The Wound Healing Process

The Journal of International Medical Research2009; 37: 1528 – 1542 [first published online as 37(5) 12]

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The Wound Healing Process: an Overviewof the Cellular and Molecular

Mechanisms

T VELNAR1,2, T BAILEY2 AND V SMRKOLJ3

1Department of Neurosurgery, University Medical Centre Maribor, Maribor, Slovenia;2Cranfield Health, Cranfield University, Cranfield, UK; 3Clinical Department of

Traumatology, University Medical Centre Ljubljana, Ljubljana, Slovenia

Wound healing remains a challengingclinical problem and correct, efficientwound management is essential. Mucheffort has been focused on wound carewith an emphasis on new therapeuticapproaches and the development oftechnologies for acute and chronic woundmanagement. Wound healing involvesmultiple cell populations, theextracellular matrix and the action ofsoluble mediators such as growth factorsand cytokines. Although the process of

healing is continuous, it may bearbitrarily divided into four phases: (i)coagulation and haemostasis; (ii)inflammation; (iii) proliferation; and (iv)wound remodelling with scar tissueformation. The correct approach towound management may effectivelyinfluence the clinical outcome. Thisreview discusses wound classification, thephysiology of the wound healing processand the methods used in woundmanagement.

KEY WORDS: WOUNDS; WOUND HEALING; CELL PROLIFERATION; INFLAMMATION; WOUND REMODELLING

IntroductionIn everyday pathology, wounds remain achallenging clinical problem, with early andlate complications presenting a frequentcause of morbidity and mortality.1,2 In anattempt to reduce the wound burden, mucheffort has focused on understanding thephysiology of healing and wound care withan emphasis on new therapeutic approachesand the continuing development oftechnologies for acute and long-term woundmanagement.3,4

The immense social and economic impactof wounds worldwide is a consequence of

their high rate of occurrence in general andtheir increasing frequency in the ageingpopulation. In addition to a high number ofacute wounds, there are also a large numberof chronic, hard-to-heal wounds associatedwith diseases and abnormalities that directlyor indirectly culminate in damage of thecutaneous coverage, including arterial,venous, diabetic and pressure ulcers. Theprevalence of these chronic wounds increaseswith age. For example, it has been estimatedthat chronic wounds affect 120 per 100 000people aged between 45 and 65 years andrises to 800 per 100 000 people > 75 years of

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T Velnar, T Bailey, V SmrkoljAn overview of the wound healing process

age.1,3 Furthermore, due to the complicationsthat accompany acute wounds, when theirhealing does not progress in a timely andorderly manner, they can convert intochronic wounds, which are more difficult tomanage.1

Wounds and wound typesA wound is defined as damage or disruptionto the normal anatomical structure andfunction.3 This can range from a simplebreak in the epithelial integrity of the skin orit can be deeper, extending intosubcutaneous tissue with damage to otherstructures such as tendons, muscles, vessels,nerves, parenchymal organs and evenbone.2

Wounds can arise from pathologicalprocesses that begin externally or internallywithin the involved organ. They can have anaccidental or intentional aetiology or theycan be the result of a disease process.Wounding, irrespective of the cause andwhatever the form, damages the tissue anddisrupts the local environment within it. Aphysiological response to the noxious factorresults in bleeding, vessel contraction withcoagulation, activation of complement andan inflammatory response.5 – 7 Normalwound healing is a dynamic and complexprocess involving a series of co-ordinatedevents, including bleeding, coagulation,initiation of an acute inflammatory responseto the initial injury, regeneration, migrationand proliferation of connective tissue andparenchyma cells, as well as synthesis ofextracellular matrix proteins, remodelling ofnew parenchyma and connective tissue andcollagen deposition.8,9 Finally, increasing thewound strength takes place in an orderedmanner and culminates in the repair ofsevered tissues.8 – 10

Wound healing begins at the moment ofinjury and involves both resident and

migratory cell populations, extracellularmatrix and the action of soluble mediators.The mechanisms underlying the processesdescribed above involve: (i) inflammatorymediators and growth factors; (ii) cell–celland cell–extracellular matrix interactionsthat govern cell proliferation, migration anddifferentiation; (iii) events involved withepithelialization, fibroplasia andangiogenesis; (iv) wound contraction; and(v) remodelling. These mechanisms areinitiated at the time of physical injury andproceed continuously throughout the repairprocess.3,8,11 – 14

