Wound-healing physiology and factors that affect the repair process

Partial-thickness wounds involve only partial loss of the skin layers; that is, they are confined to the epidermal and superficial dermal layers. Full-thickness wounds involve total loss of the skin layers (epidermis and dermis) and frequently involve loss of the deeper tissue layers as well (subcutaneous tissue, muscle, and bone) (see Figure 3-1). The timeframe for repair and the repair process itself differ significantly for partial-thickness and full-thickness wounds.

Acute wounds typically are traumatic or surgical in origin. Acute wounds occur suddenly, and in most individuals, move rapidly and predictably through the repair process and result in durable closure. In contrast, chronic wounds fail to proceed normally through the repair process. Chronic wounds frequently are caused by vascular compromise, chronic inflammation, or repetitive insults to the tissue, and they either fail to close in a timely manner or fail to result in durable closure (Brissett and Hom, 2003; Pradhan et al, 2009).

Classification of repair as primary-, secondary-, or tertiary-intention healing is based on the ideal of primary surgical closure for all wounds. Primary closure minimizes the volume of connective tissue deposition required for wound repair and restores the epithelial barrier to infection (Figure 4-1, A). Approximated surgical incisions are said to heal by primary intention; they usually heal quickly, with minimal scar formation, as long as infection and secondary breakdown are prevented. Wounds that are left open and allowed to heal by scar formation are classified as healing by secondary intention (Figure 4-1, B). These wounds heal more slowly because of the volume of connective tissue required to fill the defect. They are vulnerable to infection because they lack the epidermal barrier to microorganisms until late in the repair process. These wounds are characterized by prolongation of the inflammatory and proliferative phases of healing. Chronic wounds such as pressure ulcers and dehisced incisions typically heal by secondary intention. Wounds managed with delayed closure are classified as healing by tertiary intention, or delayed primary closure (Figure 4-1, C). This approach is sometimes required for abdominal incisions complicated by significant infection (Gabriel et al, 2010). Closure and/or approximation of the wound is delayed until the risk of infection is resolved and the wound is free of debris.

Partial-thickness wounds are shallow wounds involving epidermal loss and possibly partial loss of the dermal layer (see Plate 1). Partial-thickness wounds typically are less than 0.2 cm in depth. These wounds are moist and painful because of the loss of the epidermal covering and the resultant exposure of nerve endings. When the wound involves loss of the epidermis with exposure of the basement membrane, the wound base appears bright pink or red. In the presence of partial dermal loss, the wound base usually appears pale pink with distinct red “islets.” These “islets” represent the basement membrane of the epidermis, which projects deep into the dermis to line the epidermal appendages. These islands of epidermal basement membrane are important in partial-thickness wound healing because all epidermal cells are capable of regeneration, and each islet will serve as a source of new epithelium (Hunt and Van Winkle, 1997; Winter, 1979).

The major components of partial-thickness repair include an initial inflammatory response to injury, epithelial proliferation and migration (resurfacing), and reestablishment and differentiation of the epidermal layers to restore the barrier function of the skin (Monaco and Lawrence, 2003; Winter, 1979). If the wound involves dermal loss, connective tissue repair (granulation tissue formation) will proceed concurrently with epithelial repair (Table 4-1) (Jahoda and Reynolds, 2001).

Epidermal repair.

Tissue trauma triggers the processes that result in epidermal repair: an acute inflammatory response followed by epidermal mitosis and migration (Harrison et al, 2006). The inflammatory response produces erythema, edema, and a serous exudate containing leukocytes. When this exudate is allowed to dry on the wound surface, a dry crust, commonly referred to as a “scab,” is formed. In partial-thickness wounds, the inflammatory response is limited, typically subsiding in less than 24 hours (Winter, 1979).

Epidermal resurfacing begins as the inflammatory phase subsides and is dependent on two processes: proliferation of epidermal cells throughout the wound bed and lateral migration of the epidermal cells at the leading edge. In order to migrate laterally, the keratinocytes at the wound edge must undergo several changes. First they must acquire a migratory phenotype. This involves breakdown of their attachments to adjacent cells, the basement membrane, and the underlying dermis. These attachments normally prevent lateral migration and provide epithelial stability. The keratinocytes then must undergo cytoskeletal alterations that support lateral movement. These alterations include flattening of the cells at the advancing edge of the epithelium and formation of protrusions known as lamellipodia, which attach to binding sites in the wound bed. The migratory keratinocytes then move across the wound bed by alternately attaching to the ECM and then detaching and reattaching at a more distal point (Harrison et al, 2006; Monaco and Lawrence, 2003).

