Examples of alkalis include lime, cement, potassium hydroxide, and bleach. Acid injuries induce protein hydrolysis and do not penetrate tissue as readily as alkalis. However, acid reactions with skin are exothermic and may cause coincident thermal injury, and extensive acid injuries are associated with electrolyte imbalances. One acid worth noting is hydrofluoric acid due to its unique mechanism of injury and the ability to treat it with topical or systemic calcium. Finally, hydrocarbons such as organic solvents are capable of dissolving cell membranes and producing skin necrosis. Systemic absorption of hydrocarbons is associated with respiratory depression and hepatic toxicity [5].

Radiation injuries, which can be accidental or iatrogenic, are known to cause shortand long-term sequelae. The concept of acute radiation syndrome (ARS) was developed in recent years to describe the adverse effects of large doses of ionizing radiation on the skin. The basal skin layer is damaged, which results in inflammation, erythema, and desquamation. Blistering and ulceration may follow in days to weeks, and most wounds will heal normally, though larger doses may result in destruction of skin appendages, fibrosis, abnormal pigmentation, and ulceration or necrosis of exposed tissue. Acute ionizing radiation exposure is also associated with dysfunction of hematopoietic, gastrointestinal, and cerebrovascular, and systems [7].

Mechanisms of wound healing

Wound healing is classically divided into four phases: hemostasis, inflammation, proliferation, and remodeling. Considerable overlap exists between each phase, and a combination of biochemical and cellular events contributes to the natural continuum of tissue repair.

Hemostasis

The initial phase of wound healing is characterized by a coordinated effort between circulating platelets, soluble clotting factors, and vascular endothelium to stop hemorrhage by formation of a clot. The key sequences of events are divided into the (1) coagulation cascade and (2) platelet activation, although it

is important to remember the fundamentally integrated nature of these processes.

Hemostasis is initiated by a chain reaction of soluble serum proteins to form an insoluble fibrin mesh. The coagulation cascade has been historically grouped into intrinsic and extrinsic pathways, which have since been renamed into the contact activation and tissue factor pathways, respectively. The initial reactions of the two enzyme cascades are unique with a final common pathway consisting of factors X, V, and thrombin. The primary pathway for blood coagulation is thought to be the tissue factor pathway, with the contact activation pathway playing a secondary role. The end result of the clotting cascade is the generation of fibrin, which serves to enhance platelet aggregation and structurally reinforce the ensuing platelet plug [4]. Topical fibrin sealants have been used clinically to promote hemostasis and even skin graft adhesion in burn wounds [8].

Disruption of normal endothelium exposes subendothelial collagen and thrombogenic extracellular matrix molecules, most notably von Willebrand factor (vWF). Platelets adhere to vWF via the glycoprotein (GP) Ib receptor, which strengthens the interaction between platelets and underlying extracellular matrix. Individuals with vWF deficiency are known to have von Willebrand disease, which stands as the most common hereditary coagulation deficiency. Likewise, mutations in the GPIb receptor result in Bernard-Soulier syndrome. Both of these conditions result in bleeding tendencies because of altered platelet adhesion to exposed subendothelium [4].

Platelet adhesion leads to platelet activation, invoking the release of stored granule contents. Environmental cues from the wound environment such as hypoxia and acidosis are known to enhance platelet degranulation [9]. Alpha-granules store a number of growth factors such as platelet factor-4 (PF4), platelet derived-growth factor, fibronectin, vWF, and fibrinogen [10]. Many of these substances serve to enhance platelet adhesion or activation. PF4 binds with high affinity to endothelial-derived heparin, which serves to inactive the molecule and promote coagulation. It is the PF4-heparin complex on platelet membranes to which antibodies bind in the syndrome of heparin-induced thrombocytopenia (HIT), which can lead to dangerously low levels of platelets with a paradoxical increase in thrombosis [4].

