Wound Repair and Regeneration

In the United States, chronic wounds affect 6.5 million patients. An estimated excess of US$25 billion is spent annually on treatment of chronic wounds and the burden is rapidly growing due to increasing health care costs, an aging population and a sharp rise in the incidence of diabetes and obesity worldwide. The annual wound care products market is projected to reach $15.3 billion by 2010. Chronic wounds are rarely seen in individuals who are otherwise healthy. In fact, chronic wound patients frequently suffer from "highly branded" diseases such as diabetes and obesity. This seems to have overshadowed the significance of wounds per se as a major health problem. For example, NIH's Research Portfolio Online Reporting Tool (RePORT; http://report.nih.gov/), directed at providing access to estimates of funding for various disease conditions does list several rare diseases but does not list wounds. Forty million inpatient surgical procedures were performed in the United States in 2000, followed closely by 31.5 million outpatient surgeries. The need for post‐surgical wound care is sharply on the rise. Emergency wound care in an acute setting has major significance not only in a war setting but also in homeland preparedness against natural disasters as well as against terrorism attacks. An additional burden of wound healing is the problem of skin scarring, a $12 billion annual market. The immense economic and social impact of wounds in our society calls for allocation of a higher level of attention and resources to understand biological mechanisms underlying cutaneous wound complications.

Wound healing is a complex and dynamic biological process that involves the coordinated efforts of multiple cell types and is executed and regulated by numerous growth factors and cytokines. There has been a drive in the past two decades to study the therapeutic effects of various growth factors in the clinical management of nonhealing wounds (e.g., pressure ulcers, chronic venous ulcers, diabetic foot ulcers). For this review, we conducted an online search of Medline/PubMed and critically analyzed the literature regarding the role of growth factors and cytokines in the management of these wounds. We focused on currently approved therapies, emerging therapies, and future research possibilities. In this review, we discuss four growth factors and cytokines currently being used on and off label for the healing of wounds. These include granulocyte‐macrophage colony‐stimulating factor, platelet‐derived growth factor, vascular endothelial growth factor, and basic fibroblast growth factor. While the clinical results of using growth factors and cytokines are encouraging, many studies involved a small sample size and are disparate in measured endpoints. Therefore, further research is required to provide definitive evidence of efficacy.

Both chronic and acute dermal wounds are susceptible to infection due to sterile loss of the innate barrier function of the skin and dermal appendages, facilitating the development of microbial communities, referred to as biofilms, within the wound environment. Microbial biofilms are implicated in both the infection of wounds and failure of those wounds to heal. The aim of this review is to provide a summary of published papers detailing biofilms in wounds, the effect they have on infection and wound healing, and detailing methods employed for their detection. The studies highlighted within this paper provide evidence that biofilms reside within the chronic wound and represent an important mechanism underlying the observed, delayed healing and infection. The reasons for this include both protease activity and immunological suppression. Furthermore, a lack of responsiveness to an array of antimicrobial agents has been due to the biofilms’ ability to inherently resist antimicrobial agents. It is imperative that effective strategies are developed, tested prospectively, and employed in chronic wounds to support the healing process and to reduce infection rates. It is increasingly apparent that adoption of a biofilm‐based management approach to wound care, utilizing the “antibiofilm tool box” of therapies, to kill and prevent reattachment of microorganisms in the biofilm is producing the most positive clinical outcomes and prevention of infection.

Over the past decade, a variety of advanced silver‐based dressings have been developed. There are considerable variations in the structure, composition, and silver content of these new preparations. In the present study, we examined five commercially available silver‐based dressings (Acticoat™, Aquacel^® Ag, Contreet^® Foam, PolyMem^® Silver, Urgotul^®SSD). We assessed their cytotoxicity in a monolayer cell culture, a tissue explant culture model, and a mouse excisional wound model. The results showed that Acticoat™, Aquacel^® Ag, and Contreet^® Foam, when pretreated with specific solutes, were likely to produce the most significant cytotoxic effects on both cultured keratinocytes and fibroblasts, while PolyMem^® Silver and Urgotul^®SSD demonstrated the least cytotoxicity. The cytotoxicity correlated with the silver released from the dressings as measured by silver concentration in the culture medium. In the tissue explant culture model, in which the epidermal cell proliferation was evaluated, all silver dressings resulted in a significant delay of reepithelialization. In the mouse excisional wound model, Acticoat™ and Contreet^® Foam indicated a strong inhibition of wound reepithelialization on the postwounding‐day 7. These findings may, in part, explain the clinical observations of delayed wound healing or inhibition of wound epithelialization after the use of certain topical silver dressings. Caution should be exercised in using silver‐based dressings in clean superficial wounds such as donor sites and superficial burns and also when cultured cells are being applied to wounds.

Venous leg ulcers are common and cause considerable morbidity in the population. As healing may be slow or may never be achieved, ulcers create persistent and substantial demands on clinical resources. Great efforts have been made to accelerate tissue repair in chronic venous leg ulcers with limited success. This may at least be partly due to the limited knowledge on the microenvironment of chronic wounds. In fact, the tremendous impact of the microenvironmental conditions on the outcome of wound healing has increasingly become apparent. Oxidative stress as a consequence of an imbalance in the prooxidant‐antioxidant homeostasis in chronic wounds is thought to drive a deleterious sequence of events finally resulting in the nonhealing state. The majority of reactive oxygen species are most likely released by neutrophils and macrophages and to an unknown extent from resident fibroblasts and endothelial cells. As the inflammatory phase does not resolve in chronic wounds, the load of reactive oxygen species persists over a long period of time with subsequent continuous damage and perpetuation of the inflammation. In this article, we will critically discuss recent findings that support the role of oxidative stress in the pathophysiology of nonhealing chronic venous leg ulcers.