The epithelial cells within the wound bed continue this pattern of lateral migration until they contact epithelial cells migrating from the opposite direction. Once the epithelial cells meet, lateral migration ceases; this phenomenon is known as contact inhibition (Hunt and Van Winkle, 1997; Monaco and Lawrence, 2003).

Epithelial resurfacing is supported by increased production of basal cells just behind the advancing edge and throughout the wound bed. The process typically peaks between 24 and 72 hours after injury. The rate of reepithelialization is affected by a number of factors of clinical significance. For example, resurfacing is promoted by maintenance of a moist wound surface. Winter (1979) found that partial-thickness wounds left open to air required 6 to 7 days to resurface, whereas moist wounds reepithelialized in 4 days. This is because cells can migrate much more rapidly in a moist environment (Figure 4-2). In contrast, when the surface of the wound is covered with a scab, migration is delayed while the epithelial cells secrete enzymes known as matrix metalloproteinases (MMPs). The MMPs lift the scab by cleaving the bonds that attach it to the wound bed, creating a moist pathway that supports keratinocyte migration (Harrison et al, 2006; Monaco and Lawrence, 2003; Winter, 1979). Interestingly, hypoxic conditions in the wound bed also serve to stimulate keratinocyte migration, probably through increased production of the MMPs that promote lateral migration. This positive response to wound bed hypoxia is lost in the elderly, which may be one factor contributing to slower rates of reepithelialization in the aged (O’Toole et al, 2008). In contrast, high bacteria levels may serve as an impediment to reepithelialization. Data indicate that keratinocyte migration is inhibited by bacterial byproducts such as lipopolysaccharide (Loryman and Mansbridge, 2008). In vivo evidence strongly suggests that elevated glucose levels are another major impediment to epidermal proliferation and migration. This may be one factor contributing to delayed healing of superficial wounds in diabetic patients with poorly controlled glucose levels (Lan et al, 2008).

The newly resurfaced epithelium appears pale, pink, and dry in people of all races (see Plate 1). Because it is only a few cell layers thick, the new epithelium is very fragile and requires protection against mechanical forces such as superficial shear and friction.

Once epithelial resurfacing is complete, the epithelial cells resume vertical migration and epidermal differentiation so that normal epidermal thickness and function are restored. The normal anchors to adjacent epidermal cells and to the basement membrane are reestablished (Harrison et al, 2006). The “new” epidermis gradually repigments, matching the individual’s normal skin tone (see Plate 1). This is clinically relevant, as the wound is not completely healed until repigmentation has occurred (Monaco and Lawrence, 2003).

Regulatory factors.

The processes of epithelial proliferation and migration are regulated by a complex interplay between various MMPs and selected growth factors. Specifically, lateral migration is dependent on normal levels and function of multiple MMPs, which serve to break the attachments that bind the edge keratinocytes to adjacent cells and the underlying wound bed. In addition to playing a key role in establishment of a migratory phenotype, MMPs facilitate continued resurfacing by assisting the migrating keratinocytes to detach from the wound bed. Repetitive detachment and distal reattachment are critical to the resurfacing process (Gill and Parks, 2008). Epithelial proliferation is dependent in part on keratinocyte attachment to the ECM and in part on exposure to growth factors. Keratinocyte attachment is required for the cell to exit from the G₀ phase of mitosis, and growth factors are required to stimulate cellular reproduction. A number of growth factors promote keratinocyte proliferation includeing transforming growth factor-α (TGF-α), keratinocyte growth factor, platelet-derived growth factor (PDGF), and epidermal growth factor (EGF). See Chapter 20 for a detailed discussion of the molecular factors regulating wound repair.

Full-thickness wounds, by definition, extend at least to the subcutaneous tissue layer and possibly as deep as the fascia–muscle layer or bone (see Plate 2). Full-thickness wounds may be either acute or chronic. This section addresses acute wound healing by primary intention, such as a surgical incision.

Many steps are involved in full-thickness repair, but they are commonly conceptualized as four major phases: hemostasis, inflammation, proliferation, and remodeling. Considerable overlap exists among the phases, and the cells involved in one phase produce the chemical stimuli and substances that serve to move the wound into the next phase. Thus normal repair is a complex and well-orchestrated series of events (Figure 4-3) and is affected by a number of systemic and local factors, including the wound environment.

Hemostasis.