Despite the fact that the processes ofrepair begin immediately after an injury inall tissues and that all wounds go throughsimilar phases of healing, specialized tissuessuch as liver, skeletal tissue and the eye havedistinctive forms of regeneration and repairand follow separate pathways.14 – 17

Additionally, there are differences betweentissues in terms of the time required tocomplete regeneration. Wound healing timecan be diverse and some wounds may takeup to a year or more to heal completely.18 – 20

A completely healed wound is defined as onethat has been returned to a normalanatomical structure, function andappearance of the tissue within a reasonableperiod of time. Most wounds are usually theresult of simple injuries; however, somewounds do not heal in a timely and orderlymanner. Multiple systemic and local factorsmay slow the course of wound healing bycausing disturbances in the finely balancedrepair processes, resulting in chronic, non-healing wounds.3,10

Classification of woundsWounds can be classified according tovarious criteria.3 Time is an important factorin injury management and wound repair.Thus, wounds can be clinically categorized

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as acute and chronic according to their timeframe of healing.3,5,6

ACUTE WOUNDSWounds that repair themselves and thatproceed normally by following a timely andorderly healing pathway, with the end resultof both functional and anatomicalrestoration, are classified as acute wounds.The time course of healing usually rangesfrom 5 to 10 days, or within 30 days. Acutewounds can be acquired as a result oftraumatic loss of tissue or a surgicalprocedure.3,5 For example, an operation toremove a soft tissue tumour located in theskin and underlying parenchyma cansometimes result in a large albeit non-contaminated wound that cannot be healedby primary intention, due to the large defectwithin the tissue. Traumatic wounds are alsofrequently encountered. They may involveonly the soft tissue or they might beassociated with bone fractures. Thesecombined injuries have been classified bythe classification system of the AOFoundation (Arbeitsgemeinschaft fuerOsteosynthesefragen/Association for theStudy of Internal Fixation),21 which is one ofthe most comprehensive and widely used.Included within this classification system areclosed and open fractures with theassessment of skin, muscle, tendon andneurovascular injuries.21 A benefit of the AOFoundation’s classification system is that theextent of damage to muscles and tendons istaken into account, as it determines theprognosis of the injured limb.6 – 8

CHRONIC WOUNDSChronic wounds are those that fail toprogress through the normal stages ofhealing and they cannot be repaired in anorderly and timely manner.3,4 The healingprocess is incomplete and disturbed by

various factors, which prolong one or morestages in the phases of haemostasis,inflammation, proliferation or remodelling.These factors include infection, tissuehypoxia, necrosis, exudate and excess levelsof inflammatory cytokines.22 A continuousstate of inflammation in the wound creates acascade of tissue responses that togetherperpetuate a non-healing state. Because thehealing then proceeds in an unco-ordinatedmanner, functional and anatomicaloutcomes are poor and these woundsfrequently relapse.3,23 Chronic wounds mayresult from various causes, includingnaturopathic, pressure, arterial and venousinsufficiency, burns and vasculitis.20,23

COMPLICATED WOUNDSA complicated wound is a special entity andis defined as a combination of an infectionand a tissue defect.6 Infection poses aconstant threat to the wound. The cause ofthe defect, in contrast, evolves due to thetraumatic or post-infectious aetiology, or awide tissue resection (e.g. in tumourmanagement). Every wound iscontaminated irrespective of the cause, size,location and management. Whether or not amanifest infection develops depends on thevirulence, number and type of micro-organisms, as well as on the local bloodsupply and the patient’s inherent resistance.Typical characteristics of infection are thefive signs and symptoms that have been welldocumented: redness, heat, pain, oedemaand loss or limited function in the affectedpart. The frequency of wound infectionsdepends on the type or surgical techniqueand the location of the wound.6,23,24

Other criteria taken into account duringwound classification include aetiology,degree of contamination, morphologicalcharacteristics and communication withhollow or solid organs.3,6,20 Aetiology

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classifies wounds according to the triggerfactor into contusions, abrasions, avulsions,lacerations, cuts, stab wounds, crushwounds, shot wounds and burns.3,5,6,11

