14Wound healing

Chandan K. Sen and Sashwati Roy

SYNOPS I S

Therearethreegeneraltechniquesofwoundtreatment:primaryintention,secondaryintention,andtertiaryintention

Thefollowingoverlappingphasesdrivetheoverallhealingresponse:hemostasis,inflammation,proliferative,andremodeling

Successfulhemostasisorbloodcoagulationresultsinpreventionofbloodlossbypluggingthewoundwithinsecondsthroughvasoconstrictionandformationofahemostaticbloodclotconsistingofplateletsandfibrin.Theprocessisdividedintoinitiationandamplification.Initiationiscausedbyanextrinsicpathway,whereasamplificationisexecutedbyanintrinsicpathway

Theinflammatoryresponseduringnormalhealingischaracterizedbyspatiallyandtemporallychangingpatternsofspecificleukocytesubsets.Dysregulatedinflammationcomplicateswoundhealing.Completeresolutionofanacuteinflammatoryresponseistheidealoutcomefollowinganinsult

Infectionisacommonprobleminchronicwounds,frequentlyresultinginnonhealingwoundsandsignificantpatientmorbidityandmortality.Microorganismsdonotalwaysliveaspureculturesofdispersedsinglecellsbutinsteadaccumulateatinterfacestoformpolymicrobialaggregatessuchasfilms,mats,flocs,sludge,orbiofilms

Inanopenwoundthathasundergonecontraction,restorationofanintactepidermalbarrierisenabledthroughwoundepithelialization,alsoknownasre-epithelialization.Duringtheproliferativephaseofwoundhealing,thegranulationtissueislightredordarkpinkincolorbecauseofperfusionbynewcapillaryloops.Itissofttothetouch,moist,andgranularinappearance.Thegranulationtissueservesasabedfortissuerepair

Woundvascularizationmaybeachievedbyangiogenesisorvasculogenesis

Awoundisgenerallyconsideredchronicifithasnothealedin4weeks.Chronicwoundscanbebroadlyclassifiedintothreemajorcategories:venousandarterialulcers,diabeticulcers,andpressureulcers

Vascularcomplicationscommonlyassociatedwithproblematicwoundsareprimarilyresponsibleforwoundischemia.LimitationsintheabilityofthevasculaturetodeliverO2-richbloodtothewoundtissueleadsto,amongotherconsequences,hypoxia.Threemajorfactorsmaycontributetowoundtissuehypoxia:(1)peripheralvasculardiseasesgarrotingO2supply;(2)increasedO2demandofthehealingtissue;and(3)generationofreactiveoxygenspecies(ROS)bywayofrespiratoryburstandforredoxsignaling

SmallRNAsareanewclassofregulatorsofeukaryoticbiology.AlongsideothersmallinterferingRNAs(siRNAs),miRNAsexecuteposttranscriptionalgenesilencingthroughmRNAdestabilizationaswellastranslationalrepression.miRNAsareemergingasakeyregulatoryoftheoverallwound-healingprocess

Theregenerativepotentialofinjuredadulttissuesuggeststhephysiologicalexistenceofcellscapableofparticipatinginthereparativeprocess.Bonemarrow-derivedmesenchymalstemcells(BM-MSCs)havebeenshowntopromotethehealingofdiabeticwounds,implyingaprofoundtherapeuticpotentialforskindefectssuchaschronicwoundsandburns

Scars(alsocalledcicatrices)aremacroscopicfibroustissuethatvisiblyreplacesnormalskinafterinjury.Thereisawidespectrumofskinscarringpostwounding,includingscarlessfetalwoundhealing,fine-lineornormalscars,stretchedscars,atrophic(depressed)scars,scarcontractures,hypertrophicscars,andkeloids.

2013, Elsevier Inc. All rights reserved.

241Acutewounds

Introduction

Physical trauma represents one of the most primitive challenges that threatened survival. In other words, injury eliminated the unfit. A Sumerian clay tablet (c. 2150 BC) described early wound care that included washing the wound in beer and hot water, using poul-tices from substances such as wine dregs and lizard dung, and bandaging the wound. Ancient scriptures depicting the science of life or Ayurveda date from the sixth to seventh century BC, and represent the beginning of planned physical injury with the intent to cure.1,2 Hippocrates (c. 400 BC) detailed the importance of drain-ing pus from the wound, and Galen (c. 130200 AD) described the principle of first- and second-intention healing.3 Wound healing advanced slowly over the cen-turies, with major advances in the 19th century in the importance of controlling infection, hemostasis, and necrotic tissue.4 Today, surgical trauma, taken together with injury caused during accidents and secondary to other clinical conditions, e.g., diabetes, represents a sub-stantial cost to society.5 Any solution to wound-healing problems will require a multifaceted comprehensive approach. First and foremost, the wound environment will have to be made receptive to therapies. Second, the appropriate therapeutic regimen needs to be identified and provided while managing systemic limitations that could secondarily limit the healing response. This chapter aims to present an overall outline of the cutane-ous wound-healing process.

Acute wounds

Any violation of live tissue integrity may be regarded as a wound. Skin is the largest organ of the human body, covering about 3000 square inches (7620 cm2) in an average adult. The most important role of the skin for terrestrial animals is to protect the water-rich internal organs from the dry external environment. As a primary line of defense against external threats, maintenance of integrity of the skin is a key prerequisite for healthy survival. Thus, healthy skin can regenerate and repair itself under most common conditions. Skin wounds may be open when manifested as a tear, cut, or punc-ture. Blunt force trauma may cause closed wounds or

contusion where the skin appears to be intact but suffers from damage caused to underlying tissues.Open wounds may be generally categorized as:

Lacerations ragged tears and cuts; masses of torn tissue underneath; caused by dull knife, bomb fragments and machinery and may include crushing of tissues; frequently contaminated (Fig. 14.1)

Puncture e.g., sharp penetrations caused by nails, needles, wire, or bullets (Fig. 14.1). These are of great concern in patients with diabetes, as many of these patients have polyneuropathy and have insensate feet, leading to occult injury. Many times these patients will step on thumb tacks, safety pins, or other sharp household objects and not even know it: this, coupled with compromised vascular status, leads to chronic wound infection

Abrasions the superficial layer of the skin is removed; skinned knee or elbows and rope burns are examples; an abrasion lends itself to infection

Avulsions sections of skin torn off either in part (attached to body) or totally (detached from body); heavy bleeding is common

Amputations traumatic amputation results in nonsurgical removal of limb from the body and accompanies heavy bleeding.

There are three general techniques of wound treatment:

1. primary intention, in which all tissues, including the skin, are closed with suture material after completion of the operation

Fig. 14.1 Laceration and puncture wounds.

Laceration Puncture wound

242 Woundhealing14

2. secondary intention, in which the wound is left open and closes naturally

3. tertiary intention, in which the wound is left open for a number of days and then closed if it is found to be clean.

The wound-healing process

The entire wound-healing process may be viewed as a cascade that is governed by numerous feedback and feedforward regulatory loops driven by signals from the wound tissue itself, wound microenvironment, as well as interventions under conditions where the wound is subjected to therapy. For simplicity of understanding the several interdigitated biological processes that drive the overall healing response, wound healing is com-monly discussed as the following overlapping phases: hemostasis and inflammation, proliferative (granula-tion, vascularization, and wound closure; closure may be discussed as wound contraction and epithelializa-tion) and remodeling (can continue from weeks to years and encompasses scarring, tensile strength, and turno-ver of extracellular matrix (ECM) components). These stages, taken as a whole, are also referred to as the wound-healing cascade (Fig. 14.2).

Hemostasis

For bleeding wounds, the highest priority is to stop bleeding and this is achieved by hemostasis. Hemostasis is thus a protective physiological response to vascular injury that results in exposure of blood components to the subendothelial layers of the vessel wall. Through successful hemostasis blood loss is prevented by plug-ging the wound within seconds through vasoconstric-tion and formation of a hemostatic blood clot consisting of platelets and fibrin. Hemostasis requires both plate-lets and the blood coagulation system. The process of blood coagulation may be subdivided into initiation and amplification. Initiation is caused by an extrinsic pathway, whereas amplification is executed by an intrin-sic pathway. The intrinsic pathway consists of plasma factor XI (FXI), IX, and VIII (Fig. 14.3). Tissue factor (TF) generates a thrombin burst, a process by which

Fig. 14.2 (AC) Phases of cutaneous wound healing: a simplified representation.

Neutrophils

MacrophageFibroblastRe-epithelialization

Wound contraction

Newcapillary Granulation

tissue

Clot

24 hours

3-7 days

Weeks

Scab

A

B

C

243Hemostasis

thrombin is released instantaneously. Thrombin is a key driver of the overall coagulation cascade.The extrinsic pathway responsible for the initiation of

blood coagulation consists of the transmembrane recep-tor TF and plasma FVII/VIIa. On the other hand, the intrinsic pathway consists of plasma FXI, FIX, and FVIII.

Fig. 14.3 The blood clotting cascade. FXII, factor XII, HK, high-molecular-weight kininogen; TF, tissue factor; WBC, white blood cell; RBC, red blood cell; TM, thrombomodulin; PF3, platelet factor 3; TFPI, tissue factor pathway inhibitor.

