits inflammation by dis-
rupting the NF-κB signaling pathway and proinflammatory cytokine 
expression [32]. Furthermore, PRP may also reduce high levels of nitric 
oxide (NO). In articular cartilage, for example, elevated concentrations 
of NO has been shown to inhibit collagen synthesis and induce apoptosis 
of chondrocytes whilst increasing the synthesis of matrix- 
metalloproteinases (MMPs), which promotes a shift towards catabo-
lism [33]. 
In regards to cellular degeneration, PRP is also believed to manipu-
late autophagy in specific cell types. Upon reaching the eventual state of 
senescence, some cell populations lose quiescence and self-renewal po-
tential [34]. However, recent studies have shown that PRP therapy may 
very well reverse these detrimental states. Moussa and colleagues 
demonstrated that PRP elicits a protective role towards chondrocytes by 
increasing autophagy and anti-inflammatory markers, whilst decreasing 
apoptosis in human osteoarthritic cartilage. García-Prat et al. report that 
autophagy dictates the shift between the quiescence and senescence fate 
of muscle stem cells. According to the researchers, in vivo normalization 
of integral autophagy avoids intracellular damage accumulation, pre-
vents satellite cell senescence and functional decline in old mice and 
even in aged human stem cells, for instance [35,36]. 
Recently, Parrish and Roides also made remarkable contributions 
which further reveal the anti-inflammatory potential of PRP, this time 
focusing on the interaction of platelets and neutrophils. In their inves-
tigation, the researchers explain that arachidonic acid released by 
activated platelets is picked up by neutrophils and converted into 
leukotriene and prostaglandins, which are known inflammatory mole-
cules [3]. However, the platelet-neutrophil interaction allows for the 
conversion of leukotriene into lipoxin, which has been shown to be a 
potent anti-inflammatory protein capable of limiting neutrophil acti-
vation and preventing diapedesis, promoting succession into the ulti-
mate stages of the healing cascade [3]. 
5.2. Cytokine profile 
Cytokines in PRP have exhibited a pivotal role in manipulating the 
tissue repair process and regulating inflammatory damage. Anti- 
inflammatory cytokines are a broad range of biochemical molecules 
that mediate the pro-inflammatory cytokine response, which is induced 
mainly by activated macrophages. Anti-inflammatory cytokines interact 
with specific cytokine inhibitors and soluble cytokine receptors to 
regulate inflammation [37]. Interleukin (IL)-1 receptor antagonist, IL-4, 
IL-10, IL-11, and IL-13 are classified as predominant anti-inflammatory 
cytokines. Depending on the wound type, some cytokines such as 
interferon-alpha, leukemia inhibitory factor, TGF -β and IL-6, can 
display either pro-inflammatory or anti-inflammatory effects. TNF-α, IL- 
1 and IL-18 possess certain cytokine receptors that may inhibit pro- 
inflammatory effects of other proteins [37]. 
IL-10, one of the most potent anti-inflammatory cytokines, down- 
regulates pro-inflammatory cytokines such as IL-1, IL-6 and TNF-α, 
and up-regulates anti-inflammatory agents [38]. These counter- 
regulatory mechanisms play a key role in the production and function 
of pro-inflammatory cytokines [38,39]. In addition, certain cytokines 
may trigger specific signaling responses that stimulate fibroblasts, cells 
which are critical for tissue repair [38]. Inflammatory cytokines TGFβ1, 
IL-1β, IL-6, IL-13 and IL-33 stimulate the differentiation of fibroblasts 
into myofibroblasts, ameliorating ECM [38]. Sequentially, fibroblasts 
secrete cytokines TGF-β, IL-1β, IL-33, CXC and CC chemokines, which 
facilitate the pro-inflammatory response by activating and recruiting 
immunological cells like macrophages [39]. These inflammatory cells 
multitask at the wound site, mainly by facilitation of wound debride-
ment as well as the biosynthesis of chemokines, metabolites, and growth 
factors, which are crucial for the reconstruction of new tissue [40]. 