Dense granules harbor smaller molecules involved in platelet activation such as ADP, ATP, calcium, and serotonin. Release of these molecules into the platelet cytosol initiates a Gq-linked protein receptor cascade, which results in an increased cytosolic calcium concentration. The calcium activates protein kinase C, which, in turn, activates phospholipase A2 (PLA2), and eventually modifies the integrin membrane glycoprotein IIb-IIIa [4].

The platelet glycoprotein IIb-IIIa receptor deserves mention because of its relevance to cardiovascular medicine and disease. The natural ligand of GPIIb-IIIa is fibrinogen, which serves to link the coagulation cascade with platelet activation. Platelet activation leads to increasing its affinity to bind fibrinogen, which enhances platelet aggregation and clotting factor-mediated coagulation. The activated platelets change shape from spherical to stellate, and the fibrinogen cross-links with glycoprotein IIbIIIa receptors in neighboring platelets to promote aggregation and eventual clot formation [4].

The GPIIb-IIIA receptor is the target of several antiplatelet agents including abciximab, eptifibatide, and tirofiban. Similarly, the drug clopidogrel is known to inhibit ADP binding to the GPIIb-IIIA receptor, which results in a reduced ability of platelets to aggregate and consequently form clots. Mutations in the GPIIb-IIIa receptor lead to Glanzmann’s thrombasthenia, which leads to bleeding tendencies from impaired platelet aggregation [4].

Inflammation

Vasoconstriction occurs at the wound site immediately after injury, which may be considered the beginning of the second event in wound healing: inflammation. Vasoconstriction is primarily mediated by catecholamines (epinephrine and norepinephrine), prostaglandin F2 , and thromboxane A2. The contraction of blood vessels aids in platelet aggregation and hemostasis. Vasoconstriction is followed shortly by vasodilatation and increased vascular permeability, which allows access of inflammatory cells to the damaged tissue. Vasodilatation is mediated by prostaglandin E2, prostacyclin, histamine, serotonin, and kinins [10]. Inflammatory cells undergo a three-stage process of rolling along the vascular endothelium, integrin-mediated adhe-

sion to endothelial cells, and transmigration into the extracellular space [11].

Neutrophils are the first inflammatory cell to arrive at the wound and play a primary role in the phagocytosis of bacteria and tissue debris. A huge array of molecular signals serves as chemoattractant agents for neutrophils, including products of platelet degranulation, formyl methionyl peptides cleaved from bacterial proteins, and the degradation products of matrix proteins. Neutrophils are a major source of early cytokines in the systemic response to injury, including tumor necrosis factor (TNF)- [5].

The second cell to arrive at the wound site is the monocyte, which undergo phenotypic changes into macrophages. Macrophages can be regarded as the “master cell” involved in wound healing because of their central role in phagocytosis, inflammatory cell recruitment, and systemic inflammation. Macrophages release a variety of growth factors such as transforming growth factor- (TGF- ), platelet-de- rived growth factor (PDGF), and fibroblast growth factor (FGF), which induce fibroblast proliferation and extracellular matrix production [12]. Macrophages express specific receptors for IgG (Fc receptor), complement C3b (CR1 and CR3), and fibronectin (integrins) that facilitate recognition and phagocytosis of opsonized pathogens [13]. Importantly, macrophages also secrete cytokines such as IL-1 and TNFthat modulate the systemic response to injury. Excessive production of TNFhas been linked with multisystem organ failure as well as chronic non-healing ulcers [14]. Both IL-1 and TNFappear to play crucial roles in early wound healing, but may have an inhibitory effect on wound maturation if persistently elevated.

Emerging data also suggests a role for nerve-de- rived neuropeptides in wound repair. Stimulation of efferent nerves is known to induce local vasodilation and plasma extravasation in skin, which contributes to the local inflammatory response. The neuropeptide substance P, released from terminal endings of sensory nerves in response to noxious stimuli, is known to influence inflammatory cell chemotaxis [15, 16], angiogenesis [17, 18], and keratinocyte proliferation [19]. We have previously suggested that dysregulated neuroinflammation plays an important role in hypertrophic scarring, evident by increased levels of substance P and decreased levels of the regulating

enzyme neutral endopeptidase [20], which is responsible the exuberant matrix production, hyperemia, and pruritus seen in this condition [21].