Tissue damageVessel injury

Healing

Collagen

Prekallikrein

Polymerizationandstabilization

RBCWBC Platelet

Platelet

Commonpathway

Intrinsicpathway

Extrinsicpathway

Kallikrein

Phospholipids

Phospholipids

Prothrombin

Thrombin

Fibrinogen

AntithombinIII

Protein-S

Protein-C

FXII

FXI FXIa

TF

FIX FIXa

FVIII FVIIIa

FVII FVIIa

FX FXa

FX FVa

FV

TM

TFPI

PF3

Ca2+

Ca2+

HK

FXIIa FXIII

FXIIIa

Under physiological conditions, TF is constitutively expressed by adventitial cells surrounding blood vessels and initiates clotting. Examples of such adventitial cells include vascular smooth-muscle cells, pericytes, and adventitial fibroblasts.6,7 TF may also contribute to amplification of blood coagulation through its so-called

244 Woundhealing14

The formation of blood clot is initiated by the proteo-lytic cleavage of fibrinogen by thrombin. As a result, fibrin is produced and forms cross-links with each other. Cross-linked fibrin entraps platelets, and together they adhere to the subendothelium through adhesion mole-cules called integrins. Clot fibrin plays a key role in mounting the inflammatory process as well as in facili-tating wound angiogenesis and stromal cell prolifera-tion. Fibrin binds to integrin CD11b/CD18 on infiltrating monocytes and neutrophils. It also binds to fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor (VEGF) that help the wound tissue vascularize. Fibrin also binds to insulin-like growth factor-I and pro-motes stromal cell proliferation.11,12

Blood clot represents the seat of wound chemotaxis. Thrombin, released by platelets at the wound site, is an early mediator of clot development.13 Thrombin is a serine protease that converts soluble plasma fibrinogen into an insoluble fibrin clot. In addition, it promotes platelet aggregation. Thrombin function may be viewed as an interface between the hemostasis phase of wound healing and the ensuing inflammatory phase as it plays a potent role in mounting wound inflammation. The proinflammatory effects of thrombin include stimula-tion of vasodilation responsible for plasma extravasa-tion, edema, and an increased expression of endothelial cell adhesion molecules that helps monocytes and others cells extravasate and infiltrate the wound site. Thrombin also induces the release of proinflammatory cytokines like CCL2, interleukin-6 (IL-6), and IL-8 by endothelial cells. These cytokines induce monocyte chemotaxis.14 Furthermore, thrombin induces the release of inflamma-tory cytokines by monocytes, including IL-6, interferon-, IL-1, and tumor necrosis factor- (TNF-). These early-phase cytokines are typically proinflammatory, which may govern the differentiation of blood-derived mono-cytes into M1 wound macrophages.15 Wound chemo-taxis is also driven by the degradation of fibrin and subsequent activation of the complement system. As part of this process several chemotactic agents and cytokines are released, which in turn launch the inflam-matory phase of wound healing through chemotactic recruitment of blood-borne immune cells.16 Platelets are one of the earliest sources of cytokines which execute immune cell chemotaxis as well as macrophage activa-tion. Once trapped in the fibrin net, platelets release granules that function as a reservoir for biologically

blood-borne form, which is present as cell-derived microparticles as well as through TF expressed within platelets.68

Levels of FVIIa, a key player of the extrinsic pathway, in the circulating blood are higher than any other acti-vated coagulation factor. Following injury to the blood vessel wall, FVII comes into contact with TF expressed on TF-bearing cells (e.g., white blood cells) and forms an activated complex (TFFVIIa). TFFVIIa activates FIX and FX, resulting in FIXa and FXa, respectively. FVII is activated by thrombin, FXIa, FXII, and FXa. The acti-vation of FXa by TFFVIIa is almost immediately inhib-ited by the TF pathway inhibitor. FXa and its cofactor FVa form the prothrombinase complex, which activates prothrombin to thrombin. Thrombin is a serine protease that plays a central role in hemostasis after tissue injury by converting soluble plasma fibrinogen into an insolu-ble fibrin clot and by promoting platelet aggregation. Thrombin activates other components of the coagula-tion cascade, including FV and FVIII, which in turn activates FXI and cascades to the activation of FIX. Thrombin also activates and releases FVIII from being bound to von Willebrand factor. FVIIIa is the cofactor of FIXa, and together they form the tenase complex, which activates FX. In this way the cycle continues. The intrinsic pathway begins with formation of the primary complex on collagen by high-molecular-weight kininogen, prekallikrein, and FXII (Hageman factor). Prekallikrein is converted to kallikrein, and FXII becomes FXIIa. FXIIa converts FXI into FXIa. FXIa activates FIX, which together with its cofactor FVIIIa forms the tenase complex. The tenase complex activates FX to FXa (Fig. 14.3).The blood clot physically helps plug the wound,

minimizing blood loss. It is primarily made up of cross-linked fibrin, cells such as erythrocytes and plate-lets, as well as other ECM proteins such as fibronectin, vitronectin, and thrombospondin. Current understand-ing portrays the clot as a dynamic structural matrix containing functionally active proteins and cells. In addition to containment of blood loss, the clot serves as a first aid against microbial invasion. The clot also serves as a provisional matrix for the homing of blood-borne cells, including inflammatory as well as stem or pro-genitor cells. The provisional matrix is enriched in cytokines and growth factors which then regulate the function of the homing cells.9,10

245Inflammation

leukocytes and diapedesis (Fig. 14.5). Adhesion mole-cules such as integrins as well as P-selectin and E-selectin play a central role in enabling diapedesis of neutrophils (Fig. 14.6). These adhesion molecules bind with integrins expressed on the cell surface of neutrophils, such as CD11a/CD18 (LFA), CD 11b/CD18 (MAC-1), CD11c/CD18 (gp150, 95), and CD11d/CD18. Alongside cytokines, chemokines play a major role in mounting acute-phase inflammation after injury. Chemokines include IL-8, MCP-1, and growth-related oncogene-. In the case of an infected wound, bacterial products such as lipopolysaccharide and formyl-methionyl peptides can enhance neutrophil recruitment to the wound site (Fig. 14.7).

active proteins, such as RANTES (regulated on activa-tion, normal T cell expressed, and secreted or CCL5), thrombin, transforming growth factor- (TGF-), platelet-derived growth factor (PDGF), and VEGF. CCL5 is one of the most potent monocyte chemoattract-ants released by platelets after injury. Other cytokines and chemokines that attract monocytes to the wound bed include monocyte chemoattractant protein-1 (MCP-1) (CCL2), MIP-1 (CCL3), TGF-, fibronectin, elastin, C5a, C3a, nerve growth factor, and ECM components.17,18

Inflammation

Tissue injury triggers an acute-phase inflammation (Latin, inflammare, to set on fire) response that is meant to prepare the wound site for subsequent wound closure (Fig. 14.4). Inflammation encompasses a series of responses of vascularized tissues of the body to injury. Local chemical mediators that are biosynthesized during acute inflammation give rise to the macroscopic events characterized by Celsus in the first century, namely, rubor (redness), tumor (swelling), calor (heat), and dolor (pain). At cellular and molecular levels, inflammation results from the coordination of manifold systems of receptors and sensors that affect transcriptional and posttranslational programs necessary for host defense and resolution of infection. During normal healing, the inflammatory response is characterized by spatially and temporally changing patterns of specific leukocyte subsets.

Platelets

Formation of the clot or thrombus is dependent on platelet activation. The platelet-rich blood clot also entraps polymorphonuclear leukocytes (neutrophils). This helps amplify blood coagulation and lays the foun-dation for the subsequent acute-phase inflammatory response. In a matter of hours after injury, large numbers of neutrophils extravasate by transmigrating across the endothelial cell wall of blood capillaries to the wound site. To enable this, local blood vessels are activated by proinflammatory cytokines such as IL-1, TNF-, and interferon-. These cytokines induce the expression of adhesion molecules necessary for adhesion of

Fig. 14.4 The wound inflammatory response.

Platelets from blood release blood-clotting proteins at wound site.

Mast cells secrete factors that mediate vasodilation and vascular constriction.Delivery of blood, plasma and cells to injured area

Neutrophils and macrophages remove pathogens by phagocytosis

Macrophages secrete hormones called cytokines that attract immune system cellsto the site and activate cells involved in tissue repair

Inflammatory response continues until the foreign material is eliminated and thewound is repaired

Macrophage

Initiatetissuerepair

Cytokinessignalingpath toinjury site

Mast cell

Neutrophil

Platelets

Bacteria and other pathogens enter wound

1

2

3

45

6

1

2

3

4

5

6

246 Woundhealing14

Neutrophils

Neutrophils traverse postcapillary venules at sites of inflammation, degrade pathogens within phagolyso-somes, and undergo apoptosis. Neutrophils serve a wide range of functions, ranging from phagocytosis of infectious agents to cleansing of devitalized tissue. When coated with opsonins (generally complement and/or antibody), microorganisms bind to specific receptors on the surface of the phagocyte and invagina-tion of the cell membrane occurs with the incorporation of the microorganism into an intracellular phagosome. There follows a burst of oxygen consumption, and much, if not all, of the extra oxygen consumed is con-verted to highly reactive oxygen species. This is called

Fig. 14.5 Diapedesis. In healthy permeable capillaries the endothelium lining prevents blood cells from leaving the circulation. In response to injury, blood vessels around the wound site undergo vasodilation, increasing the permeability of capillaries. Such change enables inflammatory cells to extravasate through the capillary wall and migrate to the site of injury. This process includes release of fluid from the vessels to the extracellular space, resulting in the edematous response commonly noted during inflammation. Blood vessels possess a built-in pathway for such diapedesis to occur in response to tissue injury.