Therefore, the cytokines present in PRP play an important role in 
stimulating cell types to mediate immunological responses, driving the 
inflammatory phase through to resolution. As a matter of fact, some 
researchers have nominated this process “regenerative inflammation” 
[41], suggesting that the inflammatory phase, although discomforting 
for a patient, is a necessary key step in order for the tissue repair process 
to reach successful conclusion, considering that inflammatory signaling 
promotes epigenetic mechanisms for cellular plasticity [42]. 
The role of cytokines in the inflammatory response of fetal skin is of 
particular interest for the study of regenerative medicine. Fetal and 
adult healing mechanisms differ in the way that damaged fetal tissue can 
sometimes return to its original state depending on gestational age and 
the tissue type in question. In humans, fetal skin can undergo complete 
regeneration up to 24 weeks, whereas in adults, wound resolution re-
sults in scar formation. It is already known that scar tissue exhibits 
significantly reduced mechanical properties and limited function when 
compared to healthy tissue [43,44]. Particular attention has been 
directed towards cytokine IL-10, which was found to be highly expressed 
in amniotic fluid and fetal skin, and has been shown to play a role in a 
scarless repair to fetal skin, facilitated through pleiotropic effects of said 
cytokine. Zgheib C et al. investigated the transplantation of fetal skin 
grafts onto genetically modified knockout (KO) IL-10 mice and control 
mice. The IL-10 KO mice showed signs of inflammation and scarring 
around the graft whereas the grafts on the control group showed no 
expressive changes in biomechanical properties and scar-free healing 
occurred. 
The importance of regulating the delicate balance between anti- and 
pro-inflammatory cytokines expression is that the latter, when 
R.G. dos Santos et al. 
Cytokine 144 (2021) 155560
4
overproduced, ends up signaling cellular degradation by decreasing the 
expression of certain genes. In the case of musculoskeletal medicine, for 
instance, IL-1β downregulates SOX9, which produces an important 
transcription factor for cartilage development, regulating collagen type 
II alpha 1 (Col2A1), the gene responsible for encoding type II collagen. 
IL-1β ultimately decreases the expression of Col2A1 as well as aggrecan 
[45]. Treatment with platelet-rich products, however, have been shown 
to inhibit the effects of IL-1β, maintaining the expression of collagen 
encoding genes and reducing pro-inflammatory cytokine-induced 
chondrocyte apoptosis, remaining a viable ally for regenerative medi-
cine [45]. 
5.3. Pro-anabolic stimuli: Reconstruction of damaged tissue 
In addition to modulating the inflammatory status of injured tissues, 
the cytokines present in PRP also contribute to anabolic reactions by 
exerting their mitogenic, chemoattractive and proliferative effects [46]. 
To elaborate, an in vitro study led by Cavallo et al. [47] aimed to 
investigate the effects of different PRP formulations on human chon-
drocytes. The researchers observed that PRP products with relatively 
poor concentrations of platelets and leukocytes stimulated regular 
chondrocyte activity in a manner where it favored certain cellular 
mechanisms that promoted anabolic reactions. For instance, the 
expression of type-II collagen and aggrecan was observed. Conversely, 
high concentrations of platelets and leukocytes seemed to stimulate 
other cellsignaling pathways involving various cytokines. The authors 
proposed that this is likely attributed to the abundant presence of leu-
kocytes in that specific PRP formulation. These cells appear to be 
responsible for the increased expression of certain growth factors such as 
VEGF, FGF-b and interleukins IL-1b and IL-6, which may in turn stim-
ulate TIMP-1 and IL-10. In other words, a PRP mixture enriched with 
both platelets and leukocytes appears to promote relatively aggressive 
effects in chondrocytes when compared to a “poor” PRP formulation 
[47,48]. 
A previous study designed by Schnabel et al. [49] aimed to evaluate 
the effects of autologous biologic materials in equine tendinous tissue. 
The authors collected blood and tendon samples from 6 young adult 
horses (2–4 years of age) and focused on gene expression patterns, DNA 
and collagen content of equine flexor digitorum superficialis tendon 
explants cultured in media containing PRP or other blood products. 