A robust but appropriate inflammatory response is essential to prepare the wound bed for subsequent migration of proliferative cells. However, an overzealous inflammatory response may inhibit the formation of granulation tissue and neovascularization. Experiments in mice constitutively expressing the chemotactic cytokine interferon-inducible protein 10 demonstrate that an intense inflammatory infiltrate inhibits angiogenesis and development of healthy granulation tissue [22]. Thus, as in all homeostatic systems, a careful balance of functionalized cellular and biochemical processes is essential to proper wound healing.

Proliferation

The proliferative phase is characterized by the formation of granulation tissue, which is a pink, soft, highly vascularized platform for tissue formation. Granulation tissue is largely a product of two cell types: fibroblasts and vascular endothelial cells. Fibroblasts are evident at the wound site within 2–5 days and become the predominant cell type after the first week [4]. Migration of fibroblasts is driven by a number of chemokines secreted by macrophages including TNF- , PDGF, FGF, and TGF- . Fibroblasts begin to deposit collagen and other extracellular matrix molecules that strengthen the wound bed. Macrophages stimulate fibroblasts to produce FGF-7 (keratinocyte growth factor) and IL-6, which promote keratinocyte migration and proliferation. IL-6 is also potent stimulator of fibroblasts, which explains the decreased level found in aging fibroblasts and fetal wounds [23]. Although essential to normal wound healing, granulation tissue also harbors high bacterial counts and proteolytic activity, which may require that it is excised before skin grafting. Granulation tissue in a burn wound prevents epithelialization and likely leads to hypertrophic scar formation [24].

A number of other inflammatory cytokines may find clinical relevance in wound care. IL-8 is secreted by macrophages and fibroblasts early in wound healing and may have a stimulatory effect on keratinocytes and epithelialization. Topical application of

IL-8 to human skin grafts in a chimeric mouse model enhanced keratinocyte proliferation and re-epitheli- alization [25]. Additionally, in both human and animal studies, wound strength and healing time has been improved with topical application of PDGF [26].

TGFis expressed by platelets and fibroblasts in the wound bed and plays an important role in collagen deposition and turnover. TGFis the most potent known stimulator of fibroblast proliferation and can accelerate wound healing in steroid-treated and irradiated animals [27]. Overexpression of TGFmRNA has been found in keloid and hypertrophic scars, whereas fetal wounds contain relatively little TGF- [28]. This contrast between the heavily fibrotic scars of keloid and the scarless repair observed in utero may underscore the importance of TGFin the fibrotic response to tissue injury. A similar phenomenon has been observed in burn injuries, where higher levels of TGFcorrelate with excessive wound contraction [29]. Interestingly, exogenous application of TGF- 3 appears to reduce monocyte and macrophage recruitment to the wound site, resulting in less deposition of collagen and fibronectin in the early stages of wound healing and eventually less scarring [30]. A clinical formulation of TGF- 3 (Juvista) is currently undergoing evaluation in phase 2 trials for use in dermal scarring [31].

Coincident with fibroblast migration to the wound site is angiogenesis. Angiogenesis, or neovascularization, was historically considered a critical element of early wound healing to provide adequate transport of metabolites to and from the regenerating tissue; more recent data suggest that normal healing can occur when angiogenesis is inhibited and that most angiogenesis in the wound bed is not associated with increased blood flow to the wound [32].

Vascular endothelial cells in the wound bed arise from both preexisting blood vessels and endothelial progenitor cells (EPCs) in bone marrow. The most important regulators of angiogenesis are vascular endothelial growth factor (VEGF) and FGF-2. A dose-dependent effect of both VEGF and FGF-2 has been observed in angiogenesis [33]. VEGF is secreted as many different isoforms from a variety of stromal and mesenchymal cells, with the tyrosine kinase VEGF-receptor 2 emerging as the most preeminent in angiogenesis. VEGF/VEGFR2 signaling is involved in EPC migration from bone marrow, as well

as promotion of endothelial cell proliferation and differentiation [34].