According to the degree of contamination,wounds are classified into three groups asfollows: (i) aseptic wounds (bone and jointoperations); (ii) contaminated wounds(abdominal and lung operations); and (iii)septic wounds (abscesses, bowel operations,etc).11,20,22 Wounds may also be referred to asclosed, where the underlying tissue has beentraumatized but the skin has not beensevered, or as open, where the skin layer hasbeen damaged with the underlying tissueexposed.6,20,23,24

The wound healing processWounding and wound healing take place inall tissues and organs of the body. Many ofthese repair processes are common to alltissues. Although the process of healing iscontinuous, it is arbitrarily divided intodifferent phases in order to aidunderstanding of the physiological processesthat are taking place in the wound andsurrounding tissue.19 Healing is a complexprocess involving co-ordinated interactionsbetween diverse immunological andbiological systems. It involves a cascade ofcarefully and precisely regulated steps andevents that correlate with the appearance ofvarious cell types in the wound bed duringdistinct phases of the healing process.24 – 27

Separate parts of a wound may be atdifferent stages of healing at any onetime.6,19,20,25 Timing and interactionsbetween the components taking part in thewound healing process differ for acute andchronic wounds, although the main phasesremain the same.3,27,28 The various processesof acute tissue repair, which are triggered bytissue injury, may be united into a sequenceof four time-dependent phases: (i)

coagulation and haemostasis, beginningimmediately after injury; (ii) inflammation,which begins shortly thereafter; (iii)proliferation, which starts within days of theinjury and encompasses the major healingprocesses; and (iv) wound remodelling, inwhich scar tissue formation takes place, andwhich may last up to a year or more.22 – 26

COAGULATION AND HAEMOSTASISPHASEImmediately after injury, coagulation andhaemostasis take place in the wound.11 – 13

The principal aim of these mechanisms is toprevent exsanguination. It is a way toprotect the vascular system, keeping it intact,so that the function of the vital organsremains unharmed despite the injury. Asecond aim is a long-term one, which is toprovide a matrix for invading cells that areneeded in the later phases of healing.3,12 – 14

A dynamic balance between endothelialcells, thrombocytes, coagulation, andfibrinolysis regulates haemostasis anddetermines the amount of fibrin deposited atthe wound site, thereby influencing theprogress of the reparative processes.13,14

Noxious insult causes microvascularinjury and extravasation of blood into thewound.10,11,13 Owing to the neuronal reflexmechanism, injured vessels constrict rapidlydue to contraction of vascular smoothmuscle cells in the circular muscle layer. Thecontraction is strong enough to preventbleeding from an arteriole with a diameter of0.5 cm. The process is, however, onlyeffective in transversally interrupted vesselsand may cause complete cessation of bloodleakage. In contrast, in longitudinallysevered arterioles it increases the gap.13,14

Reflex vasoconstriction can temporarilyreduce or even stop the amount of bleeding.The vascular smooth muscle tone is,however, only useful for a few minutes until

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hypoxia and acidosis in the wound wallcause their passive relaxation, and bleedingresumes. Were it not for the formation of aninsoluble fibrin plug, the haemostaticmechanisms alone would be ineffective overthe longer term.3,10

Together with haemostatic events, thecoagulation cascade is activated throughextrinsic and intrinsic pathways, leading toplatelet aggregation and clot formation inorder to limit blood loss.12,13 As blood spillsinto the site of injury, the blood componentsand platelets come in contact with exposedcollagen and other extracellular matrixcomponents. This contact triggers the releaseof clotting factors from the platelets and theformation of a blood clot, composed offibronectin, fibrin, vitronectin andthrombospondin.3,13,14,17 The blood clot andplatelets trapped within it are not onlyimportant for haemostasis, as the clot alsoprovides a provisional matrix for cellmigration in the subsequent phases of thehaemostatic and inflammatory phases. Thecytoplasm of platelets contains α-granulesfilled with growth factors and cytokines, suchas platelet derived growth factor (PDGF),transforming growth factor-β (TGF-β),epidermal growth factor and insulin-likegrowth factors.14 These molecules act aspromoters in the wound healing cascade byactivating and attracting neutrophils and,later, macrophages, endothelial cells andfibroblasts.11,14 Platelets also containvasoactive amines, such as serotonin, thatare stored in dense bodies and causevasodilation and increased vascularpermeability, leading to fluid extravasationin the tissue that results in oedema which, inturn, potentiates itself during the followinginflammatory phase.14,19 Eicosanoids andother products of arachidonic acidmetabolism are released after injury to cellmembranes and have potent biological

functions in the immediate inflammatoryresponse.3,10,14,15

INFLAMMATORY PHASEThe humoral and cellular inflammatoryphase follows next, with the aim ofestablishing an immune barrier againstinvading micro-organisms. It is divided intotwo separate phases, an early inflammatoryphase and a late inflammatory phase.16