Tendon explants were cultured in blood, plasma, PRP, platelet-poor 
plasma (PPP), or bone marrow aspirate (BMA) at concentrations of 
100%, 50%, or 10% in serum-free DMEM with amino acids. After 
running the applicable biochemical assays, the researchers noted that 
the concentrations of TGF-β1 and PDGF-BB, in particular, were higher in 
PRP media compared to all other blood products tested. Additionally, 
tendinous tissue cultured in 100% PRP medium displayed enhanced 
gene expression of matrix proteins COL1A1, COL3A1, and COMP 
without increasing catabolic enzymes MMPs 3 and 13. At least in terms 
of tendinous structures, this in vivo study supported the use of autolo-
gous blood products, namely PRP, for the treatment of tendonitis in 
large mammals. 
Additional investigations conducted by Chen et al. [50] further 
explore the reconstructive effect of PRP. In their series of previous 
studies [45,51–53] the researchers demonstrated that in addition to 
enhanced chondrogenesis, PRP also promoted an increase in ECM syn-
thesis and the inhibition of inflammatory responses in articular cartilage 
and the nucleus pulposus, as examples. PRP is able to activate TGF-β1 
signaling via the phosphorylation of Smad2/3, therefore playing an 
important role in cell growth and differentiation [53]. Additionally, it 
has also been suggested that the fibrin clot that is formed upon activa-
tion of PRP provides a firm tridimensional structure (Fig. 2), enabling 
the adherence of cells which, in turn, may lead to the construction of 
new tissue [50]. 
Other researchers have made significant contributions in the field of 
dermatology regarding the treatment of chronic skin ulcers, which 
deserve just as much attention. The study conducted by Hesseler and 
Shyam in 2019 [2], for instance, demonstrate the value of PRP as a 
viable and effective treatment alternative for resistant chronic ulcers 
which still place a significant economic burden in health care. Diabetic 
foot ulcers, in particular, are well-known major health problems which 
make limbs susceptible to amputation [54]. A study published by Ahmed 
et al, in 2017 revealed that autologous PRP gel application can signifi-
cantly increase healing rates by stimulating acceleration of wound 
closure in patients with chronic diabetic foot ulcers via release of 
essential growth factors. Similarly, Gonchar and colleagues reviewed 
and discussed the regenerative potential of PRP and growth factor 
cocktails in terms of the improvement of diabetic foot ulcer treatment 
[55]. The researchers proposed that the use of growth factor cocktails 
may well be a possible solution which boosts the advantages of PRP and 
single growth factor use. Therefore, the combination of PRP with other 
therapeutic strategies may significantly enhance healing of chronic ul-
cers when compared to single growth factor use [55]. 
5.4. Fibrinolytic systems 
Platelets carry several factors pertaining to the fibrinolytic system, 
which may upregulate or downregulate fibrinolytic reactions. The 
temporal relationship and relative contributions of hematological 
components and the function of platelets in clot degradation are still a 
matter of great discussion in the community [56]. The literature pre-
sents many investigations solely focusing on platelets, which have long 
been known for their ability to influence the healing process. Despite 
numerous brilliant studies, other hematological components such as 
coagulation factors and the fibrinolytic system (Fig. 3) have also been 
found to be of significant contribution to efficient wound repair [57]. By 
definition, fibrinolysis is a complex biological process dependent on 
activation of certain enzymes in order to promote degradation of fibrin 
[57]. Fibrinolytic reactions have been previously appreciated by other 
authors [58] who proposed that fibrin degradation products (FDPs) may 
actually be the molecular agents responsible for stimulating tissue 
repair, preceding the sequence of vital biological events from fibrin 
deposition and removal all the way to angiogenesis, which is necessary 
for wound healing. 
The formation of a clot after injury serves as a shield to protect tis-
sues from blood loss, invasion of microbial agents, and also provides a 
provisional matrix (Fig. 2) through which cells can migrate during the 
repair process [59,60]. The clot consists of an aggregation of platelets 
embedded in a mesh of cross-linked fibrin fibers as a result of the 
cleavage of fibrinogen by serine proteases. This reaction initiates the 
polymerization of fibrin monomers, a primary event in blood clot for-
mation [57]. The clot may also serve as a reservoir of cytokines and 
growth factors which are released upon degranulation of activated 
platelets [57]. 
The fibrinolytic system (Fig. 3), which is tightly regulated by 
plasmin, exerts key roles in promoting cell migration, growth factor 
bioavailability and the regulation of other protease systems implicated 
in tissue inflammation and regeneration [61,62]. The key components 
present in fibrinolytic reactions, such as urokinase plasminogen acti-
vator receptor (uPAR) and plasminogen activator inhibitor-1 (PAI-1) are 
known to be expressed in mesenchymal stem cells (MSCs), specialized 
cell types necessary for successful wound healing [61]. 