Normal permeability of capillary

Increased permeability of capillaryduring inflammation

Capillary wall Monocyte

Monocyte squeezingthrough interstitial space

Interstitialspaces

Small amount of fluid

More fluid andantimicrobal chemicals

Fig. 14.6 Extravasation of neutrophils in response to tissue injury. Neutrophils move along the capillaries in a rolling motion which is facilitated by the binding and release of L-selectin on the neutrophil surface to sialyl-Lewis, a carbohydrate ligand expressed on the inner wall of capillaries by endothelial cells. Upon injury and/or infection the release of lipopolysaccharides, tumor necrosis factor-alpha, and interleukin-1 results in the shedding of L-selectin by the neutrophils which then strongly adhere to the inner wall of the capillary by the binding of integrin on the neutrophils to E-selectins on endothelial cells. After such adhesion, the process of diapedesis begins, allowing the neutrophils to extravasate to the wound site.

L-selectin

Sialyl-Lewis

Integrin

E-selectin

Rolling Shedding ofL-selectin

Lipopolysaccharides,interleukin-1, and

tumor necrosis factor

C3a C5a,chemokines,

histamine,prostaglandins

and leukotrienes

Activating substancesreleased bybacteriaanddamagedtissues

Adhesion Diapedesis

Neutrophil

Fig. 14.7 Diapedesis and migration of leukocytes to the wound site.

Fibrin-platelet clot

Red bloodcell

Collagen

Fibroblast

Elastin

Cytokines &growth factors

247Inflammation

Mediators present in the microenvironment that the monocyte traverses to reach the wound site interact with receptors on the monocyte cell surface, bringing forth major changes in the transcriptomic as well as proteomic make of the cell. Major examples of such receptors present on the monocyte surface include Toll-like receptors (TLRs: Fig. 14.8), complement receptors, and Fc receptors. At the wound site, macrophages function as antigen-presenting cells and phagocytes scavenging dead cells and debris. In addition, they deliver a wide range of growth factors that are known for their abilities to execute the wound-healing process. Such growth factors include TGF-, TGF-, basic FGF (bFGF), VEGF, and PDGF. These growth factors enable wound healing by causing cell proliferation and synthe-sis of ECM and inducing angiogenesis. Macrophages play a crucial role in enabling wound healing. Macrophage depletion is known to impair wound closure markedly.4,22

Mast cells

Mast cells are best known for their central role in medi-ating allergic responses. Beyond that function, it is now

respiratory burst. In addition, the cytoplasmic granules discharge their contents into the phagosome, and death of the ingested microorganism soon follows. Among the antimicrobial systems formed in the phagosome is one consisting of myeloperoxidase (MPO), released into the phagosome during the degranulation process, hydro-gen peroxide (H2O2), formed by the respiratory burst and a halide, particularly chloride. The initial product of the MPO-H2O2-chloride system is hypochlorous acid, and subsequent formation of chlorine, chloramines, hydroxyl radicals, singlet oxygen, and ozone has been proposed. These same toxic agents can be released to the outside of the cell, where they may attack normal tissue and thus contribute to the pathogenesis of disease.19 Other products delivered by neutrophils to the wound site include antimicrobials such as cationic peptides and eicosanoids as well as proteases such as elastase, cathepsin G, proteinase 3, and urokinase-type plasminogen activator. As it relates to the overall inflam-matory process elicited in response to injury, neutrophils are major players because they can modify macrophage function and therefore regulate innate immune response during wound healing.20 In the absence of neutrophils, wound site macrophages seem to lack guidance in con-ducting the healing process.21

In a healing wound, neutrophil infiltration ceases after a few days of injury. Expended neutrophils are programmed to die and dying neutrophils are recog-nized by wound site macrophages and phagocytosed. The wound site contains a small portion of macrophages that are resident. Most macrophages at the wound site are recruited from the peripheral circulation. Extravasation of peripheral blood monocytes is enabled by the interaction between endothelial vascular cell adhesion molecule-1 and monocyte very late antigen-4 (41 integrin). Factors that guide the extravasated monocyte to the wound site include growth factors, chemotactic proteins, proinflammatory cytokines, and chemokines such as macrophage inflammatory protein 1, MCP-1, and RANTES. The source of these chemoat-tractants includes clot-associated platelets, wound edge hyperproliferative keratinocytes, wound tissue fibrob-lasts and subsets of leukocyte already at the wound site. Once the monocyte leaves the blood vessel to trans-migrate into the wound site through the ECM micro-environment, the process of monocyte differentiation to macrophages has started.

Fig. 14.8 Toll-like receptors: responding to infectious agents and the wound microenvironment.

Cell membrane

Cytoplasm

Lipopolysaccharide

Gram-negativebacteria

Nucleus

Toll-likereceptor

248 Woundhealing14

diminished the accumulation of macrophages in healing skin wounds of adult guinea pigs. Such depletion resulted in impaired disposal of damaged tissue and provisional matrix, compromised fibroblast count, and delayed healing. Today, macrophages have emerged to be a pivotal driver of efficient skin repair.34,35 Macrophages are plastic and heterogeneous cells broadly categorized into two groups: classically activated or type I macro-phages, which are proinflammatory effectors, and alter-natively activated or type II macrophages.36 In the inflamed tissue, it is unclear whether the type II macro-phages that appear during the healing phase originate from newly attracted monocytes or from a switch in the activation state of previously proinflammatory macro-phages. The macrophage population first taking part in inflammation may change its phenotype and assume the role to resolve inflammation.37,38 Macrophages from diabetic wounds display dysfunctional inflammatory responses.39 A persistent inflammatory state of diabetic wound macrophages is caused by impairment in the ability of these cells to phagocytose apoptotic cells at the wound site which in turn prevents the switch from M1 to M2 phenotype.39

Resolution of inflammation

Inflammatory responses elicited by injury are only helpful to the healing process if they are timely and transient. Dysregulated inflammation complicates wound healing.39 Complete resolution of an acute inflammatory response is the ideal outcome following an insult. For resolution to ensue, further leukocyte recruitment must be halted and accompanied by removal of leukocytes from inflammatory sites. Reso-lution of inflammation is executed by a number of key factors. At the wound site successful phagocytosis of dead neutrophils by macrophages is a key factor. Impairment in macrophage function at the wound site derails the resolution of inflammation.39 Lipid media-tors, such as the lipoxins, resolvins, protectins, and newly identified maresins, have emerged as a novel genus of potent and stereoselective players that counterregulate excessive acute inflammation and stimulate molecular and cellular events that define resolution.40 Successful resolution paves the path for the healing process to progress towards successful wound closure. Prolonged inflammation may not only

known that mast cells are physiologically significant in recognizing pathogens and in regulating immune response.23 Mast cells may instantly release several proinflammatory mediators from intracellular stores. In addition, they are localized in the hostenvironment interface. These properties make mast cells key players in finetuning immune responses during infection. Recent studies using mast cell activators as effective vaccine adjuvants show the potential of harnessing these cells to confer protective immunity against micro-bial pathogens.24 Mast cell activation helps initiate the inflammatory phase of wound healing. In response to injury, mast cells at the wound site degranulate within a matter of hours and therefore become histologically silent at the wound tissue. After about 48 hours of injury, mast cells are again seen in the wound tissue and their number increases as healing progresses.25 On one hand, impaired wound healing has been reported in mast cell-deficient mice.26 On the other hand, mast cells have been implicated in skin wound fibrosis.27 With the aid of a wide array of newly formed or preformed mediators released by degranulation, the activated mast cell con-trols the key events of the healing phases: triggering and modulation of the inflammatory stage, proliferation of connective cellular elements, and final remodeling of the newly formed connective tissue matrix. The impor-tance of the mast cell in regulating healing processes is also demonstrated by the fact that a surplus or deficit of degranulated biological mediators causes impaired repair, with the formation of exuberant granulation tissue (e.g., keloids and hypertrophic scars), delayed closure (dehiscence), and chronicity of the inflam-matory stage.28

Macrophages

Macrophages represent the predominant cell type in a healing wound 35 days following injury. The primary acute function of wound macrophages, which arrive at an injury site hours later than neutrophils, is to operate as voracious phagocytes cleansing the wound of all matrix and cell debris, including fibrin and apoptotic neutrophils. Macrophages also produce a range of cytokines, growth and angiogenic factors that drive fibroblast proliferation and angiogenesis.4,2932 In a classic study, Leibovich and Ross33 demonstrated that antimacrophage serum combined with hydrocortisone

249Infection

the mid-1990s showed that pathogen recognition by the innate immune system is instead actually specific, relying on germline-encoded pattern recognition recep-tors (PRRs) that have evolved to detect components of foreign pathogens, referred to as pathogen-associated molecular patterns (PAMPs).45 TLRs regulate innate and adaptive immune responses and are important modula-tors of inflammation during wound-healing responses. The finding that there is activation of TLR signaling during tissue damage in several disease situations in the absence of infection suggests that endogenous mole-cules serve as TLR agonists, although it is unclear whether this response is biologically important for maintenance of homeostasis, such as tissue repair, or whether this recognition is simply accidental. It is note-worthy that microbial infection triggers the production of modified endogenous molecules (such as high-mobility group protein B1, oxidized phospholipids, -defensin 2, and nucleic acids) that are recognized by TLRs or other cytosolic PRRs. This may suggest that these endogenous molecules, along with PAMPs, act as adjuvants to activate innate immune programs via TLRs and/or other PRRs, and have key roles in facilitating adaptive immunity against infecting microbes.Chronic wounds have a complex colonizing flora that