Early inflammatory phaseStarting during the late phase of coagulationand shortly thereafter, the earlyinflammatory response has many functions.It activates the complement cascade andinitiates molecular events, leading toinfiltration of the wound site by neutrophils,whose main function is to preventinfection.11 The neutrophils start with thecritical task of phagocytosis in order todestroy and remove bacteria, foreignparticles and damaged tissue. Phagocytoticactivity is crucial for the subsequentprocesses, because acute wounds that have abacterial imbalance will not heal.3,7,16

The neutrophils begin to be attracted tothe wound site within 24 – 36 h of injury byvarious chemoattractive agents, includingTGF-β, complement components such as C3aand C5a, and formylmethionyl peptidesproduced by bacteria and platelet products.3

Due to alterations in the regulation of surfaceadhesion molecules, neutrophils becomesticky and, through a process of margination,begin to adhere to the endothelial cells in thepost-capillary venules surrounding thewound.14,16 Then, the neutrophils roll alongthe surface of the endothelium being pushedforward by the blood flow. These adhesionsand rolling mechanisms are mediated byselectin-dependent interactions and areclassified as weak attachments.16,17

Chemokines secreted by endothelial cells

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rapidly activate a stronger adhesion system,which is mediated by integrins.17,18 Cells stoprolling and migrate out of the venules,squeezing between the endothelial cells by aprocess known as diapedesis.16 – 18 Thesubsequent migration now depends onchemokines and other chemotactic agents.Once in the wound environment, neutrophilsphagocytose foreign material and bacteria,destroying them by releasing proteolyticenzymes and oxygen-derived free radicalspecies.17 – 19,25

Neutrophil activity gradually changeswithin a few days of wounding, once all thecontaminating bacteria have beenremoved.16,17 Upon completing the task, theneutrophils must be eliminated from thewound prior to progression to the next phaseof healing. Redundant cells are disposed ofby extrusion to the wound surface as sloughand by apoptosis, allowing elimination ofthe entire neutrophil population withouttissue damage or potentiating theinflammatory response.16,26 The cellremnants and apoptotic bodies are thenphagocytosed by macrophages.3,25 – 28

Late inflammatory phaseAs part of the late inflammatory phase, 48 –72 h after injury, macrophages appear in thewound and continue the process ofphagocytosis.16,29 – 34 These cells areoriginally blood monocytes that undergophenotypic changes on arrival into thewound to become tissue macrophages.Attracted to the wound site by a myriad ofchemoattractive agents, including clottingfactors, complement components, cytokinessuch as PDGF, TGF-β, leukotriene B4 andplatelet factor IV, as well as elastin andcollagen breakdown products, macrophageshave a longer lifespan than neutrophils andcontinue to work at a lower pH.35,36 Thesecells are fundamental for the late stages of

the inflammatory response, acting as keyregulatory cells and providing an abundantreservoir of potent tissue growth factors,particularly TGF-β, as well as othermediators (TGF-α, heparin bindingepidermal growth factor, fibroblast growthfactor [FGF], collagenase), activatingkeratinocytes, fibroblasts and endothelialcells.25 – 28,33 – 35 Clearly the depletion ofmonocytes and macrophages from thewound causes severe healing disturbancesdue to poor wound debridement, delayedfibroblast proliferation and maturation, aswell as delayed angiogenesis, resulting ininadequate fibrosis and a more weaklyrepaired wound.17,31 – 33

The last cells to enter the wound site in thelate inflammatory phase are lymphocytes,attracted 72 h after injury by the action ofinterleukin-1 (IL-1), complement componentsand immunoglobulin G (IgG) breakdownproducts.16,25,26 The IL-1 plays an importantrole in collagenase regulation, which is laterneeded for collagen remodelling, productionof extracellular matrix components andtheir degradation.16,29,30