5.4.1. Cell migration 
The activation of plasminogen via uPA – uPAR association is a pro-
cess that facilitates migration of inflammatory cells since it enhances 
pericellular proteolysis [63]. Due to the fact that uPAR lacks trans-
membrane and intra-cellular domains, this protein requires co-receptors 
such as integrins and vitronectin in order to regulate cell migration. 
Elaborating further, uPA – uPAR binding leads to an increase in affinity 
of uPAR for vitronectin and integrins, promoting cell adhesion [64]. 
Plasminogen activator inhibitor-1 (PAI-1) in turn detaches cells by 
R.G. dos Santos et al. 
Cytokine 144 (2021) 155560
5
disrupting the uPAR-vitronectin and integrin–vitronectin interactions 
when it binds uPA from the uPA–uPAR–integrin complexes on the cell 
surface [65]. 
In the regenerative medicine context, MSCs are known to be mobi-
lized from the bone marrow under conditions of severe organ injury and 
may therefore be found in circulation of patients with multiple bone 
fractures, for example. However, under specific scenarios such as end- 
stage kidney failure, end-stage liver failure, or during rejection 
episode after heart transplantation these cells may not be detected in 
blood [66]. Interestingly, these human bone marrow-derived mesen-
chymal (stromal) progenitor cells cannot be detected in the blood of 
healthy individuals [67]. 
A role for uPAR in MSC mobilization has also been previously put 
forward, similar to the occurrencein hematopoietic stem cell (HSC) 
mobilization. Vallabhaneni et al. demonstrated that the administration 
of granulocyte colony-stimulating factor in uPAR deficient mice caused 
failure in MSC mobilization [68], which once again reinforces the sup-
portive role of fibrinolytic systems in cell migration. Further research 
also shows that the glycosylphosphatidylinositol-anchored uPA receptor 
regulates adhesion, migration, proliferation, and differentiation by 
activating certain intracellular signaling pathways, as follows: the pro-
survival phosphatidylinositol4,5-bisphosphate 3-kinase/Akt and ERK1/ 
2 signaling, and the focal adhesion kinase (FAK) [69,70]. 
5.4.2. MSC behavior 
In the wound healing context, fibrinolytic factors have demonstrated 
further importance. Plasminogen deficient mice, for instance, exhibited 
a severe delay in wound healing events, suggesting the essential 
participation of plasmin in this process [71]. In humans, loss of plasmin 
also leads to complications in wound healing. Disrupted blood flow can 
significantly halter tissue regeneration, which would also explain why 
these regenerative processes are much more challenging in diabetic 
patients [72,73]. 
MSCs are recruited into the wound site with the purpose to accelerate 
the healing of wounds [74]. Under stable conditions these cells express 
uPA uPARand PAI-1 [75]. The last two proteins are hypoxia inducible 
factor α (HIF-1α) targets [76] which is quite convenient because the 
activation of HIF-1α in MSCs promotes the upregulation of FGF-2 and 
HGF; HIF-2 α in turn upregulates VEGF-A [77] which collectively 
contribute to wound healing as suggested by Table 1. Furthermore in a 
synergistic fashion HGF also appears to enhance the recruitment of 
MSCs into the wound site [78]. 
It must be noted that ischemic and hypoxic conditions have been 
shown to significantly interfere with wound repair. Despite the fact that 
MSCs tend to inhabit tissues which offer low oxygen levels, the in vivo 
survival of transplanted MSCs becomes limited because transplanted 
cells often die under such adverse conditions as observed in wounded 
tissues [79]. 
The fate of adhesion and survival of MSCs under hypoxic conditions 
is dependent on the secretion of fibrinolytic factors by these cells. PAI-1 
has higher affinity for vitronectin and therefore competes for uPAR and 
integrins binding to vitronectin, which inhibits cell adhesion, migration 
Fig. 1. Regenerative Effects of PRP. 
Fig. 2. The Microarchitecture of the Fibrin Clot (PRF). 
R.G. dos Santos et al. 