changes over time. Staphylococcus aureus and coagulase-negative staphylococci are the most commonly isolated organisms. Chronic wounds are colonized by multiple bacterial species and many persist in the wound once they are established. In chronic venous leg ulcers the most common bacteria noted, in order of abundance, were S. aureus, Enterococcus faecalis, Pseudomonas aerugi-nosa, coagulase-negative staphylococci, Proteus spp., and anaerobic bacteria. Resident (colonizing) bacterial species are commonly present in ulcers. The longer an ulcer remains unhealed, the more likely it will acquire multiple aerobic organisms and a significant anaerobic population. Chronic wounds are commonly compli-cated by underlying ischemia. Thus, they tend to have a low tissue oxygen level. This facilitates the growth of anaerobes in ischemic wounds. Adequate delivery of oxygen to the wound tissue is vital for optimal healing and resistance to infection.46 Hospitalization, surgical procedures, and prolonged or broad-spectrum anti-biotic therapy may predispose patients to coloniza-tion or infection, or both, with resistant organisms, including S. aureus (methicillin-resistant S. MRSA) or

compromise wound closure but may also worsen scar outcomes.41,42

Infection

Infection is a common problem in chronic wounds, fre-quently resulting in nonhealing and significant patient morbidity and mortality.43 Wound infection and the sub-sequent release of proinflammatory modulators result in pain and delayed healing. The pain, in turn, compro-mises the immune response to infection.44 All wounds become contaminated by bacteria from the surrounding skin, the local environment, and autologous patient sources. The local environment is particularly relevant for hospitalized patients. Colonization is defined as the presence of proliferating bacteria without a noticeable host response. Colonization of the wound may enhance or impede wound healing, depending upon the bacte-rial load. Bacterial loads in excess of 105 organisms/gram of tissue are a threat to wound healing, although this threshold may be altered by the status of the host immune system and the number and types of bacterial species present. The concept of critical colonization is controversial and not universally accepted. Critical colonization is characterized by increased bacterial burden or covert infection, and the wound at this stage may enter a nonhealing, chronic inflammatory state. Substantial colonization may not cause the obvious signs of inflammation but will likely affect wound healing with failure to heal or slowing of progression. Signs of critical colonization are atrophy or deteriora-tion of granulation tissue, discoloration of granulation tissue to deep red or gray, increased wound friability, and increased drainage. The transition to infection occurs when bacterial proliferation overcomes the hosts immune response and host injury occurs. Several factors determine transition from colonization to infection: the bioburden itself, the virulence of the organisms, the syn-ergistic action of different bacterial species, and the ability of the host to mount an immune response.43

During the past decade, there has been rapid progress in the understanding of innate immune recognition of microbial components and its critical role in host defense against infection. The early concept of innate immunity was that it nonspecifically recognized microbes; however, the discovery of TLRs (Fig. 14.8) in

250 Woundhealing14

drug and other physiologic changes that could impair drug effectiveness.43

Vascularization

Wounds larger than can be closed by diffusion of oxygen from neighboring intact blood vessels or wounds com-plicated by underlying ischemia largely rely on wound vascularization for their closure. Wound vascularization may be achieved by angiogenesis or vasculogenesis. Angiogenesis represents sprouting of capillaries from existing blood vessels in the wound edge tissue. Vasculogenesis relies on the formation of new blood vessels by mobilization of bone marrow-derived endothelial stem cells.Wound vascularization is regulated by all phases in

wound healing hemostasis, inflammation, tissue for-mation, as well as tissue remodeling. The hemostatic plug provides a bed for blood-borne cells to home. Once cells entangle in this plug, the ECM environment modi-fies cell function towards successful healing. Platelets in the clot serve as a source of growth factors and cytokines which recruit several cell types, including endothelial cells, to the wound site. During the inflammatory phase, leukocytes at the wound site serve as a major source of proangiogenic factors such as VEGF-A and IL-8 that lay the early foundation for successful wound tissue vascu-larization. As neutrophils are expended and undergo cell death, the number of macrophages at the wound site substantially increases. Macrophage-derived TGF-, TGF-, bFGF, PDGF, and VEGF play a key role in driving skin wound angiogenesis. Growing evidence demonstrates that no single angiogenic factor is singu-larly effective in significantly influencing wound out-comes. Wound vascularization is a sophisticated process requiring dynamic, temporally and spatially regulated interaction between cells, angiogenic factors, and the ECM.Key processes in tissue vascularization are depicted

in Figure 14.9. Angiogenic cues are elicited by micro-environmental signals such as hypoxia and are ampli-fied by angiogenic factors such as VEGF expressed by and released from cells at the wound site. VEGF was originally identified as an endothelial cell-specific growth factor-stimulating angiogenesis and vascular permeability. Some family members, VEGF C and D, are

vancomycin-resistant enterococci.43 Because inflamma-tory responses to microbial invasion may be diminished in persons with diabetes, clinical signs of infection are often absent in persons with diabetic foot ulcers when infection is limited to localized tissue.47

Biofilm

Microorganisms do not always live as pure cultures of dispersed single cells but instead accumulate at inter-faces to form polymicrobial aggregates such as films, mats, flocs, sludge, or biofilms. The biofilm state of microorganisms may lead to an increase in virulence and propensity to cause infection. In most biofilms, the microorganisms account for less than 10% of the dry mass, whereas the matrix can account for over 90%. The matrix is the extracellular material, mostly produced by the organisms themselves, in which the biofilm cells are embedded. It consists of a conglomeration of different types of biopolymers known as extracellular poly-meric substances (EPS) that forms the scaffold for the three-dimensional architecture of the biofilm and is responsible for adhesion to surfaces and for cohesion in the biofilm. The formation of a biofilm allows a lifestyle that is entirely different from the planktonic state. Although the precise and molecular interactions of the various secreted biofilm matrix polymers have not been defined, and the contributions of these components to matrix integrity are poorly understood at a molecular level, several functions of EPS have been determined, demonstrating a wide range of advantages for the biofilm mode of life.The architecture of biofilms is influenced by many

factors, including hydrodynamic conditions, concentra-tion of nutrients, bacterial motility, and intercellular communication, as well as exopolysaccharides and pro-teins.48 Chronic wounds offer attractive conditions for biofilm production because proteins (collagen, fibronec-tin) and damaged tissues are present, which allow attachment. The biofilm impedes healing of chronic wounds. Most of the chronic wound pathogens, such as MRSA and Pseudomonas spp., are typical biofilm pro-ducers. Compared to bacteria in the unattached free-living planktonic form, bacteria that reside within mature biofilms are highly resistant to traditional anti-biotic therapies. Bacteria in biofilms grow more slowly, and slower growth may lead to decreased uptake of the

251Vascularization

process in which cells move in a given direction either in response to changes in the extracellular environment or as a consequence of an intrinsic propensity for direc-tional movement. ECM remodeling by proteases pro-motes cell migration, a critical event in the formation of new vessels. Temporal and spatial regulation of ECM remodeling events allows for local changes in net matrix deposition or degradation, which in turn contributes to control of cell growth, migration, and differentiation during different stages of angiogenesis. Matrix-bound growth factors released by proteases and/or by ang-iogenic factors promote angiogenesis by enhancing endothelial migration and growth. Matrix molecules promote endothelial cell growth and morphogenesis, and/or stabilize nascent blood vessels. Hence, ECM molecules and ECM remodeling events play a key role in regulating angiogenesis.51

The formation of the capillary-like tubes is specific to endothelial cells and integral to the process of angio-genesis. The basement membrane represents a biologi-cally functional highly specialized ECM on which the basal nonluminal surface of endothelial cells rests. This matrix forms a continuous sleeve around the endothe-lial cells, and maintains the tube-like structures of the blood vessels. More than 20 years ago Kubota et al. observed that endothelial cells plated on a reconstituted

specifically involved in lymphangiogenesis. Ligation of these angiogenic factors with their corresponding recep-tors elicits a multitude of cell-signaling processes that activate microvascular endothelial cells. For example, VEGF and their endothelial tyrosine kinase receptors are central regulators of tissue vascularization. VEGF signaling through VEGFR-2 is the major angiogenic pathway. VEGFR-3 has also been shown to be important for angiogenesis, acting together with VEGF/VEGFR-2 and Dll4/Notch signaling to control angiogenic sprout-ing.49 The biological significance of other angiogenic factors such as EGF and bFGF is mediated by their cor-responding receptors, which also belong to the family of tyrosine kinase receptors EGFR, FGFR-1, FGFR-2, FGFR-3, and FGFR-4.Other tyrosine kinase receptors of outstanding sig-

nificance in this regard are Tie-1 and Tie-2. Along with the VEGF receptor, these are the only known endothelial cell-specific receptor tyrosine kinases. Tie-2 is induced on the endothelium of neovessels in skin wounds and downregulated as newly formed vessels regress. As an indicator of Tie-2 activation, Tie-2 phosphorylation is detected in skin wounds at all stages of the healing process.50 Activated microvascular endothelial cells respond by proliferating, which is followed by direc-tional migration of these cells. Migration is a complex

Fig. 14.9 Neoangiogenesis: the formation of new blood vessels. VEGF, vascular endothelial growth factor; ECM, extracellular matrix.