PROLIFERATIVE PHASEWhen ongoing injury has ceased,haemostasis has been achieved and animmune response successfully set in place, theacute wound shifts toward tissue repair.16 – 19

The proliferative phase starts on the thirdday after wounding and lasts for about 2weeks thereafter. It is characterized byfibroblast migration and deposition of newlysynthesized extracellular matrix, acting as areplacement for the provisional networkcomposed of fibrin and fibronectin. At themacroscopic level, this phase of woundhealing can be seen as an abundantformation of granulation tissue. The diverseprocesses that take place in the proliferativephase are briefly discussed below.27 – 29

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Fibroblast migration

Following injury, fibroblasts andmyofibroblasts in the surrounding tissue arestimulated to proliferate for the first 3 days.33

They then migrate into the wound, beingattracted by factors such as TGF-β and PDGF,that are released by inflammatory cells andplatelets.37 Fibroblasts first appear in thewound on the third day after injury and theiraccumulation requires phenotypicmodulation. Once in the wound, theyproliferate profusely and produce the matrixproteins hyaluronan, fibronectin,proteoglycans and type 1 and type 3procollagen. All of their products aredeposited in the local milieu.3,33,35

By the end of the first week, abundantextracellular matrix accumulates, whichfurther supports cell migration and isessential for the repair process.30,35 Now,fibroblasts change to their myofibroblastphenotype. At this stage, they contain thickactin bundles below the plasma membraneand actively extend pseudopodia, attachingto fibronectin and collagen in theextracellular matrix.35,37 Wound contraction,which is an important event in thereparative process that helps to approximatethe wound edges, then takes place as thesecell extensions retract. Having accomplishedthis task, redundant fibroblasts areeliminated by apoptosis.38 – 40

Collagen synthesisCollagens are an important component inall phases of wound healing. Synthesized byfibroblasts, they impart integrity andstrength to all tissues and play a key role,especially in the proliferative andremodelling phases of repair.39 – 41 Collagensact as a foundation for the intracellularmatrix formation within the wound.Unwounded dermis contains 80% type 1 and25% type 3 collagen, whereas wound

granulation tissue expresses 40% type 3collagen.3

Angiogenesis and granulation tissueformationModelling and establishment of new bloodvessels is critical in wound healing and takesplace concurrently during all phases of thereparative process. In addition to attractingneutrophils and macrophages, numerousangiogenic factors secreted during thehaemostatic phase promote angio-genesis.38,42,43

Resident endothelial cells are responsiveto a number of angiogenic factors, includingFGF, vascular endothelial growth factor(VEGF), PDGF, angiogenin, TGF-α and TGF-β.A fine balance is kept by the action ofinhibitory factors, such as angiostatin andsteroids.44 – 48 Inhibitory and stimulatoryagents act on proliferating endothelial cellsdirectly as well as indirectly, by activatingmitosis, promoting locomotion and bystimulating the host cells to releaseendothelial growth factors.49,50 Underhypoxic conditions, molecules are secretedfrom the surrounding tissue, promotingproliferation and growth of endothelial cells.In response, a four-step process takes place:(i) production of proteases by endothelialcells for degradation of the basal lamina inthe parent vessel in order to crawl throughthe extracellular matrix; (ii) chemotaxis; (iii)proliferation; and (iv) remodelling anddifferentiation. FGF and VEGF play centralregulatory roles in all of theprocesses.43,45,46,49 – 51 Initially, there is novascular supply in the wound centre, soviable tissue, which is limited to the woundmargins, is perfused by uninjured vesselsand by diffusion through undamagedinterstitium.3,26,52 Capillary sprouts from thesurrounding edges invade the wound clotand, within a few days, a microvascular

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network composed of many new capillariesis formed.

Chemotaxis is the ability of cells to movealong a chemical gradient.53 This biochemicalmechanism enables cells to reply properly toenvironmental stimuli that determineproliferation, differentiation and migration.Chemotactic agents act on cell surfacereceptors to direct the cell migration that isinvolved in angiogenesis during woundhealing.54,55 The contributing factors act asmediators for neovascularization and vesselrepair at the injury site. They are alsoimportant modulators of cell growth anddifferentiation, and they include endothelialcell growth factor, TGF-α, VEGF, angiopoietin-1, fibrin and lipid growth factors.56 – 58

Migration is the consequence of chemotacticactivity and is necessary for angiogenesis.59,60