Cytokine 144 (2021) 155560
6
and proliferation [80]. The interaction between PAI-1 and PAs culmi-
nates in loss of PAI-1 affinity for vitronectin, allowing cell migration 
[61]. This dictates either adhesive or migratory effect on MSCs and also 
influences their survival. 
5.4.3. Mononuclear cell recruitment and regeneration system 
There is plenty of discussion about the role of mononuclear cells in 
wound healing according to the literature. Macrophages are mostly 
derived from blood monocytes and play an important role in regenera-
tive medicine [81]. These cells usually penetrate wound sites approxi-
mately 24–48 h following injury as a result of the secretion of IL-4, IL-1, 
IL-6 and TNF-α by neutrophils [82]. Platelets release thrombin and 
platelet factor 4 (PF4), two chemokines which promote recruitment of 
monocytes as well as their differentiation into macrophages and den-
dritic cells [83–85]. A notable feature of macrophages is their plasticity, 
which is their ability to switch phenotypes and also transdifferentiate 
into other cell types such as endothelial cells, subsequently displaying 
various functions in response to different biochemical stimuli in the 
wound microenvironment [86–88]. This inflammatory cell expresses 
two major phenotypes, either M1 or M2, depending on the local mo-
lecular signals as a source of stimuli. M1 macrophages are induced by 
microbial agents, therefore assuming a more pro-inflammatory role. The 
M2 macrophage, conversely, is generally produced by type 2 responses 
and takes on an anti-inflammatory property typically characterized by 
an increase in IL-4, IL-5, IL-9 and IL-13 [87,89]. It is also associated with 
tissue repair via the production of growth factors [57]. The transition 
from M1 into the M2 subtype is largely driven by the late stages of 
wound healing where M1 macrophages trigger apoptosis of neutrophils 
and initiate clearance of these cells [90]. The phagocytosis of neutro-
phils activates a sequence of events in which production of cytokines is 
switched off, allowing macrophages to undergo polarization and release 
TGF-β1. This growth factor is a key regulator of myofibroblast differ-
entiation and therefore wound contraction, which allows resolution of 
inflammation and initiation of the proliferative phase in the healing 
cascade [57]. Another highly relevant protein involved in cellular pro-
cesses is serglycin (SG). This hematopoietic cell secretory granule pro-
teoglycan has been found to be essential for the storage of specific 
secretory proteins in immune cells such as mast cells, neutrophils, and 
cytotoxic T lymphocytes [91]. Whilst many non-hematopoietic cells also 
synthesize serglycin, all inflammatory cells produce high amounts of this 
protein and store it in granules for further interaction with other in-
flammatory mediators, including proteases, cytokines, chemokines and 
growth factors [92]. The negatively charged glycosaminoglycan (GAG) 
chains in SG appear to be crucial for secretory granule homeostasis 
Fig. 3. Fibrinolytic Cascade. 
Table 1 
The biological role of growth factors present in PRP. 
Name Abbreviation Biological role 
Platelet-derived 
growth factor 
PDGF Increases expression of collagen, 
proliferation of bone cells, fibroblast 
chemotaxis and proliferative activity, 
macrophage activation. 
Vascular endothelial 
growth factor 
VEGF Triggers angiogenesis, chemotaxis of 
macrophages and neutrophils, migration 
and mitosis of endothelial cells, and 
increases permeability of blood vessels. 
Transforming growth 
factor-β 
TGF-β Augments production of collagen type 1, 
stimulates angionesis and chemotaxis of 
immune cells, inhibits osteoclast 
formation and bone resorption 
Epidermal growth 
factor 
EGF Stimulates proliferation and 
differentiation of epithelial cells, promotes 
secretion of cytokines by mesenchymal 
and epithelial cells. 
Insulin-like growth 
factor 
IGF Promotes cell growth and differentiation, 
stimulates collagen synthesis and recruits 
cells from bone, endothelium, epithelium 
and other tissues. 
Fibroblast growth 
factor 
FGF Promotes the growth, proliferation and 
differentiation of chondrocytes and 
osteoblasts, and stimulates proliferation of 
mesenchymal cells. 
Hepatocyte growth 
factor 
HGF Secreted by mesenchymal cells, HGF 
stimulates mitogenesis, cell motility, and 
matrix invasion. 