1

34

5

6

10

9

8

72

Angiogenicfactors bind

to endothelialcell receptors

Endothelial cellactivation

Angiogenicfactor (VEGF)

production

Release

Tie-2

y3

Pericyte

Endothelial cellproliferation

Directionalmigration

Loopformation

Vascularstabilization

Tubeformation

ECMremodeling

252 Woundhealing14

acquire smooth-muscle cell characteristics and differen-tiate into contractile myofibroblasts. The first pheno-typic transition of wound edge fibroblasts into so-called protomyofibroblasts is characterized by neoformation of contractile -cytoplasmic actin stress fibers and occurs in response to profibrotic cytokines and to altered prop-erties of the ECM. In the presence of TGF-1 in a mechanically restrained environment, these cells express -smooth-muscle actin de novo, which significantly increases their contractile activity and is a hallmark of the differentiated myofibroblast.59 Wound contraction may significantly contribute to wound closure, although this contribution is much larger in loose-skinned rodents than in humans.In an open wound that has undergone contraction,

restoration of an intact epidermal barrier is enabled through wound epithelialization, also known as re-epithelialization.60,61 A wound that is not epithelial-ized is not considered healed, no matter how perfectly restored the underlying dermal structures may be. Thus, wound epithelialization, also called re-epithelialization, is a critical and defining feature of wound repair. Re-epithelialization of the wound can be conceptually viewed as the result of three overlapping keratinocyte functions: migration, proliferation, and dif-ferentiation. The sequence of events by which keratino-cytes accomplish the task of re-epithelialization is generally believed to begin with dissolution of cellcell and cellsubstratum contacts. This is followed by the polarization and initiation of directional migration in basal and a subset of suprabasilar keratinocytes over the provisional wound matrix. A subset of keratinocytes immediately adjacent to, but not within, the wound bed then undergoes mitosis. Finally, there is multilayering of the newly formed epidermis and induction of dif-ferentiation specific gene products to restore the func-tionality of the epidermis. The most limiting factor in wound re-epithelialization is migration, since defects in this function, but not in proliferation or differentiation, are associated with the clinical phenotype of chronic nonhealing wounds.62 The process of epithelialization continues until the barrier is re-established and the wound is covered. The process of re-epithelialization is accelerated by a moist environment63,64 and is facilitated by the enzyme matrix metalloproteinase 1, a collagenase which lessens the affinity of the collagenintegrin contacts.65

basement membrane matrix, rapidly attached, aligned, and formed capillary-like tubules. The cells did not pro-liferate. The vessels that are thus formed contain a lumen and tight cellcell contacts. The cells are polar-ized with the nuclei basally located towards the base-ment membrane matrix. Furthermore, the capillary-like structures take up acetylated low-density lipoprotein, which is a marker of differentiation for these cells.52 Angiogenesis not only depends on endothelial cell inva-sion and proliferation, but also requires pericyte cover-age of vascular sprouts for vessel stabilization. These processes are coordinated by VEGF and PDGF through their cognate receptors on endothelial cells and vascular smooth-muscle cells, respectively.53 Structural support to blood vessels is provided to pericytes and vascular smooth-muscle cells. Normal pericytes are embedded within the basement membrane of capillaries, either as solitary cells or a single-cell layer, where they coordinate intercellular signaling with endothelial cells and other components of the blood vessel wall to prevent leakage. In contrast, vascular smooth-muscle cells form single or multiple layers around arteries and veins to mediate vascular tone and contraction. Pericyte coverage is required for the stabilization of immature endothelial tubes.54 Pericytes have function beyond angiogenesis that are relevant to wound healing. Skin pericytes may act as mesenchymal stem cells (MSCs), exhibiting the capacity to differentiate into bone, fat, and cartilage line-ages. Thus, pericytes represent a potent stem cell popu-lation in the skin that is capable of modifying the ECM microenvironment and promoting epidermal tissue renewal from nonstem cells.55

Wound closure

Wound contraction and re-epithelialization contribute to closure of wounds that heal by secondary intention. Wound contraction represents an early response to injury that is aimed at juxtaposing the edges of an open wound.56 This early phase of wound closure appears to be mediated by a contractile purse-string force pro-duced by a circumferentially arranged band of fusiform-shaped epidermal cells situated in the wound margin.57 Fibroblasts at the wound edge tissue are recognized to play a key role in enabling wound contraction.58 During the inflammatory phase of wound healing, fibroblasts

253Chronicwounds

surgical textbooks define chronic wounds as wounds which have not healed in 3 months. Chronic wounds can be broadly classified into three major categories: venous and arterial ulcers, diabetic ulcers, and pressure ulcers.

Venous ulcers

Venous ulcers (stasis ulcer or varicose ulcers) are wounds that are thought to occur due to improper functioning of venous valves, usually of the legs. They are the major cause of chronic wounds, occurring in 5070% of chronic wound cases.68 Venous ulcers develop mostly along the medial distal leg, and can be very painful. According to the revised clinical, etiology, anatomy, and pathophysi-ology (CEAP) classification of chronic venous disease published in 2004, a venous ulcer is defined as a full-thickness defect of skin, most frequently in the ankle region, that fails to heal spontaneously and is sustained by chronic venous disease.69 Systematically, venous ulcer may be defined as a defect in the skin with sur-rounding pigmentation and dermatitis, located in the lower leg (usually in the gaiter region) that has been present for greater than 30 days, characterized by persistent venous hypertension and abnormal venous function (result of venous reflux and/or obstruction confirmed by hemodynamic and/or physiologic assess-ment), without a primary or associated arterial, immu-nologic, endocrine, or systemic cause. It is recognized that ulcers can be caused purely by venous pathology such as venous reflux or obstruction. When these abnor-malities are combined with additional pathologic condi-tions, they contribute to the causation and perpetuation of the ulcer. The latter situation includes comorbid con-ditions such as arterial ischemia, swelling and lymph-edema, trauma, autoimmune disorders, neurotrophic conditions, and diabetic vascular disease. These ulcers are categorized as of mixed origin, in which the venous component may or may not play a dominant role. Successful treatment of such ulcers includes not only the venous component but also concomitant management of the comorbid condition.

Arterial ulcers

Because both arterial and venous ulcers typically occur on the lower leg, differentiating between them can be

Proliferative phase

The proliferative phase starts around 2 days after injury and normally lasts up to 3 weeks in a healing cutaneous wound. This phase overlaps with the inflammatory phase and supports re-epithelialization, the formation of new blood vessels, and the influx of fibroblasts and laying down of the ECM. By the time this phase begins, the degradation of the fibrin clot by the macrophages has begun and invading endothelial cells and fibroblasts rapidly fill that space. Migrating fibroblasts produce the cytokines that induce keratinocytes to migrate and proliferate.66 Activated macrophages produce several cytokines, such as PDGF and TNF-, which also induce fibroblasts to produce keratinocyte growth factor which in turn induces wound re-epithelialization.60,61

Granulation tissue

The fibrin clot formed during hemostasis participates in the early inflammatory phase and is replaced by a per-fused, fibrous connective tissue that grows from the base of a wound and is able to fill wounds of almost any size. During the proliferative phase of wound healing, this granulation tissue is light red or dark pink in color because of perfusion by new capillary loops. It is soft to the touch, moist, and granular in appearance. The gran-ulation tissue serves as a bed for tissue repair. The ECM of granulation tissue is created and modified by fibrob-lasts. Initially, it consists of a network of type III colla-gen, a weaker form of the structural protein that can be produced rapidly. This is later replaced by the stronger, long-stranded type I collagen, as evidenced in scar tissue. Formation and contraction of the granulation tissue represent integral aspects of the healing wound. In ischemic wounds, contraction of the granulation tissue is impaired because of faulty myofibroblast function, cells responsible for granulation tissue contraction.67

Chronic wounds

A wound is generally considered chronic if it has not healed in 4 weeks. Chronic wounds have been also defined as wounds that have not shown a 2040% reduc-tion in area after 24 weeks of optimal therapy. Standard

254 Woundhealing14

challenging for wound care practitioners. However, they have very different pathophysiologies and man-agement pathways. The most common cause of arterial ulcers is atherosclerosis. Risk factors for the develop-ment of atherosclerosis include age, smoking, diabetes mellitus, hypertension, dyslipidemia, family history, obesity, and sedentary lifestyle.70 Ischemia and necrosis are common consequences. Both acute and chronic arte-rial insufficiency can lead to the formation of lower-extremity ulcers. Arterial insufficiency can occur at any level, from large arteries to arterioles and capillaries. Tissue ischemia that leads to leg ulcers tends to occur more in the setting of large-vessel or mixed disease. Vascular claudication, with exercise, at night, or while one is resting, is often the most distinguishing charac-teristic of arterial ulcers. Determining the ankle brachial index give an indication of a patients ability to heal. However, diabetic patients may have falsely elevated ankle brachial index results secondary to vessel calcifi-cation. Patients with arterial ulcers must have increased/adequate blood supply to heal and benefit most from revascularization procedures. It should be noted that arterial insufficiency might act in concert with other pathological mechanisms, leading to tissue necrosis and ulceration. Diabetic foot ulcers, for example, may result from the combination of neuropathy, trauma, and arte-rial insufficiency.

Diabetic ulcers

Diabetic ulcers are the most common foot injuries leading to lower extremity amputation (Figs 14.10 and 14.11). In diabetics, the effects of peripheral neuropathy, peripheral vascular disease, and infection often combine to facilitate the development of diabetic ulcers that can lead to gangrene and amputation. Diabetic persons, like people who are not diabetic, may develop athero-sclerotic disease of large- and medium-sized arteries, such as aortoiliac and femoropopliteal atherosclerosis. However, significant atherosclerotic disease of the infra-popliteal segments is particularly common in the diabetic population. Underlying digital artery disease, when compounded by an infected ulcer in close proxim-ity, may result in complete loss of digital collaterals and precipitate gangrene. The reason for the prevalence of this form of arterial disease in diabetic persons is thought to result from a number of metabolic abnormalities,

Fig. 14.10 The ischemic diabetic foot.

Healthy foot Diabetic foot

Blood vessel damage in the feet may cause tissue damagesuch as sores, lesions and poor circulation that can lead to amputation

Fig. 14.11 Diabetic foot ulcer.

