burn injury is a major cause of trauma to the human body , causing death and disability , with a long healing period and high health care costs . the mortality rate of burn injury is decreasing with new treatment modalities , although secondary infections and long healing periods still affect mortality rates . early debridement and skin grafting are successful , but insufficient graft donor area and unsuitable patient circumstances for surgery hinder skin grafting . in these circumstances , using products that increase the wound - healing process often affect patient morbidity and mortality . for this purpose , different kinds of dressings and pharmacotherapies have been developed , but most of them are costly , and the mechanisms underlying these therapies have not been fully documented . growth factors have been implicated in mediating the normal healing process as well as playing a role in impaired wound healing . certain growth factors , such as platelet - derived endothelial cell growth factor ( pdgf ) and insulin - like growth factor - i ( igf - i ) , inhibit the apoptosis pathways necessary for the rapid turnover of cells required to facilitate the different stages of physiologic wound healing . the wound healing effects of growth hormone ( gh ) appear to be directly and indirectly associated with igf-1 expression . poffenbarger and haberal reported that a glycoprotein named nonsuppressible insulin - like activity ( nsila ) exhibits insulin - like activity on fibroblasts in tissue and modulates the fibroblastic response in wound healing . platelet - rich plasma ( prp ) is a new adjunct that is increasingly used to treat soft and bony tissue defects to accelerate bone formation , especially in oral and maxillofacial bone reconstructive surgery , and in managing chronic nonhealing wounds [ 25 ] . platelet - rich plasma is produced from centrifuged blood combined with thrombin and calcium chloride as a viscous coagulum gel , which is a rich source of growth factors released by activated platelets . they release multiple growth factors and cytokines , including transforming growth factor- ( tgf-1 and 2 ) , transforming growth factor- ( tgf- ) , vascular endothelial growth factor ( vegf ) , platelet - derived endothelial cell growth factor ( pdgf ) , epidermal growth factor ( egf ) , platelet thromboplastin , thrombospondin , coagulation factors , fibroblast growth factor ( fgf ) , platelet activating growth factor-4 , calcium , serotonin , histamine , and hydrolytic enzymes with degranulation triggered by proteins such as thrombin . it has been demonstrated that platelet - derived growth factors are successful in treating nonhealing cutaneous human wounds . growth factors attract cells to the wound site ; they stimulate cell proliferation and increase differentiation and extracellular matrix deposition . transforming growth factor- is important in tissue repair , and it was demonstrated that tgf- has therapeutic value in the treatment of chronic , nonhealing wounds . platelet - derived endothelial cell growth factor improves dermal regeneration , promotes protein and collagen synthesis causing endothelial migration or angiogenesis , and increases expression of tgf- . transforming growth factor- and pdgf levels are higher in platelet - rich plasma compared with platelet - poor plasma ( ppp ) . using the effects of tgf- on wound healing ( e.g. , promoting differentiation and proliferation ) , we aimed to achieve enhanced wound healing and more - organized collagen in burn wounds treated with prp gel compared to controls , without excessive deposition of connective tissue or scar formation . although the positive effects of prp on graft maturation and wound healing are well known , a similar effect on burn healing is not clear . this study sought to investigate the effects of prp on burn wound healing in an experimental partial - thickness hot water burn injury . this study was performed at the experimental research center of baskent university with the permission of the experimental research ethical committee of baskent university faculty of medicine . all rats were brought to the experimental research center 1 week before the experiment and kept in special cages under controlled temperature ( 22c ) , humidity , and lighting conditions ( 12-hour day , 12-hour night ) . group 1 rats ( study group ; n=10 ) were exposed to prp application after burn injury ; group 2 rats ( control group ; n=10 ) were exposed to burn injury only ; and group 3 rats ( blood donor group ; n=10 ) were used as blood donors for the preparation of prp . all animals were anesthetized by an intraperitoneal injection of 50 mg / kg ketamine hydrochloride and 10 mg / kg xylazine hydrochloride . under sterile conditions , a 333 cm sponge ( vileda , procter & gamble company , cincinnati , oh ) was kept in boiling water and touched to the rats shaved dorsum for 35 seconds with its own weight to induce a partial - thickness burn injury . the burn site was covered with opsite drape ( smith & nephew , inc . , wound management , st . after experimentation , all rats were kept in special cages under controlled temperature , humidity , and lighting conditions . rats were given 0.002 g / kg fentanyl citrate twice a day for postoperative analgesia . all rats were killed after administration of anesthesia ( 60 mg / kg intraperitoneal ketamine ) on the seventh day after burn injury . platelet - rich plasma was derived after 2-step centrifugation of blood taken from the rats in group 3 . with the first soft , short spin ( 1500 rpm , 20c , 10 minutes ) , then , the plasma fraction was separated with a hard , long spin ( 2000 rpm , 20c , 15 minutes ) into the prp and the plasma - poor platelets . after this , the prp was combined with thrombin ( diathrombin , diamed , switzerland ) and calcium chloride ( calcium chloride , diamed , switzerland , 0.02 mol / l ) to activate platelets to produce sufficient clot formation that could be applied on the burn site . the mixture ratio was 1 ml : 0.1 ml : 1 ml ( prp - thrombin - ca chloride ) . after the rats were killed on the seventh day , the burn site was excised and tissue samples were sent to the biochemistry department for detection of tissue hydroxyproline levels and to the pathology department for histopathologic examinations . who was included in the study and blinded to all information regarding the groups performed the histopathologic examination of the tissues . tissue specimens were fixed in a 10% formaldehyde solution , and then embedded in paraffin . histopathologic examinations for wound healing were assessed by light microscope with hematoxylin - eosin staining , and collagen deposition was assessed by light microscopy with trichrome staining in 100 and 200 magnification areas . the wound healing process was assessed with a modified ehrlich - hunt numeric scale for polymorphonuclear leucocytes ( pmnl ) , fibroblasts , vessel proliferation , and collagen deposition as follows : 0 ( none ) , 1 ( focal ) , 2 ( mild ) , 3 ( moderate ) , and 4 ( significant ) . all animals were anesthetized by an intraperitoneal injection of 50 mg / kg ketamine hydrochloride and 10 mg / kg xylazine hydrochloride . under sterile conditions , a 333 cm sponge ( vileda , procter & gamble company , cincinnati , oh ) was kept in boiling water and touched to the rats shaved dorsum for 35 seconds with its own weight to induce a partial - thickness burn injury . the burn site was covered with opsite drape ( smith & nephew , inc . , wound management , st . after experimentation , all rats were kept in special cages under controlled temperature , humidity , and lighting conditions . rats were given 0.002 g / kg fentanyl citrate twice a day for postoperative analgesia . all rats were killed after administration of anesthesia ( 60 mg / kg intraperitoneal ketamine ) on the seventh day after burn injury . platelet - rich plasma was derived after 2-step centrifugation of blood taken from the rats in group 3 . with the first soft , short spin ( 1500 rpm , 20c , 10 minutes ) , then , the plasma fraction was separated with a hard , long spin ( 2000 rpm , 20c , 15 minutes ) into the prp and the plasma - poor platelets . after this , the prp was combined with thrombin ( diathrombin , diamed , switzerland ) and calcium chloride ( calcium chloride , diamed , switzerland , 0.02 mol / l ) to activate platelets to produce sufficient clot formation that could be applied on the burn site . the mixture ratio was 1 ml : 0.1 ml : 1 ml ( prp - thrombin - ca chloride ) . after the rats were killed on the seventh day , the burn site was excised and tissue samples were sent to the biochemistry department for detection of tissue hydroxyproline levels and to the pathology department for histopathologic examinations . who was included in the study and blinded to all information regarding the groups performed the histopathologic examination of the tissues . tissue specimens were fixed in a 10% formaldehyde solution , and then embedded in paraffin . histopathologic examinations for wound healing were assessed by light microscope with hematoxylin - eosin staining , and collagen deposition was assessed by light microscopy with trichrome staining in 100 and 200 magnification areas . the wound healing process was assessed with a modified ehrlich - hunt numeric scale for polymorphonuclear leucocytes ( pmnl ) , fibroblasts , vessel proliferation , and collagen deposition as follows : 0 ( none ) , 1 ( focal ) , 2 ( mild ) , 3 ( moderate ) , and 4 ( significant ) . the others were compared using the t test . values for p less than 0.05 were accepted as significant . two rats from each group died during the experiment , so the statistical analysis was performed with 8 rats in each group . in the study group , mean tissue hydroxyproline ( hyp ) level was detected as 0.950.35 g hyp / g , whereas in the control group mean tissue hyp level was detected as 0.690.08 g hyp / g . the hyp levels were significantly higher in the study group than in the control group ( p=.03 ) ( figure 1 ) . in both groups , partial - thickness burn injury