Fibroblast growth 
factor 
FGF Regulates cellular proliferation, survival, 
migration, and differentiation. 
Insulin-like growth 
factor-1 
IGF-1 By eliciting anabolic effects, this hormone 
plays a key role in cellular growth. 
R.G. dos Santos et al. 
Cytokine 144 (2021) 155560
7
because they can bind and promote storage of basically charged granule 
components in cell-, protein-, and GAG chain–specific manners [93]. In 
regards to their involvement in PRP, Woulfe and colleagues have pre-
viously shown that SG deficiency is well associated with morphological 
alterations in platelets; defects inplatelet factor 4, β-thromboglobulin 
and PDGF storage in platelets; poor platelet aggregation and secretion in 
vitro as well as defective thrombi formation in vivo. The researchers 
have therefore concluded that this proteoglycan appears to be a major 
regulator of thrombosis. 
6. Conclusion 
Platelet-rich products can be obtained via the collection and centri-
fugation of an individual’s whole blood, separating the mixture into 
distinct layers containing plasma, platelets, leukocytes and erythrocytes. 
An elevated concentration of platelets above the basal value enables 
accelerated growth of bone and soft tissues with minimal side effects. 
The application of autologous PRP products is a relatively new 
biotechnology undergoing expansion which continues to reveal opti-
mistic results in the stimulation and enhanced healing of various sorts of 
tissue injuries. 
The efficacy of this alternative therapeutic may be attributed to the 
local delivery of a broad range of growth factors and proteins, 
mimicking and supporting physiologic wound healing and reparative 
tissue processes. Furthermore, it is evident that the fibrinolytic system 
holds significant influence in tissue repair as a whole. In addition to 
being able to modify the cellular recruitment of inflammatory cells and 
MSCs, it also regulates proteolytic activity in areas of wound healing and 
regenerative processes of mesodermal tissues including bone, cartilage, 
and muscle, thus being a key component in musculoskeletal medicine, 
for example. 
Significant contributions have been made by many authors regarding 
PRP and its broad range of effects in the context of regenerative medi-
cine. Accelerated healing is a highly sought goal among many pro-
fessionals in the medical field and PRP represents an orthobiologic tool 
that continues to deliver promising developments in inciting and 
orchestrating the cascade of regenerative events. However, further 
studies are warranted as this therapeutic tool remains complex, espe-
cially due to the release of a myriad of bioactive factors and their various 
interactive mechanisms and signaling effects. 
Authors’ note 
The use of autologous PRP-derived components (fibrin, in this 
particular instance) may be an ideal biological tool in the application of 
regenerative medicine due to its relatively low cost and low risk as a 
supportive therapeutic agent in tissue healing. The fibrin clot offers a 
biological delivery of bioactive molecules and growth factors and 
functions as a natural bioscaffold, providing support for cell transplants. 
For these reasons, when combined with additional PRP components, it 
may enhance the regenerative effects because the fibrin matrix itself has 
a strong affinity for platelets. This allows platelets to degranulate and 
release their contents for an extended amount of time in the local wound 
microenvironment to effectively modulate inflammation and recruit 
more cell types, therefore facilitating the progression into the final 
stages of the healing cascade. 
Declaration of Competing Interest 
The authors declare that they have no known competing financial 
interests or personal relationships that could have appeared to influence 
the work reported in this paper. 
Appendix A 
See Figs. 1–3 and Table 1. 
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	The regenerative mechanisms of platelet-rich plasma: A review
	1 Introduction
	2 The evolution of PRP
	3 Obtaining PRP
	4 The healing cascade
	4.1 Hemostasis
	4.2 Inflammation
	4.3 Cellular and matrix proliferation
	4.4 Remodeling
	5 Enhancing wound healing
	5.1 Inflammatory modulation
	5.2 Cytokine profile
	5.3 Pro-anabolic stimuli: Reconstruction of damaged tissue
	5.4 Fibrinolytic systems
	5.4.1 Cell migration
	5.4.2 MSC behavior
	5.4.3 Mononuclear cell recruitment and regeneration system
	6 Conclusion
	Authors’ note
	Declaration of Competing Interest
	Appendix A Declaration of Competing Interest
	References