This study was undertaken to determine if healing rates are reliable early predictors of ultimate complete wound closure in venous leg ulcers and diabetic foot wounds. We conducted a retrospective analysis of 306 venous leg ulcers and 241 diabetic foot ulcers enrolled in two large controlled, prospective, randomized pivotal trials to compare topical wound treatments, to determine whether certain early markers of healing could be correlated with later total wound closure. Two‐sided tests at 95% confidence demonstrated that wound margin advance, initial healing rate, percent wound surface area reduction, and wound healing trajectories (all p<0.001) were powerful predictors of complete wound healing at 12 weeks. Wounds with poor healing progress by these criteria at 4 weeks were highly likely to remain unhealed after 8 additional weeks of treatment. Analysis of the diabetic foot ulcers and venous leg ulcers subgroups separately demonstrated consistent statistical test results with high significance; similarly, the results remained valid independent of the topical treatment used. The early prediction of eventual wound healing or nonhealing using early healing rates may enable more efficient triage of patients to advanced healing technologies. We believe that these surrogate markers are robust predictors of healing regardless of wound etiology and that they merit wider use in clinical trials and routine patient care.

Honey is a popular natural product that is used in the treatment of burns and a broad spectrum of injuries, in particular chronic wounds. The antibacterial potential of honey has been considered the exclusive criterion for its wound healing properties. The antibacterial activity of honey has recently been fully characterized in medical‐grade honeys. Recently, the multifunctional immunomodulatory properties of honey have attracted much attention. The aim of this review is to provide closer insight into the potential immunomodulatory effects of honey in wound healing. Honey and its components are able to either stimulate or inhibit the release of certain cytokines (tumor necrosis factor‐α, interleukin‐1β, interleukin‐6) from human monocytes and macrophages, depending on wound condition. Similarly, honey seems to either reduce or activate the production of reactive oxygen species from neutrophils, also depending on the wound microenvironment. The honey‐induced activation of both types of immune cells could promote debridement of a wound and speed up the repair process. Similarly, human keratinocytes, fibroblasts, and endothelial cell responses (e.g., cell migration and proliferation, collagen matrix production, chemotaxis) are positively affected in the presence of honey; thus, honey may accelerate reepithelization and wound closure. The immunomodulatory activity of honey is highly complex because of the involvement of multiple quantitatively variable compounds among honeys of different origins. The identification of these individual compounds and their contributions to wound healing is crucial for a better understanding of the mechanisms behind honey‐mediated healing of chronic wounds.

Upon stimulation, mast cells release a heterogeneous group of factors that promote inflammation and influence cell proliferation. Mast cells accumulate at sites of injury, further suggesting a critical role in wound healing. To assess the importance of mast cells in tissue repair, we compared wound healing in mast cell–deficient WBB6F1/J‐Kit^W/Kit^W–v (Kit^W/Kit^W–v) and wild type WBB6F1/++ (WT) mice. During the inflammatory phase, neutrophil infiltration into wounds of the Kit^W/Kit^W–v mice was significantly less than that of WT mice (84.6 ± 10.3 vs. 218 ± 26.0 cells/10 high‐power fields at day 3, p < 0.001), while wound macrophage and T cell infiltration were similar in both strains. The decrease in neutrophils could not be explained by changes in tumor necrosis factor‐α or macrophage inflammatory protein‐2 levels, because the amounts of these two neutrophil chemoattractants were similar in both Kit^W/Kit^W–v and WT mice. Surprisingly, the absence of mast cells had no effect on the proliferative aspects of wound healing, including reepithelialization, collagen synthesis, and angiogenesis. Although mast cells are known to release proangiogenic mediators, vascular endothelial growth factor levels were similar in WT and Kit^W/Kit^W–v mice. Moreover, levels of fibroblast growth factor‐2 were increased in Kit^W/Kit^W–v mice (4206 ± 107 vs. 1865 ± 249 pg/ml, p < 0.01). These results suggest that mast cells modulate the recruitment of neutrophils into sites of injury, yet indicate that mast cells are unlikely to exert a major influence on the proliferative response within healing wounds. (WOUND REP REG 2003;11:46–54)

Wound healing in the oral mucosa is clinically distinguished by rapid healing and lack of scar formation compared with dermal wounds. Mechanisms of favorable mucosal healing are yet to be elucidated. Utilizing a murine model of equivalent‐size mucosal and skin wounds, we verified the rapid reepithelializaton and reduction in scarring of oral wounds reported in humans. Collagen fibrillar structure in oral wounds rapidly approached the size of normal collagen fibrils, while the collagen ultrastructure in skin remained immature through the later phases of healing. To determine whether the transforming growth factor‐β (TGF‐β) contributes to the lack of scar formation in oral mucosa, we compared the expression and production in oral and skin wounds. The RNase protection assay demonstrated significantly lower levels of TGF‐β1 expression in oral wounds compared with dermal wounds, and no changes were observed in the expression levels of TGF‐β2 or TGF‐β3. ELISA analysis confirmed that oral wounds contained lower levels of TGF‐β1 levels compared with dermal wounds, along with a significant increase in the ratio of TGF‐β3 to ‐β1. These findings showed reduced scarring in oral wounds at the ultrastructural level, and provide evidence that site‐specific differences in TGF‐β production contributes to the superior healing of oral wounds.