Diabetic foot ulcer

255Chronicwounds

motor nerves can lead to clawing of the toes because of a resulting imbalance of intrinsic and extrinsic muscles. The long-term morbidity of nerve injuries of the foot is predominantly related to sensory nerve injury, except when a tibial nerve injury causes intrinsic muscle func-tion loss. The two main problems associated with inju-ries to these nerves are the lack of sensation in the distal distribution of the nerve (Fig. 14.12) and the formation of a painful neuroma. When the nerve injury occurs in a weight-bearing area of the foot, the presence of a painful neuroma often leads to complex regional pain syndrome type I.74

Pressure ulcers

A pressure ulcer is a localized injury to the skin or underlying tissue, usually over a bony prominence, as a result of unrelieved pressure. Pressure ulcers or pres-sure sores represent a common health problem, particu-larly among the physically limited or bedridden elderly. Pressure ulcers on the buttocks affect nearly all wheel-chair users. Pressure ulcer is a general term covering a number of different tissue injuries, from superficial heel sores to deep pressure sores under the buttocks. Because compressive forces, shearing forces, and/or friction are the major underlying causes some call them decubitus ulcer.75 Pressure ulcers occur in approximately 515% of patients in home care, healthcare facilities, and hospi-tals. Pressure ulcers are painful, decrease the quality of life, increase susceptibility to infections, risk of death and nursing workload, and create significant costs.76 Predisposing factors are classified as intrinsic (e.g., limited mobility, poor nutrition, comorbidities, aging skin) or extrinsic (e.g., pressure, friction, shear, mois-ture). When an ulcer occurs, documentation of each ulcer (i.e., size, location, eschar and granulation tissue, exudate, odor, sinus tracts, undermining, and infection) and appropriate staging (I through IV) are essential to the wound assessment.77

While there is little doubt that pressure ulcers are related to the mechanical insult of soft tissue, several hypotheses have been developed pertaining to the link between mechanical loading and tissue necrosis. The two major hypotheses deal with tissue deformation and ischemia.78 Pressure-induced soft-tissue deformation causes cell necrosis. There is a time/strain relationship, meaning that time does indeed play a role. In other

including high low-density lipoprotein and very- low-density lipoprotein levels, elevated plasma von Willebrand factor, inhibition of prostacyclin synthesis, elevated plasma fibrinogen levels, and increased plate-let adhesiveness.71 When peripheral arterial insuffi-ciency complicates neuropathy there is a 10-fold risk of ulceration progressing to infection, gangrene, and amputation.72

Peripheral sensory neuropathy affects 50% of diabetic patients and is attributed to chronic hyperglycemia. It is a major cause of foot ulceration and lower limb ampu-tation. In addition to poor glucose control, traditional cardiovascular risk factors for macrovascular disease are independent-risk factors for incident peripheral neuropathy. In addition, data from the EURODIAB cohort suggest that female sex may be an independent-risk factor. This makes it difficult to identify specific diabetes components of neuropathy.73 Nerve injuries of the foot can also be caused by penetrating wounds. The sequelae of such injuries depend on the nerve injured and the level of the injury. In the foot and ankle, the main function of the nerves is to provide sensation. Generally, the tibial nerve and its branches (i.e., the medial and lateral plantar nerves) innervate the intrin-sic musculature, although the deep peroneal nerve innervates the extensor digitorum brevis and extensor hallucis brevis muscles (Fig. 14.12). Denervation of these

Fig. 14.12 Cutaneous innervations of the foot.

Lateral plantar nerve

Saphenous nerve

Deep peroneal nerve

Superficial peronealnerve

Medial plantar nerve

Calcaneal branch(tibial nerve)Sural nerve

Dorsal surface Plantar surface

256 Woundhealing14

role in the regulation of vascular tone as well as in ang-iogenesis. In a wound setting, large amounts of molecu-lar oxygen are partially reduced to form reactive oxygen species (ROS). ROS includes oxygen free radicals such as superoxide anion as well as its nonradical derivative, hydrogen peroxide (H2O2). Superoxide anion radical is the one-electron reduction product of oxygen. NADPH oxidases represent one major source of superoxide anion radicals at the wound site. NADPH oxidases in phago-cytic cells help fight infection. Superoxide anion also drives endothelial cell signaling such as required during angiogenesis. In biological tissues, superoxide anion radical rapidly dismutates to hydrogen peroxide, either spontaneously or facilitated by enzymes called super-oxide dismutases. Endogenous hydrogen peroxide drives redox signaling, a molecular network of signal propagation that supports key aspects of wound healing such as cell migration, proliferation, and angiogenesis. Neutrophil-derived hydrogen peroxide may be utilized by MPO to mediate peroxidation of chloride ions, result-ing in the formation of hypochlorous acid (HOCl), a potent disinfectant (Fig. 14.13).Three major factors may contribute to wound tissue

hypoxia: (1) peripheral vascular diseases garroting O2 supply; (2) increased O2 demand of the healing tissue;

words, external tissue loading for a long time or on a chronic basis results in ulcer formation. Prevention includes identifying at-risk persons and implementing specific prevention measures such as following a patient-repositioning schedule, keeping the head of the bed at the lowest safe elevation to prevent shear, and using pressure-reducing surfaces. Ischemia is a state caused by lack of blood supply to any given tissue. The hypoth-esis proposes that a mechanical loading of the tissue impinges on the arterial blood vessels, thereby causing local ischemia. Since cells depend on oxygen, heat, and nutrients transported by the blood, they will become hypoxic and subsequently necrotic.

Ischemia and tissue oxygenation

Vascular complications commonly associated with problematic wounds are primarily responsible for wound ischemia. Limitations in the ability of the vascu-lature to deliver O2-rich blood to the wound tissue lead to, among other consequences, hypoxia. Hypoxia is a reduction in oxygen delivery below tissue demand, whereas ischemia is a lack of perfusion, characterized not only by hypoxia but also by insufficient nutrient supply.79 Hypoxia, by definition, is a relative term. It is defined by a lower tissue partial pressure of oxygen (pO2) compared to the pO2 to which the specific tissue element in question is adjusted under healthy condi-tions in vivo. Depending on the magnitude, cells con-fronting hypoxic challenge either induce an adaptive response that includes increasing the rates of glycolysis and conserve energy or undergo cell death. Generally, acute mild to moderate hypoxia supports adaptation and survival. In contrast, chronic extreme hypoxia leads to tissue loss.While the tumor tissue is metabolically designed to

thrive under conditions of hypoxia, hypoxia of the wound primarily caused by vascular limitations is intensified by coincident conditions (e.g., infection, pain, anxiety and hyperthermia) and leads to poor healing outcomes. Oxygen and its reactive derivatives (Fig. 14.13) are required for oxidative metabolism-derived energy synthesis, protein synthesis, and the maturation (hydroxylation) of extracellular matrices such as collagen. Molecular oxygen is also required for nitric oxide (NO) synthesis which in turn plays a key

Fig. 14.13 Molecular oxygen and its derivatives in wound healing. ATP, adenosine triphosphate; NO, nitric oxide.

Red bloodcells

Mature collagen

Endothelial cells

Redoxsignals

H2O2

O2

Superoxideanion radical

NO

HOCl

Infectiouspathogens

Neutrophil

Proteinsynthesis

Macrophage

ATP

257Ischemiaandtissueoxygenation

of VEGF and its receptors leads to insufficient skin ang-iogenesis.81 Whether cells in the pockets of extreme hypoxia are O2-responsive is another concern. Even if such cells may have passed the point of no return in the survival curve, correction of tissue oxygenation is likely to help clean up the dead or dying tissue and replace the void with proliferating neighboring cells. Pockets of moderate or mild hypoxia are likely to be the point of origin of successful angiogenic response as long as other barriers such as infection and epigenetic alterations are kept to a minimum.

Limited supply and high demand: the oxygen imbalance

Peripheral vascular disease can affect the arteries and the veins as well as the lymph vessels. The most common and important type of peripheral vascular disease is peripheral arterial disease, or PAD, which affects about 8 million Americans. The ankle brachial pressure index represents a simple noninvasive method to detect arte-rial insufficiency within a limb. Arterial diseases, espe-cially those associated with diabetes, represent a major complicating factor in wound healing. PAD is the only identifiable etiology in approximately 10% of leg ulcers. In an ischemic limb, peripheral tissues are deprived of blood supply as PAD progresses, causing tissue loss, ulcers, and gangrene.

and (3) generation of ROS by way of respiratory burst and for redox signaling.80 Other related factors, such as arterial hypoxia (e.g., pulmonary fibrosis or pneumonia, sympathetic response to pain, hypothermia, anemia caused by major blood loss, cyanotic heart disease, high altitude), may contribute to wound hypoxia as well. Depending on factors such as these, it is important to recognize that wound hypoxia may range anywhere from near anoxia to mild to modest hypoxia. In this context it is also important to appreciate that point measurements performed in the wound tissue may not provide a complete picture of the wound tissue biology because it is likely that the magnitude of wound hypoxia is not uniformly distributed throughout the affected tissue, especially in large wounds. This is most likely the case in chronic wounds presented clinically as opposed to experimental wounds which are more con-trolled and homogeneous in nature. In any single problem wound presented in the clinic, it is likely that there are pockets of near anoxia as well as that of differ-ent grades of hypoxia (Fig. 14.14). As the weakest link in the chain, tissue at the near-anoxic pockets will be vulnerable to necrosis which in turn may propagate secondary tissue damage and infection. Pockets of extreme hypoxia may be flooded with hypoxia-inducible angiogenic factors but would fail to vascularize func-tionally because of insufficient O2 that is necessary to fuel the repair process. Indeed, uncontrolled expression

Fig. 14.14 Heterogeneous distribution of oxygen in the wound tissue: pockets of graded levels of hypoxia. Shade of blue represents graded hypoxia. Shade of red or pink represents oxygenated tissue. Tissue around each blood vessel is dark pink in shade, representing regions that are well oxygenated (oxygen-rich pockets). Bacteria and bacterial infection are presented by shades of green on the surface of the open wound.