statistically , except for the inflammatory cell ( pmnl ) infiltration , there was no significant difference between the groups ( figure 2 ) . in the study group , the mean pmnl score was 10.53 , whereas in the control group it was 2.3750.51 ( mean sem ) . therefore , inflammatory cell infiltration was statistically higher in the control group ( p=.005 ) . moderate inflammatory cell infiltration from the control group and mild inflammatory cell infiltration from the study group are shown in figures 3 and 4 . in the study group , the mean fibroblast score was 0.6250.51 , whereas in the control group it was 0.3750.51 ( mean sem ) . although the fibroblast score was higher in the study group , there was no significant difference between the groups ( p=.31 ) . focal fibroblast infiltration from the study group is shown in figure 5 . in the study group the mean vessel proliferation score was 0.750.7 , whereas in the control group it was 0.250.46 ( mean sem ) . vessel proliferation score was higher in the study group ; however , no significant difference was detected between the groups ( p=.26 ) . mild vessel proliferation from the study group is shown in figure 6 . in the study group , the mean collagen deposition score was 0.6250.51 , while in the control group it was 0.3750.51 ( mean sem ) . collagen deposition scores were higher in the study group but no significant difference was detected between the groups ( p=.31 ) . mild collagen deposition from the study group is shown in figure 7 . in the study group the epithelization rates were higher in the study group , but there was no significant difference between the groups ( p>.05 ) . in the study group , mean tissue hydroxyproline ( hyp ) level was detected as 0.950.35 g hyp / g , whereas in the control group mean tissue hyp level was detected as 0.690.08 g hyp / g . the hyp levels were significantly higher in the study group than in the control group ( p=.03 ) ( figure 1 ) . in both groups , partial - thickness burn injury was confirmed with macroscopic and microscopic examination . statistically , except for the inflammatory cell ( pmnl ) infiltration , there was no significant difference between the groups ( figure 2 ) . in the study group , the mean pmnl score was 10.53 , whereas in the control group it was 2.3750.51 ( mean sem ) . therefore , inflammatory cell infiltration was statistically higher in the control group ( p=.005 ) . moderate inflammatory cell infiltration from the control group and mild inflammatory cell infiltration from the study group are shown in figures 3 and 4 . in the study group , the mean fibroblast score was 0.6250.51 , whereas in the control group it was 0.3750.51 ( mean sem ) . although the fibroblast score was higher in the study group , there was no significant difference between the groups ( p=.31 ) . in the study group the mean vessel proliferation score was 0.750.7 , whereas in the control group it was 0.250.46 ( mean sem ) . vessel proliferation score was higher in the study group ; however , no significant difference was detected between the groups ( p=.26 ) . in the study group , the mean collagen deposition score was 0.6250.51 , while in the control group it was 0.3750.51 ( mean sem ) . collagen deposition scores were higher in the study group but no significant difference was detected between the groups ( p=.31 ) . in the study group the epithelization rate was 37.5% , while in the control group it was 25% . the epithelization rates were higher in the study group , but there was no significant difference between the groups ( p>.05 ) . patient age , burn depth , and size of burn area are the most - important factors that influence mortality and morbidity after thermal injuries . burn depth is also the most important factor affecting patients long - term appearance and functioning . malnutrition , immunosuppression , and extensive burn area provide a suitable environment for microorganisms ; such infections are the most important cause of mortality and morbidity . in laboratory studies it has been shown that topical administration of bone marrow cells increases dermal fibroblast population , vessel proliferation , and wound healing . for this reason , wound covering materials and artificial skin substitutes were developed and are used , but their indications are not yet clear . the first laboratory products used for this purpose were epithelial cells produced in a laboratory environment , but approximately 3 weeks are needed to produce the few thousand cm area required in extensive burns . desired properties of wound cover materials include forming a barrier against fluid loss and microorganisms , supporting increasing cells in wound healing , and allowing vessel proliferation , keratinocyte adhesion , and differentiation ; unfortunately , no such material currently exists . platelet - rich plasma contains a high level of platelets , clotting factors , cytokines , and growth factors . they start releasing these activated factors 10 minutes after clotting , and more than 95% of growth factors are released in the