As a complex process that involves co-ordinated changes in cytoskeletalorganization, signal transduction and celladhesion, migration is dependent on theactin-rich network beneath the plasmamembrane and is regulated by physical andchemical factors in the vascular system.59

Regulation is achieved by three mechanisms:(i) chemotaxis (migration towards theconcentration gradient of the chemoattractivesubstance); (ii) mechanotaxis (migrationinduced by mechanical forces); and (iii)haptotaxis (migration in response to agradient of immobilized ligands).59,60

Cellular motility requires three distinctactions: protrusion at the cell front; adhesion,to attach the actin cytoskeleton to thesubstratum; and finally traction, propellingthe trailing cytoplasm forward.61 – 63

ProtrusionWith three types of interconnected filaments,the cytoskeleton is anchored at cell–celljunctions and cell–extracellular matrixadhesions, providing mechanical support for

the cell.59 The actin network is well known forits dynamic reorganization, it acts as amechano-effector and it is important for co-ordinating cell migration. During the first stepof locomotion, actin polymerization takesplace at the leading edge, determined by thehighest concentration of chemoattractivesubstance, pushing the plasma membraneoutward. A protruding structure forms, in thecase of the endothelial cell these are known asfilopodia, and they are filled with filamentousactin.64 Unidirectional movement of the cell ismaintained through the action of a cyclicassembly and disassembly of actin filamentsin front of and well behind the leading edge,respectively.63,65 Multiple signalling pathwaysand regulatory proteins control actindynamics and the changes of cellmorphology.59,62,66

AdhesionAdhesion to a solid substratum is aparticularly important step in cellmigration.14,65,67 It is mediated by integrins,which act as the primary receptors forextracellular matrix proteins and areconsequently required for cell motility.61,68 Inaddition, these molecules are also involvedin signal transduction, and in regulatingand stimulating migration.68,69

Adhesion and migration are inverselyproportional; an optimal rate of migration isachieved with increasing adhesion, butmobility is reduced with furtherattachment.61,68 Endothelial cells can adjusttheir adhesion intensity, with weaklyadhesive cells moving faster than highlyadhesive ones. After attachment to theextracellular matrix, the cell changes itsmorphology from an oval- or spindle-shapeto an irregular, flattened one. Thesealterations in shape are governed by integrinsignalling and depend on integrin contactswith the extracellular matrix in focal

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complexes, forming initially at the ends ofthe filopodia.61,67,68 Endothelial cells migratefastest immediately after injury, then theyenter a slower migration rate, which ismaintained during the healing process.66,70

TractionContractile forces, transmitted through theintegrin–cytoskeletal connections, allow thecell to pull the cytoplasm forward bygenerating traction to the substratum.59 Theforce for movement is provided by myosinmotor proteins, linked to contractile actinbundles along the cell. Interactions betweenmyosin and actin fibres pull the cell bodyforwards. At the same time, the extracellularmatrix-binding proteins on the trailing edgeof the moving cell must release theirconnections.67,68

The degree of strength of the integrincoupling to the cytoskeleton is influenced bythe rigidity of the substratum. With strongercouplings to a firm surface, force can betransmitted through the migrating cell moreefficiently.68 During locomotion, the tractionforces generated at the sites of contact can behigh enough to deform the extracellularmatrix and to rearrange it significantly.53 – 58

The direction of migration requires initialpolarization of the cell and both physicaland chemical stimuli influence it, as hasbeen discussed above.44,71 – 75

Macrophages, proliferating fibroblastsand vascularized stroma, together withcollagen matrix, fibrinogen, fibronectin andhyaluronic acid, constitute the acutegranulation tissue that replaces the fibrin-based provisional matrix.33,39 With collagenaccumulation, the density of the bloodvessels diminishes and the granulation tissuegradually matures to produce a scar.51,64,76

EpithelializationMigration of epithelial cells starts from the

wound edges within a few hours ofwounding. A single layer of cells initiallyforms over the defect, accompanied by amarked increase in epithelial cell mitoticactivity around the wound edges. Cellsmigrating across them attach to theprovisional matrix below. When theadvancing epithelial cells meet, migrationstops and the basement membrane starts toform.26,28,39