Any acute injury extending beyond the epidermis causes bleeding, which activates a series of overlapping events designed to control blood loss, establish bacterial control, and seal the defect. Immediately upon injury, disruption of blood vessels exposes the subendothelial collagen to platelets, which trigger platelet activation and aggregation. Simultaneously, injured cells in the wound area release clotting factors that activate both the intrinsic and extrinsic coagulation pathways. As part of the coagulation pathway, circulating prothrombin is converted to thrombin, which is used to convert fibrinogen to fibrin. The end result is formation of a clot composed of fibrin, aggregated platelets, and blood cells. Hemostasis is further accomplished by a brief period of vasoconstriction mediated by thromboxane A₂ and prostaglandin 2-α, substances released by the damaged cells and activated platelets. Clot formation seals the disrupted vessels so that blood loss is controlled. The clot also provides a temporary bacterial barrier, a reservoir of growth factors, and an interim matrix that serves as scaffolding for migrating cells (Brissett and Hom, 2003; Monaco and Lawrence, 2003; Phillips, 2000).

Clot formation, followed by fibrinolysis (clot breakdown), is a critical event in the sequence of wound healing (Figure 4-4). The activation and degranulation of platelets cause the α-granules and dense bodies of the platelets to rupture, releasing a potent “cocktail” of energy-producing compounds and cytokines/growth factors (including complement factor C5a). These substances attract the cells needed to begin the repair process. They also provide fuel for the energy-intensive process of wound healing. The platelet-derived substances thought to be most critical to repair include PDGF, TGF-β, and fibroblast growth factor-2. Thus hemostasis, which is the body’s normal response to tissue injury, actually initiates the entire wound-healing cascade. The importance of hemostasis to wound healing is underscored by the finding that inadequate clot formation is associated with impaired wound healing and that the extrinsic coagulation pathway is critical to repair (Brissett and Hom, 2003; Monaco and Lawrence, 2003).

Proliferation.

The third phase of acute full-thickness wound healing is the proliferative phase. During this phase, the wound surface is covered with new epithelium that restores the bacterial barrier, vascular integrity is restored, and the incisional defect is mended with new connective tissue. The key components of the proliferative phase are epithelialization, neoangiogenesis, and matrix deposition/collagen synthesis. Limited contraction of the newly formed ECM also may occur.

Epithelialization.

Epithelialization of a full-thickness wound healing by primary intention begins within hours after injury and typically is complete within 24 to 48 hours. This “neoepithelium” is only a few cells thick but is sufficient to provide a closed wound surface and a bacterial barrier. This process is the basis for the Centers for Disease Control and Prevention’s recommendation that new surgical incisions be covered with a sterile dressing for the first 24 to 48 hours postoperatively. Until reepithelialization is complete, the potential for bacterial invasion exists, and the wound should be managed with sterile wound care and a cover dressing that provides a bacterial barrier (Mangram et al, 1999; Yamamoto et al, 2014). As with partial-thickness wound repair, the processes of lateral migration, vertical migration, and differentiation continue throughout the proliferative phase and gradually reestablish epidermal thickness and function (Braiman-Wiksman et al, 2007; Myers et al, 2007). In full-thickness wounds, the new epidermis is slightly thinner than the original epidermis. Because the neoepidermis is covering scar tissue rather than normal dermis, the rete pegs that normally dip into the dermis are lacking (Monaco and Lawrence, 2003).

Granulation tissue formation.

A hallmark outcome of the proliferative phase is the formation of granulation tissue, which begins as the inflammatory phase subsides, at 3 to 4 days postinjury (Braiman-Wiksman et al, 2007). Granulation tissue is composed primarily of capillary loops and newly synthesized connective tissue proteins and is often referred to as the ECM. Fibroblasts and inflammatory cells also are present in this new matrix. Neoangiogenesis and connective tissue synthesis occur simultaneously in a codependent fashion to form the new ECM that will fill the wound defect. Through angiogenesis, new capillaries are formed and joined with existing severed capillaries, thus restoring the delivery of oxygen and nutrients to the wound bed. At the same time, a new “provisional” ECM is formed through the synthesis of connective tissue proteins such as type III collagen (Figure 4-5) (Pradhan et al, 2009).

Neoangiogenesis.

Endothelial cells typically are quiescent. Neoangiogenesis requires stimulation by growth factors, which convert the quiescent cells to actively proliferating cells. Molecules in the ECM also play a critical role by influencing the response of endothelial cells to angiogenic growth factors (Cheresh and Stupack, 2008). Neoangiogenesis occurs by two mechanisms: production of new vessels by local endothelial cells and recruitment of circulating stem progenitor cells to form new vessels de novo (Thom, 2009).