Pocket of oxygenation

Pocket of hypoxia

Near-anoxic necrotic core

Endothelialprogenitor cell

Endothelialcell

Myofibroblast

Basal layer

Dermis

Keratinocyte inepidermis

Stratumgranulosum

Stratum corneum

Neutrophil

Bacteria

Macrophage

258 Woundhealing14

systems have been studied for their effect on wound healing. While these approaches may compensate for the deficiency of ATP per se in the ischemic wound tissue, they will fail to address the other essential func-tions of O2 and its derivatives in wound healing, as discussed below.Absolute requirements for O2 arise in several points

along the angiogenic sequence. For instance, all vessels require a net or sheath of ECM, mainly collagen and proteoglycans, to guide tube formation and resist the pressures of blood flow. Conditions for collagen deposi-tion and polymerization can be created only if molecu-lar O2 is available to be incorporated into the structure of nascent collagen by prolyl- and lysyl hydroxylases. Without the obligatory extracellular hydroxylated col-lagen, new capillary tubes assemble poorly and remain fragile.8284 This has a convincing clinical correlate in scurvy, i.e., ascorbate deficiency. Scurvy results from insufficient intake of ascorbate which is required for correct collagen synthesis in humans. Ascorbate is required for the posttranslational hydroxylation of col-lagen that enables the matured collagen molecules to escape to the extracellular space and provide the neces-sary tensile strength. In scurvy, the collagenous sheath cannot form because, under ascorbate-deficient condi-tions, collagen cannot be hydroxylated. Consequently, new vessels fail to mature. Older vessels weaken and break, and wounds fail to heal. Thus, while hypoxia is a proved instigator of molecular signals for angiogen-esis, it is also a proven enemy of vessel growth itself in nontumor tissues. Collagen deposition proceeds in direct proportion to pO2 across the entire physiologic range, from zero to hundreds of mmHg. The Km for O2 for this reaction is approximately 25 and the Vmax is approximately 250 mmHg, suggesting that new vessels cannot even approach their greatest possible rate of growth unless the wound tissue pO2 is high.85 Angiogenesis is directly proportional to pO2 in injured tissues.83 Hypoxic wounds deposit collagen poorly and become infected easily, both of which are problems of considerable clinical significance.79

Redox signaling

Additional high demand for oxygen is placed by a family of enzymes known as NADPH oxidases, which are known to be highly active at the wound site.86 Recent

Venous insufficiency, on the other hand, is the root cause of most leg ulcers. Chronic venous insufficiency, characterized by the retrograde flow of blood in the lower extremity, is associated with changes in the venous wall and valves generally caused by inflammatory dis-orders induced by venous hypertension and associated fluid shear stress. Factors causing arterial hypoxemia may also limit O2 supply to the wound tissue. Compromised pulmonary health, loss of hepatic func-tion, hemodialysis, anemia, altitude hypoxemia, nitro-glycerin therapy, nasal packing, critical illness, pain, and hypothermia are examples of conditions associated with arterial hypoxemia. Vasoconstricting drugs may contribute to tissue hypoxia as well.79

Increased energy demand of the healing tissue leads to a hypermetabolic state wherein additional energy is generated from oxidative metabolism, increasing the O2 demand of the healing tissue. Adenosine triphosphate (ATP) thus generated powers tissue repair. At the injury site, extracellular ATP may be contributed by platelets and other disintegrating cells. Extracellular ATP liber-ated during hypoxia or inflammation can either signal directly to purinergic receptors or, after phosphohydro-lytic metabolism, can activate surface adenosine recep-tors. Purinergic signaling may influence numerous aspects of wound biology, including immune response, inflammation, vascular as well as epithelial biology. ATP may be immunostimulatory or vice versa, depend-ing on extracellular concentrations as well as on expres-sion patterns of purinergic receptors and ectoenzymes. Extracellular ATP induces receptor activation in epithe-lial cells. ATP, released upon epithelial injury, acts as an early signal to trigger cell responses, including an increase in heparin-binding epidermal growth factor (EGF)-like growth factor shedding, subsequent transac-tivation of the EGF receptor and its downstream signal-ing, resulting in wound healing. ATP released from the injured epithelial cells is now known also to turn on NADPH oxidases, the activity of which is critically required to produce the redox signals required for wound healing.80 Human endothelial cells are rich in purinergic receptors and therefore responsive to extra-cellular ATP as well. ATP induces endothelium-dependent vasodilation. Both ATP as well as adenosine regulate smooth muscle and endothelial cell prolifera-tion. Recognizing that hypoxia limits ATP synthesis in the ischemic wound tissue, therapeutic ATP delivery

259MicroRNAs

inherent tissue repair program that seeks to restore the injured tissue both structurally as well as functionally. There are two key steps that separate a protein-coding gene from its corresponding protein. First, the DNA hosting the gene must transcribe to mRNA. Finally, the mRNA must be translated to protein. Work emerging during recent years demonstrates that both of these critical steps are subject to robust and redundant regula-tion by microRNAs (miRNAs: 1922 nucleotides long), which are noncoding RNAs found in all eukaryotic cells. Work during the past decade recognizes small RNAs as a new class of regulators of eukaryotic biology. Alongside other small interfering RNAs (siRNAs), miRNAs execute posttranscriptional gene silencing through mRNA destabilization as well as translational repression. In simple words, whether a gene would code a protein or not is decided upon by miRNAs for which the gene is a target. miRNAs form basepairs with specific sequences in protein-coding mRNAs. Near-perfect pairing induces cleavage of the target mRNA, whereas partial pairing results in translational repres-sion and mRNA decay through deadenylation path-ways.105 According to the miRbase database, the human genome encodes 1048 miRNAs. This count is rapidly growing. These miRNAs may regulate more than a third of all protein-coding genes and virtually all biological processes. Mammalian cells express cell type-specific miRNAs which silence unique subsets of target genes within the cell. While miRNAs are mostly known for being functional in the cytoplasm, nuclear miRNAs may also participate in gene regulation. Initially considered an oddity, miRNA-dependent control of gene expres-sion is now accepted as being integral to the normal function of cells and organisms. miRNAs are emerging as key regulators of the overall wound-healing process.106

Inflammation

Disruption of miRNA biogenesis has a major impact on the overall immune system. Emerging studies indicate that miRNAs, especially miR-21, miR-146a/b, and miR-155, play a key role in regulating several hubs that orchestrate the inflammatory process.107 miRNAs have been directly implicated in the pathogenesis of inflam-matory diseases such as osteoarthritis and rheumatoid arthritis. Resolvin-regulated specific miRNAs target genes involved in resolution of inflammation establish

work has identified that oxygen is not only required to disinfect wounds and fuel healing but that oxygen-dependent redox-sensitive signaling processes repre-sent an integral component of the healing cascade.80 The widely held notion that biological free radicals are necessarily agents of destruction is now facing serious challenge.87 Over a decade ago it was proposed that in biological systems oxidants are not necessarily always the triggers for oxidative damage and that oxidants such as H2O2 could actually serve as signaling messen-gers and drive several aspects of cellular signaling.88 Today, that concept is much more developed and mature. Evidence supporting the role of oxidants such as H2O2 as signaling messenger is compelling.8999

Nitric oxide

At the wound site, NO is generated by an oxygen-dependent biosynthetic process. In the late 1970s, research was unfolding that implicated NO involve-ment in the process of vasodilation. By 1986, research culminated in the identification of NO as the endothelium-derived relaxing factor responsible for the maintenance of vascular tone, thus implicating NO as a potential wound-healing agent.100 Maximal NO synthase activity is noted early in cutaneous wound healing, with sus-tained production up to 10 days after wounding. Wound macrophages represent a major source of NO produc-tion in the early phase of wound healing.101 Inhibition of wound NO synthesis lowered wound collagen accu-mulation and wound-breaking strength, suggesting that NO synthesis is critical to wound collagen accumulation and acquisition of mechanical strength. Later it was demonstrated that wound fibroblasts are phenotypi-cally altered during the healing process to synthesize NO, which, in turn, regulates their collagen synthetic and contractile activities.102 The blockade of NO synthe-sis impairs cutaneous wound healing, acting in early and late phases of wound repair.103 Interestingly, impaired diabetic wound healing is associated with decreased wound NO synthesis.104

MicroRNAs

Wound healing is largely dependent on injury-inducible protein-coding genes as they serve as drivers of an

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hypoxamir. Expression of hypoxia-inducible factor 1 (HIF-1) is also controlled by specific miRNAs. In turn, HIF-1 controls the expression of hypoxamirs, which are induced in the injured tissue.117 Hypoxamirs are also induced by HIF-independent pathways. Although hypoxamirs generally favor angiogenesis, their meta-bolic and cell cycle arrest functions are in conflict with wound healing, especially in an ischemic setting. Silencing specific hypoxamirs may therefore represent a prudent approach to facilitate tissue repair. miRNA-210 represses mitochondrial respiration116 and exaggerates production of undesired mitochondrial ROS.118 These outcomes are not compatible with the higher energy demands associated with tissue repair. miR-210 also silences signaling via FGF,119 a key contributor to wound healing. The injured tissue is highly rich in ROS.86 In addition, at the site of injury transition metal ions are released from a protein-bound state. Conditions such as these cause DNA damage which opposes tissue repair. DNA repair systems are therefore of key significance in enabling tissue repair. miR-210 antagonizes DNA repair.120 This is another hypoxamir function that is in conflict with wound healing. Compatible with the common observation that ischemic wounds are refrac-tory to healing response, elevated miRNA-210 in ischemic wounds attenuated keratinocyte proliferation and impaired wound closure.106,121