Hypoxia is a potent inducer of both angiogenesis and fibroblast proliferation. The major player in hypoxic gene expression has emerged as hypoxia-induc- ible factor 1 (HIF-1), a DNA-binding transcription factor that is known to alter gene transcription of a number of proteins involved in metabolism, angiogenesis, migration, and proliferation [35]. Cultured endothelial cells upregulate expression of several pro-angiogenic molecules when cultured in hypoxia, including endothelin-1, VEGF, and PDGFchain [36]. Fibroblast replication and longevity are increased in low oxygen tension culture [37], as is TGFsecretion [36]. These observations highlight the contribution of hypoxia in the wound bed in proliferative cell signaling.

Recently, the role of T-lymphocytes in wound healing is under increased investigation. T-cells migrate into the wound bed during the late proliferative and early remodeling phase. Mice deficient in T- and B-cells have a reduced capacity to scar [38], though contradictory reports exist concerning the beneficial effects of CD4+ and CD8+ lymphocytes on wound healing [39, 40]. Additionally, a unique type of T-cell exists in the skin, known as dendritic epidermal T- cells, which are thought to modulate many aspects of wound healing such as inflammation, host defense, and maintenance of tissue integrity. Mice lacking or defective in dendritic epidermal T-cells show delayed wound closure and decreased keratinocyte proliferation at the wound site [41, 42].

Epithelialization

Epithelialization is the third important concomitant event in wound repair and the most clinically significant evidence of wound closure. Keratinocytes migrate from wound edges and dermal appendages such as hair follicles, sweat glands, and sebaceous glands. Subsequent proliferation of these cells at the wound site provides a neo-epidermal covering. A discrete sequence of events has been identified in keratinocyte migration and proliferation, which involves disassembly of hemidesmosomes and desmosomes, retraction of intracellular tonofilaments and keratin filaments, and formation of focal contacts and cytoplasmic actin filaments [43]. The interplay

between laminin, MMPs, integrins, and soluble growth factors has been extensively studied in this process [44].

Renewal of keratinocytes during normal homeostasis and wound repair is a defining feature of reepithelialization. The upper region of hair follicles below the sebaceous gland (known as the bulge) contains multipotent progenitor cells that contribute to maintenance and renewal of epithelium [45, 46]. Additionally, epidermal cells are known to migrate from neighboring unwounded epidermis or from the infundibulum, the portion of the hair follicle between the epidermis and the sebaceous gland [47]. The role of epidermal appendages is especially evident in partial thickness burns, where advancement of the epidermal tongue is limited to approximately 1 cm from an epidermal appendage source. Full-thickness burns greater than 2 cm rarely heal other than by contraction.

The relative contributions of follicular stem cells and epidermal stem cells to re-epithelialization is debatable, although genetic analyses have confirmed that the epidermis has intrinsic capacity for self-renewal and does not depend on follicule-de- rived multipotent progenitor cells [48, 49]. Further evidence for this notion comes from reports of de novo hair follicle generation in the healing skin of adult mice [50]. This phenomenon, which has never been observed in human, is contingent upon Wntmediated signaling, which is also involved in pattern formation and the epithelial-mesenchymal transformation during embryogenesis [51]. Elucidation of the overlapping pathways in wound repair and development is a central principle of efforts toward scarless repair and skin regeneration.

Remodeling

The remodeling phase depicts the replacement of granulation tissue with scar. A key feature of tissue remodeling that emerges during this stage of wound healing is the balance between ECM synthesis and degradation. While fibrogenic growth factors such as PDGF and FGF stimulate fibroblast matrix deposition, resident cells induce continuous degradation of extracellular matrix by matrix metalloproteases (MMPs). MMPs are a family of zinc-proteases that are capable of degrading a variety of ECM compo-