The growth factors that stimulate production of new vessels are produced by the cells in the wound bed (injured endothelial cells, macrophages, fibroblasts, keratinocytes) and include vascular endothelial growth factor (VEGF), basic fibroblast growth factor, and possibly PDGF. The most important angiogenic growth factor is VEGF; in addition to stimulating local endothelial cells to proliferate and migrate, VEGF attracts stem progenitor cells to the wound bed and stimulates them to differentiate into endothelial cells (Braiman-Wiksman et al, 2007; Gill and Parks, 2008; Thom, 2009). A number of ECM factors also affect neoangiogenesis. One is the provisional matrix itself, which affects the migration of endothelial cells (Gill and Parks, 2008). Another is the level of TIMP-3, which has a very negative effect on angiogenesis. TIMP-3 binds to the receptor sites for VEGF and blocks its angiogenic effects (Gill and Parks, 2008). A third factor is the level of nitric oxide, which promotes angiogenesis by activating MMP-13 (Lizarbe et al, 2008).

Clinical factors affecting neoangiogenesis include oxygen levels within the wound bed, patient age, gender, diabetes, coronary artery disease, radiation therapy, and chemotherapy. Hypoxia acts as an initial stimulus to angiogenesis, but persistent hypoxia interferes with endothelial cell proliferation and new vessel formation. Hyperglycemia and glycolysis are associated with production of substances that are toxic to endothelial cells. Diabetes, aging, female gender, coronary artery disease, radiation therapy, and chemotherapy all interfere with angiogenesis by reducing mobilization of stem progenitor cells, thus compromising the potential for de novo development of new vessels. Aging is also associated with dysfunction and impaired mobilization of local endothelial cells, partly due to impaired expression of VEGF and other angiogenic stimuli. Of interest, some data suggest that exercise can partially reverse these age-related changes (Hoenig et al, 2008; Rodriguez et al, 2008; Thom, 2009). Therapeutic modalities that may promote angiogenesis include exogenous application of basic fibroblast growth factor and topical or systemic administration of erythropoietin (EPO). Animal studies and limited human case reports indicate potent stimulation of angiogenesis and granulation tissue formation with each of these therapies; EPO has also been associated with inhibition of excessive inflammation (Hamed et al, 2014; O’Goshi and Tagami, 2007).

Matrix deposition/collagen synthesis.

Fibroblasts are responsible for synthesis of the connective tissue proteins that compose the provisional ECM; therefore fibroblasts are critical to the repair process. Fibroblasts migrate into the wound bed from the surrounding tissues in response to growth factors and interleukins released by degranulating platelets, activated leukocytes (neutrophils and macrophages), and keratinocytes. Migration requires up-regulation of binding sites (integrin receptors) on the cell wall, which is mediated by PDGF and TGF-β. This up-regulation of binding sites is essential because fibroblasts migrate by maintaining attachment to one binding site while extending lamellipodia in search of another site. Once the fibroblast is able to bind to a new site, it releases the original attachment and “moves” in the direction of the wound bed (Monaco and Lawrence, 2003; Myers et al, 2007).

Fibroblasts begin to appear in the wound bed toward the end of the inflammatory phase, 2 to 3 days after injury. By day 4, fibroblasts are the predominant cells in the wound matrix (Dubay and Franz, 2003). Once fibroblasts arrive at the wound site, growth factors bind to fibroblast receptor sites and trigger intracellular processes that move the fibroblasts into the reproductive phase of the cell cycle, thus stimulating proliferation of the fibroblast. Finally, the fibroblasts are converted into “wound fibroblasts” by TGF-β, a growth factor secreted by macrophages. These wound fibroblasts differ from typical dermal fibroblasts in that they exhibit decreased proliferative behavior but increased synthesis of connective tissue proteins such as collagen (Frank et al, 2002).

Collagen synthesis follows the established process for synthesis of any protein. The collagen molecule is characterized by a glycine-X-Y repeating sequence. The collagen molecule must undergo a series of intracellular modifications before the molecule is secreted into the extracellular environment and becomes part of the ECM. One of the most critical modifications is “cross-linking” of the proline and lysine molecules, a process known as hydroxylation. These cross-links are essential for the development of tensile strength. A number of factors are required for normal hydroxylation, including oxygen, ascorbic acid, iron, copper, and selected enzymes. Thus hypoxia, vitamin C deficiency, copper deficiency, and iron deficiency all can compromise hydroxylation and the development of tensile strength. High-dose corticosteroids also can impair tensile strength because corticosteroids suppress the enzymes needed for hydroxylation. Following intracellular modifications, the collagen molecule is secreted into the extracellular environment as the triple helix procollagen. It then undergoes additional steps that culminate in the formation of cross-linked fibrils. The enzyme lysyl oxidase is essential to these processes and to the development of stable collagen fibers (Monaco and Lawrence, 2003).

