Discussion:

Hypertrophic scar model in the rabbit ear: a reproducible model for studying scar tissue behavior with new observations on silicone gel sheeting for scar reduction

Hypertrophic scarring poses a clinically relevant problem as it can be cosmetically disfiguring and functionally debilitating. A lack of animal models has hindered an understanding of the pathogenesis and development of new treatment strategies therefore has largely been empiric. Our group has developed a unique hypertrophic scar (HS) model in the rabbit ear. The model has been reproducible, quantifiable, and measurable over a time period of 1 month. We describe the development as well as the reliability and responsiveness of this model to different therapeutic agents, such as TGF-beta blockade, silicone occlusion, and application of collagen-synthesis inhibitors. Moreover, it has given insights into the mechanism of action of silicone sheeting occlusive treatment and ultimately suggests that the epidermis plays a critical role in the development of HS. Additionally, we will present new data supporting the importance of the epidermis and further clarify the mechanism of action of silicone sheeting. When a semi-occlusive polyurethane film was left in place for an additional time period, scar formation was reduced. HSs of this model covered with silicone sheets and five layers of Tegaderm showed a significant scar reduction by 80% compared with wounds with only one layer of Tegaderm. The HS model in the rabbit ear is a highly reliable, responsive, and practical model for studying scar tissue behavior. Furthermore, our data suggest that the degree and the duration of occlusion are most important for reducing scar tissue formation.

Hydrated keratinocytes reduce collagen synthesis by fibroblasts via paracrine mechanisms

Regulating collagen metabolism can control hypertrophic scars in cutaneous wounds. Hypertrophic scars can be reduced by occlusive dressings such as silicone sheeting; however, their mechanism is still unknown. We hypothesized that hydration of keratinocytes reduces the collagen secretion of fibroblasts by modifying the cytokine levels. Stratified human epidermal keratinocytes and confluent human dermal fibroblasts were co-cultured serum free for 72 hours. Keratinocytes were either kept at the air interface or hydrated. Messenger RNA (mRNA) levels of interleukin-1 (IL-1)alpha, IL-1beta, tumor necrosis factor alpha (TNF-alpha), keratinocyte growth factor (KGF), and procollagen-1 were analyzed by real-time reverse transcription-polymerase chain reaction. Secretion of cytokines into conditioned media was quantified by enzyme-linked immunosorbent assay and collagen content by Western blot. The content of collagen-I decreased by 44% in the presence of hydrated keratinocytes. Co-culture with air-treated keratinocytes decreased collagen-I only by 23%. Co-cultured hydrated keratinocytes had significantly higher TNF-alpha mRNA (172%) than hydrated keratinocytes. At the protein level, there was an overall trend toward increased TNF-alpha levels in hydrated cultures. IL-1beta secretion decreased significantly under hydration (42% monoculture, 58% co-culture). Co-culture stimulated a 240% increase of KGF mRNA in fibroblasts compared with monocultured fibroblasts. Fibroblasts secreted 4.5-fold more KGF in hydrated co-cultures and sixfold more KGF in air-treated co-cultures. Hydration of keratinocytes modifies important paracrine interactions between keratinocytes and fibroblasts and reduces collagen-1, which supports the hypothesis that hydration of the epidermis and restoration of water barrier function play an important role in scar formation.