REMODELLING PHASEAs the final phase of wound healing, theremodelling phase is responsible for thedevelopment of new epithelium and finalscar tissue formation. Synthesis of theextracellular matrix in the proliferative andremodelling phases is initiatedcontemporarily with granulation tissuedevelopment. This phase may last up to 1 or2 years, or sometimes for an even moreprolonged period of time.33,35 Theremodelling of an acute wound is tightlycontrolled by regulatory mechanisms withthe aim of maintaining a delicate balancebetween degradation and synthesis, leadingto normal healing. Along with intracellularmatrix maturation, collagen bundlesincrease in diameter and hyaluronic acidand fibronectin are degraded.29,39 The tensilestrength of the wound increases progressivelyin parallel with collagen collection.39,41

Collagen fibres may regain approximately80% of the original strength compared withunwounded tissue. The acquired finalstrength depends on the localization of therepair and its duration, but the originalstrength of the tissue can never beregained.3,41

Synthesis and breakdown of collagen aswell as extracellular matrix remodelling takeplace continuously and both tend toequilibrate to a steady state about 3 weeksafter injury.30,41 Matrix metalloproteinase

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enzymes, produced by neutrophils,macrophages and fibroblasts in the wound,are responsible for the degradation ofcollagen. Their activity is tightly regulatedand synchronized by inhibitory factors.Gradually, the activity of tissue inhibitors ofmetalloproteinases increases, culminating ina drop in activity of metalloproteinaseenzymes, thereby promoting new matrixaccumulation.15,41,77

Although the initial deposition ofcollagen bundles is highly disorganized, thenew collagen matrix becomes more orientedand cross-linked over time. Its subsequentorganization is achieved during the finalstages of the remodelling phase, to a greaterextent by the wound contraction that hasalready begun in the proliferative phase. Theunderlying connective tissue shrinks in sizeand brings the wound margins closertogether, owing to fibroblast interactionswith the extracellular matrix. The process isregulated by a number of factors, with PDGF,TGF-β and FGF being the most important.41,42

As the wound heals, the density of fibroblastsand macrophages is further reduced byapoptosis.40,76 With time, the growth ofcapillaries stops, blood flow to the areadeclines and metabolic activity at the woundsite decreases.39,41,78 The end result is a fullymatured scar with a decreased number ofcells and blood vessels and a high tensilestrength.78 – 80

HEALING BY PRIMARY ANDSECONDARY INTENTIONSuccessful wound healing depends on thetimely and optimal functioning of manydiverse processes, cell types, molecularmediators and structural elements. Differentcells dominate various phases of the repair,and cellular patterns vary according to thedifferent types of injury and the extent oftissue damage.3,81 – 85 During normal wound

healing, closed incisions and open woundswith tissue defects progress through a seriesof co-ordinated molecular and cellularevents, resulting either in regeneration ortissue repair. The least complicated exampleof wound repair is the healing of cleanwounds without loss of tissue and uninfectedsurgical incisions approximated by sutures.This is referred to as healing by primaryintention.8,24,40 Death of a limited number ofepithelial and connective tissue cells occursbecause of small disruptions in basementmembrane continuity. It is a fast process andcontrasts markedly with the healing of anopen wound with extensive loss of tissue.8,10

Here, the reparative process is morecomplicated because large tissue losses haveto be filled, which occurs during healing bysecondary intention. In comparison tohealing by first intention, this process takeslonger and large amounts of granulationtissue are formed in order to fill the tissuedefect.3,9,33,52,84

Numerous pathophysiological andmetabolic factors can affect wound healingand result in a poor outcome.26 They includelocal causes, such as oedema, ischaemia,tissue hypoxia, infection, necrosis andgrowth factor imbalance, as well as systemiccauses including metabolic disease,nutritional status and general perfusiondisturbances or pre-existing illness. Thesefactors act by altering the wound repairenvironment, impeding healing and turningthe acute wound into a chronic one. Allprocesses, from wound closure time,inflammatory cell influx and fibroblastmigration, to collagen and extracellularmatrix deposition are delayed in thissituation.22,26,29 – 31,84

Management of woundsThe correct approach to treating woundsshould effectively assist the healing process

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and it can have an important impact on thefinal clinical outcome. Physiological,endocrine and nutritional support at aclinical level significantly influence repairand, without them, wound healing oftenfails completely.6,24,29