The early granulation tissue is a provisional matrix characterized by unstructured collagen and high levels of fibronectin. This provisional matrix also includes vibronectin, laminin, and proteoglycans such as hyaluronic acid (Braiman-Wiksman et al, 2007; Gill and Parks, 2008; Schneider et al, 2008). As the ECM matures, collagen becomes the predominant protein, representing a little over 50% of the new ECM. Proteoglycans such as hyaluronic acid are present in small amounts but serve critical functions. For example, hyaluronic acid facilitates cell migration, protects cells against free-radical and proteolytic damage, and contributes viscoelastic properties to the new matrix (Chen and Abatangelo, 1999; Monaco and Lawrence, 2003).

Wounds healing by primary intention, such as sutured incisions, require a limited amount of connective tissue to mend the defect. In these wounds, ECM production is essentially complete within 14 to 21 days (Hunt and Van Winkle, 1997; Monaco and Lawrence, 2003). Although the granulation tissue is not visible in these wounds, a “healing ridge” can be palpated just under the intact suture line by days 5 to 9. The healing ridge is produced by the newly formed connective tissue proteins. Absence of this healing ridge indicates impaired healing and increased risk for dehiscence (Hunt and Van Winkle, 1997).

Factors affecting granulation tissue formation.

Factors affecting granulation tissue formation include perfusion status, oxygen levels, nutritional status, and glucose levels/diabetes. Granulation tissue formation is very much an oxygen-dependent process. Hypoxia can act as a stimulus to fibroblast proliferation and initial collagen synthesis, but adequate levels of oxygen (30–40 mm Hg) are absolutely essential for the latter steps in collagen synthesis and for the cross-linking that provides tensile strength. Adequate oxygen levels are also required for prevention of infection. Infection rates in surgical wounds are inversely proportional to oxygen levels (Rodriguez et al, 2008). Tissue oxygenation of acute wounds is discussed in more detail in Chapters 28 and 34. Nutritional status affects granulation because granulation tissue formation requires adequate levels of protein and micronutrients such as ascorbic acid, iron, and copper. Diabetes is associated with delayed deposition of granulation tissue and insufficient production of collagen to establish a mature matrix (Braiman-Wiksman et al, 2007).

Contraction.

Contraction occurs when specialized fibroblasts known as myofibroblasts exert tractional forces on the ECM, reducing the size of the wound. Wounds healing by primary intention have limited or no contraction because the wound edges have been surgically approximated (Monaco and Lawrence, 2003).

Although full-thickness wounds healing by secondary intention (e.g., pressure ulcers) proceed through the same phases as full-thickness wounds healing by primary intention, key differences do exist within each phase. These differences are summarized in Table 4-2 and discussed here.

Absence of hemostasis.

The absence of hemostasis has a tremendous impact on the healing trajectory of wounds. Bleeding and hemostasis do not occur in wounds healing by secondary intention. Failure to bleed and clot compromises the repair process because the wound-healing sequence of events is normally initiated by clot breakdown and the subsequent release of growth factors. One theorized benefit of surgical debridement of chronic wounds is to trigger clot formation and the release of growth factors so that the repair process is reactivated.

Prolonged inflammatory phase.

The inflammatory phase frequently is prolonged. Because the goal of this phase is to establish a clean wound bed and to obtain bacterial balance, the wound will remain in this phase until necrotic tissue has been eliminated and bacterial loads have been controlled. Wounds healing by secondary intention frequently are characterized by large amounts of devitalized tissue and heavy bacterial loads. Thus the inflammatory phase generally lasts considerably longer than the 3 days that are typical in the approximated incision.

Prolonged proliferative phase.

The proliferative phase is prolonged, and the sequence of events is different. In wounds healing by primary intention, epithelialization occurs first, followed by angiogenesis and formation of a limited volume of connective tissue proteins (ECM); contraction does not occur or is very limited. These processes usually are complete within 14 to 21 days. In secondary-intention wound healing, the proliferative phase begins with granulation tissue formation (to fill in the soft tissue defect), followed by contraction (to minimize the defect); epithelialization is the final phase.

The volume of granulation tissue required to fill the defect (and the time required for this phase of repair) is determined by the size of the wound and the degree to which contraction is able to reduce the size of the defect. Because the wound bed is visible, the clinician is able to assess progress in healing. Healthy granulation tissue presents as a red, vascular, granular wound bed as a result of the numerous capillary loops in combination with the newly synthesized ECM proteins (see Plate 3).

Increased amount of contraction.

Contraction is much more important in secondary-intention wound healing than in closed wounds because contraction reduces the size of the soft tissue defect and thus reduces the amount of granulation tissue required. The rate of contraction for open wounds averages 0.6 to 0.7 mm/day (Gabbiani, 2003; Monaco and Lawrence, 2003). The degree to which a specific wound will contract is determined partly by the mobility of the surrounding tissue. For example, the tissue surrounding sacral and abdominal wounds is quite mobile and can contract easily. In contrast, the tissue surrounding a wound on the extremity or overlying a bony prominence has limited potential for contraction. Contraction is considered undesirable in some wounds because it can cause cosmetic deformities or flexion contractures of joints.