first hour . several in vivo and in vitro studies showed that all cells in the wound healing process are sensitive to the growth factors . epidermal growth factor is a chemotactic factor for fibroblasts , and topical application of egf increases epidermal regeneration and strength of wound tension . platelet - derived endothelial cell growth factor stimulates fibroblast and smooth muscle cell migration and proliferation , increases collagen deposition , and is also a chemotactic factor for neutrophils and monocytes . furthermore , it has been shown that chondrocyte , osteoblast , and periosteal cell proliferation is promoted by pdgf and bfgf . transforming growth factor-1 regulates cell differentiation , proliferation , chemotaxis , and synthesis of several extracellular matrix proteins . in animal models , it has been shown that topical application of tgf-1 increases collagen synthesis , granulation tissue , and strength of wound tension . transforming growth factor- triggers collagen synthesis and quickens maturation of collagen in the early period of wound healing . in addition , using tgf- and pdgf together increases collagen deposition more than using tgf- alone . it has been demonstrated that prp increases wound healing in acute trauma wounds , chronic nonhealing wounds , and incisional wounds , and is effective in soft and hard tissue reconstructions [ 2,3,5,6,1820 ] . it has been shown that the inflammatory phase is reduced , and prolonged inflammation ( which often leads to bacterial infections and scar formation ) does not exist in wounds treated with prp . . showed that using prp with acellular xenogeneic dermal matrix for treatment of deep second - degree burns decreased infection rate and increased wound healing . furthermore , an animal study and several case reports showed improved burn wound - healing time after the application of prp . a deep dermal burn could benefit from prp through its hemostatic antimicrobial abilities and the positive effects seen in wound healing . a recent review showed a significant benefit of platelet - rich plasma on wound healing in acute wounds and it improved long - term outcome of laser therapies . there are several studies suggesting that addition of prp to the graft site enhances wound healing , promoting epithelization and angiogenesis in split - thickness skin grafts and donor sites . . demonstrated that the viscoelastic properties of scars treated with the combination of split - thickness skin grafting ( stsg ) plus autologous platelet concentrate return more rapidly to the plateau state than areas treated with stsg only . in a recent study , prp increased the speed of repair of the extracellular matrix and its components in deep second - degree burn wounds in horses , and treatment with 2 applications of prp accelerated formation of extracellular matrix during the first half of wound healing . we have shown that in the study group , pmnl infiltration was significantly lower than in the control group , whereas there were no statistically significant differences between groups in terms of fibroblast infiltration , vessel proliferation , collagen deposition , or epithelization . the difference between the wound healing processes in the burn wounds versus regular burn wound healing may be the cause of this result . as opposed to regular wound healing , in burn wounds this hemorrhagic infill constructs a skeleton structure for cells coming into the wound site , including neutrophils , monocytes , fibroblasts , and endothelial cells . moreover , in burn wounds , blood supply is reduced because of the damaged vessels . at the burn site it is hard to define healed parts because of this lack of hemorrhage , which determines the borders of the wound . although in partial - thickness burn injuries cellular damage generally exists in the upper dermis , in the lower dermis the structural matrix damage is minimal . thus , structural matrix replacement is limited , and wound healing in partial - thickness wounds usually does not involve replacement of damaged structural matrix . further evidence supporting our conclusion is the statistically higher tissue hydroxyproline levels in the prp - treated group , which is an important indicator in wound healing . in the vertebrata , nearly all the hydroxyproline is present in collagen ; therefore , determining the hydroxyproline level is an adequate method for assessing collagen metabolism . hydroxyproline levels were statistically higher in the prp group than in the control group ( p=.03 ) . histopathologically , there was significantly less inflammatory cell infiltration ( p=.005 ) and there were no statistically significant differences between groups in fibroblast development , collagen production , vessel proliferations , or epithelization . platelet - rich plasma seems to partially improve burn wound healing in this experimental burn injury model . use of platelet - rich plasma can be an option for patients who have extensive burn areas with no chance for grafting to accelerate the burn healing process .