Stem cells

Endogenous miRNA-binding sites have been identified in murine embryonic stem cells (ESCs). miRNAs govern ESCs function by serving as control hubs managing regulatory networks. A central importance of such gov-ernance is highlighted by the observation that ESCs lacking miRNAs lose their stemness. ESCs with defi-cient miRNA biogenesis systems switch to a mode of ongoing cell division. They do not differentiate on demand because of failure to turn off the pluripotency regulatory program.122 miRNAs conduct the orchestra of critical gene regulatory networks controlled by pluripotency factors within stem cells. Individual miRNA-dependent pathways that promote the repro-gramming of somatic cells into induced pluripotent stem (iPS) cells have been now identified. Manipulation of specific cellular miRNAs helps enhance reprogram-ming of somatic cells to an ESC-like phenotype, helping

a novel resolution circuit involving RvD1 receptor-dependent regulation of specific miRNAs.108 Fur-thermore, the brain-specific microRNA-124 can tame inflammation by turning off activated microglial cells and macrophages.109 Of relevance to tissue repair is also the regulatory loop where cytokines, including those elicited following injury, are regulated by miRNAs as well as regulate miRNA expression.110,111

Angiogenesis

In 20052008, the first series of observations establishing key significance of miRNAs in the regulation of mam-malian vascular biology came from experimental studies involved in arresting miRNA biogenesis to deplete the miRNA pools of vascular tissues and cells.112 Dicer-dependent biogenesis of miRNA is required for blood vessel development during embryogenesis. Mice with endothelial cell-specific deletion of Dicer, a key enzyme supporting biogenesis of miRNAs, display defective postnatal angiogenesis. NADPH oxidase-derived ROS drive wound angiogenesis. Endothelial NADPH oxidase is subject to control by miRNAs.113 Hypoxia is widely recognized as a cue that drives angiogenesis as part of an adaptive response to vascularize the oxygen-deficient host tissue. Hypoxia-sensitive miR-200b is involved in such induction of angiogenesis via directly targeting Ets-1.114 Various aspects of angiogenesis, such as prolif-eration, migration, and morphogenesis of endothelial cells, can be regulated by specific miRNAs in an endothelial-specific manner. miRNAs known to regu-late angiogenesis in vivo are referred to as angiomiRs.115 miRNA-126 is specific to endothelial cells and regulates vascular integrity and developmental angiogenesis. Manipulating angiomiRs in the setting of tissue repair represents a new therapeutic approach that could be effective in promoting wound angiogenesis.

Hypoxia response

Tissue injury is often associated with disruption of vas-cular supply to the injury site. Thus, the injured tissue often suffers from insufficient oxygen supply or hypoxia. Under conditions of additional underlying ischemia, hypoxia is severe and seriously limits wound healing.79 Hypoxia induces specific miRNAs, collectively referred to as hypoxamirs.116 miRNA-210 is a classical

261Stemcells

lost. Transdifferentiation is another naturally occurring mechanism that takes dedifferentiation a step further and sees cells regressing to a point where they can switch lineages, allowing them to differentiate into another cell type. Furthermore, reprogramming aims to revert differentiated cells to pluripotency. From here, they can differentiate into almost any cell type. Although reprogramming occurs naturally during fertilization to produce totipotent cells that can differentiate into any

to generate iPS cells.123 Expression of miRNAs is also subject to control by epigenetic factors.124 Such control influences the balance between proliferation and dif-ferentiation of stem cells. In executing such control, the miRNA element of epigenetics cross-talks with changes in chromatin structure as well as with changes in DNA methylation. Collectively, this provides for a mecha-nism by which the tissue injury microenvironment can influence miRNA-dependent reparative and regenera-tive processes.

Stem cells

The regenerative potential of injured adult tissue sug-gests the physiological existence of cells capable of par-ticipating in the reparative process. The epithelium of the skin, the epidermis, is in a continuous equilibrium of growth and differentiation and has the remarkable capacity to self-renew completely, which relies on reser-voirs of stem cells. In mammals, there are two broad types of stem cells: ESCs that are isolated from the inner cell mass of blastocysts, and adult stem cells that are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenished in adult tissues. Stem cells have the prop-erty of self-renewal without differentiation and have the potential to differentiate into any type of cell. Totipotent stem cells have the ability to give rise to a whole organ-ism due to their ability to differentiate into embryonic and extraembryonic cells. Pluripotent stem cells are derived from totipotent cells and can differentiate into all cell types but cannot give rise to a new organism (Fig. 14.15). ESCs are derived from the developing embryo, usually from the inner cell mass of a blastocyst or earlier morula stages. Lost or damaged cells can be replaced by differentiation, dedifferentiation, or transdifferentiation (Fig. 14.16). Recent advances have shown that the addi-tion of a group of genes can not only restore pluripo-tency in a fully differentiated cell state (differentiation) but can also induce the cell to proliferate (dedifferentia-tion) or even switch to another cell type (transdifferen-tiation). Dedifferentiation is represented by a terminally differentiated cell reverting back to a less-differentiated stage from within its own lineage. This process allows the cell to proliferate again before redifferentiating, leading to the replacement of those cells that have been

Fig. 14.15 Stem cell renewal.

White blood cellsPlateletsErythrocytes

Totipotent cells

Pluripotent stem cell

Other committedstem cells

Bloodstem cells

Specialized cells

Fig. 14.16 Stem cell differentiation, dedifferentiation, and transdifferentiation.

White bloodcells

Muscle Nerve Bone

Othertissues

Red bloodcells

Totipotentcells

Trans-differentiation

De-differen

tiation

Retro-differen

tiation

Differentiation

Re-differentiationPluripotentstem cell

Progenitor cells

Bloodstemcells

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stem/progenitor cells derived from the bone marrow could home to injured tissues and participate in the repair/regeneration. Moreover, culture-expanded bone marrow-derived MSCs have been shown to promote the healing of diabetic wounds, implying a profound thera-peutic potential for skin defects such as chronic wounds and burns.126

During the inflammatory phase, leukocytes migrat-ing to the wound site are hematopoietic cells derived from the bone marrow. In the skin the bulge of the hair follicle (Fig. 14.18) plays a role as a reservoir of stem cells. In mice it has been shown that the bulge region contains hematopoietic cells that are identical to the

cell type, it has not yet formally been shown to be a genuine regenerative response. Furthermore, repro-gramming sidesteps the necessity of using embryos for regenerative therapies by using differentiated cells taken from a patient. From a clinical perspective, this comes with the additional bonus of circumventing the immunological problems such as transplant rejec-tion and graft-versus-host disease associated with engraftment.125

The bone marrow (Fig. 14.17), home of stem and pro-genitor cells, contributes a significant proportion of cells in the skin. The normal skin has long been known to contain bone marrow-derived cells that are involved in host defense and inflammatory processes, including wound healing. However, recent studies demonstrate that the bone marrow contributes not only inflamma-tory cells, but also keratinocytes and fibroblast-shaped cells to the skin. Similar to leukocytes in trafficking,

Fig. 14.17 Bone marrow stem cells. CNS, central nervous system.

Blood cells

Skeletalmuscle

Fat cell

Bone marrowstromal cell

Cardiac muscle

Neuron

Glial cell

Epithelialcell

Liver

CNSstem cells

Bloodvessel

Fig. 14.18 Stem cells in the epidermis. Hair follicles act as reservoirs of stem cells in the skin. (A) Cross-sectional view of a hair follicle. The matrix stem cells differentiate into various parts of the hair while the long-term stem cells are present in the bulge region. The stem cells from the bulge maintain the sebaceous gland and epidermal stem cells. (B) A mammalian gut crypt. Stem cells are located at the basal region with the Paneth cells. The transit amplifying (TA) cells are stem cell progeny which move upwards and differentiate.

TA cell

Hair shaft

Outer root sheath

Inner root sheath

Dermal papilla

Basementmembrane

Paneth cell

Basement membrane

Stem cell

Mesenchymal cell

Matrix stem cells

Bulge stem cells

Sebaceous gland

A

B

263Stemcells

protective barrier against its external environment. With rare exceptions (such as palms and eyelids), hair folli-cles are present all over the skin and play an important role in physiological tissue renewal and regeneration after injury. Hair follicles represent an autonomous min-iorgan, which provides an excellent model system for studying the biology of adult stem cells.141 There are obvious differences between human and mouse skin, yet the knowledge gained from lineage-tracing experi-ments in mice has greatly expanded our understanding of the cellular behavior of the different keratinocyte populations during physiological and injury-induced tissue regeneration. As in most of the bodys organs, the skin experiences constant renewal. The upper kerati-nized layer of the interfollicular epidermis, which con-sists of terminally differentiated cells, is shed and replaced by cells originating from the actively prolifer-ating layer beneath. In contrast, hair follicles undergo cycles of growth (anagen) and rest (telogen). The poten-tial for proliferation to sustain the lifelong replenish-ment of normal cell loss and to repair occasional tissue damage lies within the population of epidermal stem cells. The hair follicle bulge harbors stem cells that may help in renewal and repair of the skin. The term bulge, originally called der Wulst, was introduced in 1903 by a German morphologist, P. Sthr, to describe an eminent structure at the site of attachment of the arrector pili muscle in human hair follicles.141 Similar to many o