Contraction is mediated by myofibroblasts, modified fibroblasts that contain actin and myosin monofilaments and smooth muscle proteins. Differentiation of fibroblasts into myofibroblasts is stimulated by growth factors such as TGF-β1 and PDGF. Substances within the ECM itself are also thought to contribute to the development of myofibroblasts. Intracellular actin filaments and extracellular fibronectin work jointly to establish a contractile force that compresses and “shrinks” the ECM, thus pulling the wound edges toward each other (Demidova-Rice et al, 2012; Gabbiani 2003).

Delayed epithelialization.

Because full-thickness wounds involve loss of the deep dermis and epidermal appendages (along with their epithelial lining), epithelialization in these wounds proceeds from the periphery of the wound inward in a centripetal fashion. Epithelial migration requires an open, proliferative wound edge. Closed, nonproliferative wound edges, also known as epibole, are sometimes seen in open wounds healing by secondary intention, probably due to premature keratinization of the wound edges (Figure 4-6 and Plate 4). In these wounds, an open edge must be reestablished, by either surgical excision or chemical cauterization, before epithelial migration can occur (Schultz, Sibbald, Falanga, 2003).

Prolonged remodeling.

The remodeling process for wounds healing by secondary intention is essentially the same as that for wounds healing by primary intention. Clinicians and caregivers must remain acutely aware that newly “healed” wounds initially lack tensile strength, and measures should be initiated to minimize stress on the remodeling wound until tensile strength has developed, which occurs 2 to 3 months after closure. For example, the patient with a newly healed pressure ulcer should remain on a therapeutic support surface and should minimize time spent lying on the involved surface.

The nature of the injury differs between acute and chronic wounds. Acute wounds usually begin with a sudden, solitary insult and proceed to heal in an orderly manner. In contrast, chronic wounds are commonly caused by an underlying pathologic process, such as vascular insufficiency, that produces repeated and prolonged insults to the tissues. Failure to correct or control the underlying pathology can result in a persistent cycle of injury that causes repetitive tissue damage. In contrast, correction of the underlying pathology frequently can shift the wound to a healing pathway (Goldman, 2004).

Chronic wounds frequently are complicated by impediments to healing, such as ischemia, necrotic tissue, heavy bacterial loads, and high levels of proinflammatory MMPs. These factors prolong the inflammatory phase of wound healing by continuing to recruit macrophages and neutrophils into the wound bed. In addition, high levels of proinflammatory MMPs are associated with ongoing destruction of the ECM (Demidova-Rice et al, 2012; Gill and Parks, 2008; Goldman, 2004). Studies indicate that the levels of inflammatory substances in chronic wounds are 100 times higher than the levels in acute wounds (Berg Vande and Robson, 2003). Fortunately, this cycle can be interrupted by elimination of the noxious stimuli, that is, by debridement of necrotic tissue and control of the bacterial burden (Goldman, 2004).

In addition to high levels of inflammatory proteases, low levels of growth factors commonly characterize the environment of chronic wounds. Normal levels of growth factors are critical to repair, because a “threshold level” of growth factors is required to move target cells out of the quiescent G₀ phase and into the reproductive cycle (Berg Vande and Robson, 2003). The low levels of growth factors commonly found in chronic wounds may be the result of inadequate production by the cells of the wound bed (or insufficient numbers of “producer” cells). Additional potential causes include rapid breakdown of growth factors by the high levels of MMPs or “binding” of the growth factors by the ECM (Berg Vande and Robson, 2003; Steed, 2003). Although this imbalance between inflammatory and proliferative stimuli usually is thought to be the cause of impaired healing, some investigators suggest that the imbalance instead may be a result of chronicity. When the wound begins to heal, the ratio of inflammatory to proliferative stimuli normalizes (Goldman, 2004; Henry and Garner, 2003).

Additional “host factors” that trigger chronicity in a wound include ischemia, malnutrition, and comorbidities such as diabetes (Steed, 2003). Malnutrition is a particularly common contributor to wound chronicity. Fibroblasts that lack the requisite raw ingredients cannot synthesize connective tissue proteins. The importance of nutritional status is reflected in studies documenting consistently impaired wound healing in patients whose albumin levels were lower than 2.0 g/L (Burns et al, 2003). In addition, a study on wound fluid as a predictor of healing found the only constituents to reliably predict healing were total protein and albumin, that is, albumin levels greater than 2.0 g/L (James et al, 2000; Schultz et al, 2003).