The first stage of wound managementshould be a thorough assessment of thewound and the patient. The process beginswith a diagnosis of the wound’s aetiology andcontinues with optimizing the patient’smedical condition, particularly blood flow tothe wound area.26,85,86 An acute wound in astable patient with normal blood flow shouldheal successfully if appropriate care is given.Guidelines of the medical procedures to useduring wound management have beenextensively described elsewhere.6,10,20,23,25,28 – 30

The wound will need to be debrided anddressed correctly.29,33,46 Sufficient debride-ment, defined as the removal of non-viable,infected and hyperkeratotic tissue, forms thebasis of non-delayed as well as delayedwound healing. Debridement is essential asit accelerates wound healing and differenttechniques exist.26,86 In chronic wounds, themeasures used to reverse medicalabnormalities are complex and the aetiologyof the wound is not easy to identify. Correctdebridement helps to convert a chronicwound into an acute one, which can thenprogress through the normal stages ofhealing.40,77,86,87

The accumulation of devitalized tissue inthe wound promotes bacterial colonizationand impairs the body’s ability to fightinfection, thereby preventing completerepair of the wound.3 The aim ofdebridement is to remove ischaemic andnecrotic tissue, which presents potential forinfection and contamination of the tissue bybacteria and foreign bodies. During theoperation, necrotic and vital tissues aredistinguished by a lack of capillary refill,

colour and clear demarcation.84 – 86 Deadmuscle fibres do not contract on stimulationand are poorly perfused. Special tissues, suchas tendons and fascia, are not removeddespite not being vital because they promotethe healing process in the wound; a surgeondoes so only in case of severecontamination.88,89

The next important step is the lavage ofmicro-organisms, dead tissue and foreignbodies, which can further decrease tissuebacterial counts. Commonly, a bacitracinsolution is used. In contrast, Patzakis89

recommends irrigation with large quantitiesof saline. Low pressure irrigation onlyremoves contamination on the surface.Although high pressure irrigation reducesbacterial colonization of the wound and thefrequency of infections, a high pressure jetcan damage fine tissues and push dirtparticles deeper into the wound or even intothe bone. High pressure irrigation also causesthe soaking of wound margins with liquid,reducing the ability of the wound to resistinfection. High pressure irrigation must beemployed cautiously and is onlyrecommended for very contaminatedwounds.88 Necrectomy and irrigation tend tocomplement each other. When it is notpossible to irrigate all of the bacteria and dirtfrom the wound or the infection has spread,necrectomy removes contaminated anddevitalized tissue, which would otherwiseweaken the wound defence mechanisms.88 – 90

Novel management optionsfor woundsVarious medical approaches and therapeuticinterventions can affect the differentprocesses involved in the wound healingcascade. Following wounding, the healingtime may be shorter when there is less injuredtissue, for example during minimallyinvasive surgery, which reduces the amount

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• Received for publication 14 April 2009 • Accepted subject to revision 30 April 2009• Revised accepted 24 August 2009

Copyright © 2009 Field House Publishing LLP

of soft tissue damage and post-operativemorbidity. Novel techniques of topical growthfactor application and incisional primingwith PDGF or IL-1 can optimize both thecellular and molecular environment, thusdecreasing healing time by modifyinginflammation and accelerating theproliferative phase.91,92 Electrical fieldstimulation may optimize the remodellingphase by promoting more efficient fibroblastrecruitment and collagen deposition,93 – 96

prosthetic materials can favour tissuerepair,4,97 and gene therapy,98 which iscurrently in pre-clinical development, may beable to provide a way for selective healing.

ConclusionWounds occupy a remarkable place in

everyday pathology. They are of diverseaetiology and different classification criteriaexist. Any wounding damages the tissue andaffects the local environment. The host’sresponse to wounding involves variousprocesses of tissue healing that are triggeredby tissue injury, and encompasses fourcontinuous phases including coagulationand haemostasis, inflammation,proliferation and wound remodelling withscar tissue deposition. Correct clinicalmanagement may positively influence thewound healing course and reduce potentialcomplications.

Conflicts of interestThe authors had no conflicts of interest todeclare in relation to this article.

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Author’s address for correspondenceDr Tomaz Velnar

Department of Neurosurgery, University Medical Centre Maribor, Ljubljanska ulica 5,2000 Maribor, Slovenia.

E-mail: [email protected]

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The Wound Healing Process