Pale, pink tissue that is smooth rather than granular is indicative of delayed healing and a compromised ability to synthesize collagen and other connective tissue proteins (see Plate 5).

Denervation is another potential cause of failure to heal. Sensory nerves secrete neuropeptides (e.g., substance P) that are highly chemotactic for inflammatory cells. Therefore denervated wounds are subject to impaired inflammation and compromised healing. Denervation may be one of the factors contributing to chronicity of pressure ulcers in patients with spinal cord injury and neuropathic ulcers in diabetic patients (Macri and Clark, 2009; Richards et al, 1997).

In summary, differences in the healing trajectory for acute and chronic wounds stem from the nature of the injury, the cellular events that follow injury, and miscellaneous host factors. In general, healing wounds are characterized by high mitotic activity, therapeutic levels of inflammatory cytokines, low levels of proteases, and mitotically competent cells. In contrast, chronic wounds exhibit low mitotic activity, excessive levels of inflammatory cytokines, high levels of proteases, and senescent or mitotically incompetent cells.

Bioactive molecules include growth factors and cytokines. They are produced by the cells in the wound bed (e.g., platelets, neutrophils, macrophages, and fibroblasts) and act as “directors” of cell function and thus of the repair process. They do so by attracting the needed cells to the wound bed, stimulating them to proliferate, and then directing the cells to carry out specific aspects of repair. For example, TNF-α is a proinflammatory cytokine that attracts neutrophils and macrophages to the wound bed and is present in high concentrations during the inflammatory phase, whereas VEGF is a growth factor that supports proliferation and migration of endothelial cells and therefore is important during angiogenesis (Gill and Parks, 2008; Macri and Clark, 2009; Thom, 2009).

Growth factors and cytokines act as “controllers” for the cells critical to the repair process. However, it is increasingly apparent that proteins within the ECM control the levels and function of growth factors and cytokines. Key categories of matrix proteins include MMPs, ADAMs, and tissue inhibitors of metalloproteinase (TIMPs). Matrix metalloproteinases and ADAMs can “up-regulate” the levels of growth factors and cytokines by cleaving them from the cell and thus activating them; however, they also can “down-regulate” the levels of growth factors and cytokines by degrading them or by inhibiting their release. Tissue inhibitors of metalloproteinase control the activity of MMPs by binding to them, thus blocking their effects (Gill and Parks, 2008).

The ECM serves as a scaffold for migrating cells and as a repository for growth factors. It also influences the response of the cells to growth factors. For example, keratinocyte proliferation requires both attachment to the ECM and stimulation by growth factors. Cell migration is another aspect of repair that is dependent on both matrix proteins and integrins within the matrix itself. Matrix proteins promote migration by degrading the bonds between the migrating cells and the underlying wound bed, and the matrix promotes reattachment through the expression of binding sites (integrins) within the matrix. The matrix also supports cell-to-cell communication. The ECM needs to be porous and pliable in order to support the movement of cells, nutrients, and growth factors through the wound environment (Cheresh and Stupack, 2008; Schultz and Wysocki, 2009). Dermal matrix dressings (see Chapter 19) may be beneficial to support cell migration and granulation tissue formation, in select wounds (Nataraj et al, 2007).

A number of host factors influence cells’ response to growth factors, cytokines, and matrix proteins. One of these factors is the type of receptors on the cell wall. This is an important factor because all cells within the wound bed are exposed to the same mix of regulatory substances, but only select cells respond. In addition, different cells may exhibit different responses to the same regulatory substance because regulatory substances exert their effects primarily through binding with cell receptors. Therefore only cells with the specific receptor sites respond to the regulatory substance, and the effects of receptor binding vary based on cell type. For example, PDGF stimulates migration of some cells and mitosis in others, but it has no effect on other cells (Martin, 1997; Witte and Barbul, 1997). Furthermore, fibroblasts and keratinocytes in elderly individuals have a decreased number of receptor sites, which may explain why elderly patients tend to exhibit a diminished response to some regulatory substances (Ashcroft et al, 2002).

Other factors that impact an individual’s ability to heal normally are systemic factors such as nutritional status, perfusion, oxygenation, and comorbidities such as diabetes. These factors are addressed briefly in this chapter and in greater depth in other chapters in this text.

As mentioned earlier in this chapter, the early-gestation fetus typically heals without scarring, an ability that is lost at 22 to 24 weeks of gestation (Samuels and Tan, 1999). Features and characteristics of early-gestation healing are listed in Box 4-2. One significant difference is a markedly diminished inflammatory response. Multiple studies using various models have demonstrated that inflammation is minimal or essentially absent in early-gestation (scarless) healing. This finding is consistent with other studies showing a strong link between the intensity and duration of the inflammatory response and the subsequent development of scar tissue (Wilgus, 2007). The specific factors thought to contribute to this very minimal inflammatory response include the following: (1) Fetal platelets release much lower levels of proinflammatory growth factors and cytokines, most specifically the profibrotic isoforms of TGF-β. TGF-β is responsible for attracting inflammatory cells to the wound bed, for promoting angiogenesis and fibroblast activity, and for remodeling of the ECM; (2) Marked reduction in migration of inflammatory cells (neutrophils, macrophages, and T cells); this is partially due to the lower levels of proinflammatory growth factors and partially due to the fact that the fetal immune system is poorly developed; (3) Marked reduction in angiogenic stimuli linked to scar formation (VEGF, TGF-β1, PDGF, and prostaglandin E2) (Walraven et al, 2014; Wilgus, 2007).

Another difference in early-gestation healing is the rate at which epithelial resurfacing occurs. Rapid up-regulation of the adhesion molecules within the ECM allows cells to move laterally, which promotes early keratinocyte migration (Wilgus, 2007).

Most importantly, a number of differences in fetal healing contribute to production of a new ECM that retains the characteristics of unwounded tissue. One contributing factor may be the high levels of hyaluronic acid and hyaluronic-acid-stimulating activity factor. This factor is significant because hyaluronic acid supports rapid cell migration. In addition, high levels of MMPs compared with TIMPs favor degradation of the ECM, which is thought to help prevent overproduction of collagen. Finally, fetal fibroblasts retain the ability to produce new matrix proteins that retain the basket-weave configuration characteristic of normal dermis (Wilgus, 2007).

In summary, early-gestation fetal repair is characterized by a significantly reduced inflammatory response and a rapid and balanced proliferative phase that restores the dermal architecture without scarring (see Box 4-2).

Excessive or hyperproliferative scarring is a complication of acute full-thickness wound healing that presently is not well understood. The two types of hyperproliferative scarring are hypertrophic scars and keloid scars (see Plate 7, C and D). Both types appear raised, are red or pink, and are pruritic. Hypertrophic scars are confined to the original incisional or scar area, whereas keloids expand beyond the incision into the surrounding tissue. This expansion, sometimes described as a mushroom effect, is the result of continued proliferation of connective tissue proteins that may continue indefinitely (Atiyeh, 2005; Slemp and Kirschner, 2006).

Hypertrophic scars are characterized by increased vasculature, increased numbers of white blood cells and fibroblasts, and a thickened epidermal layer (Atiyeh, 2005). The collagen fibers in hypertrophic scars are organized and oriented parallel to the epidermal layer; however, they contain an abundance of myofibroblasts, which leads to contracture formation. One positive feature of hypertrophic scars (compared with keloids) is their potential to eventually regress during the remodeling phase (Slemp and Kirschner, 2006).

Keloids are a much more serious problem than hypertrophic scars. Keloids may continue to expand, creating both functional and cosmetic deficits. In addition, scars that initially appear to be “normal” may deviate into a pattern of keloid growth over time. Biochemical analysis of keloid scars reveals increased numbers of receptor sites for growth factors and major abnormalities in the behavior of fibroblasts and keratinocytes, which may partially explain the excessive production of ECM. Most importantly, the balance between synthesis and degradation of connective tissue proteins is lost. This loss of balance is theorized to result from abnormalities in cell-to-cell communication and immune function, failure of apoptosis (spontaneous cell death), and the effects of hypoxia and oxygen free radicals. Keloid scars are characterized by disorganized collagen bundles; however, these scars do not contain myofibroblasts and are not associated with contracture formation (Atiyeh, 2005; Hahn et al, 2013; Slemp and Kirschner, 2006).

Risk factors for hyperproliferative scarring include a strong family history (particularly important with keloid formation), age 10 to 30 years, and darkly pigmented skin. In the United States, individuals of African American descent have a 5% to 15% increased risk compared with Caucasians (Atiyeh, 2005; Slemp and Kirschner, 2006). Exposure to mechanical stretch may be a trigger for keloid production (Ogawa et al, 2012).

Strategies designed to prevent or correct hyperproliferative scarring have provided inconsistent and often suboptimal results. Surgical excision, intralesional steroids (either single or combination steroids), and topical silicone sheeting are the most commonly prescribed therapies. Table 4-3 lists current treatment options. Research is ongoing, with the goals of accurately identifying the cellular and biochemical abnormalities that produce these scars and designing therapies to prevent and correct them (Kieran et al, 2013; Sayed and Bayat, 2013; Yamamoto and Kiyosawa, 2012).