Hypertrophic scar (HTS) following thermal injury and other forms of trauma is a dermal fibroproliferative disorder that leads to considerable morbidity. Because of the lack of an ideal animal model, research is difficult. We have established an HTS model that involves transplanting human split-thickness skin graft (STSG) or full-thickness skin graft (FTSG) onto the backs of nude mice. The animals developed raised, firm, and reddish scars 2 months following transplantation. Histology and micromeasurement indicate raised, thickened engrafted skin with STSG and FTSG. In contrast, thickening was not observed with full-thickness rat skin grafts used as controls. Masson's trichrome staining demonstrates increased accumulations of collagen fibrils in the dermis in both scars grafted with STSG and FTSG. Staining cells with toludine blue and an antibody for F4/80 showed an increase in the infiltration of mast cells and macrophages. Quantification of fibrocytes reveals increased fibrocytes. Moreover, STSG grafted skin had significantly more macrophages, mast cells, and fibrocytes than FTSG. Real-time polymerase chain reaction analysis showed significantly elevated mRNA levels for type I collagen, transforming growth factor-β, connective tissue growth factor and heat shock protein 47 in both types of engrafted skin. These data demonstrate that human skin grafted onto nude mice develops red raised and thickened scars having intrinsic properties that closely resemble HTS formation as seen in humans. Interestingly, STSG developed more scar than FTSG. Furthermore, inflammatory cells and bone marrow-derived fibrocytes may play a critical role in HTS development in this animal model.

Hypertrophic scar (HSc) is a fibroproliferative disorder that occurs following deep dermal injury. Lack of a relevant animal model is one barrier toward better understanding its pathophysiology. Our objective is to demonstrate that grafting split-thickness human skin onto nude mice results in survival of engrafted human skin and murine scars that are morphologically, histologically, and immunohistochemically consistent with human HSc. Twenty nude mice were xenografted with split-thickness human skin. Animals were euthanized at 30, 60, 120, and 180 days postoperatively. Eighteen controls were autografted with full-thickness nude mouse skin and euthanized at 30 and 60 days postoperatively. Scar biopsies were harvested at each time point. Blinded scar assessment was performed using a modified Manchester Scar Scale. Histologic analysis included hematoxylin and eosin, Masson's trichrome, toluidine blue, and picrosirius red staining. Immunohistochemistry included anti-human human leukocyte antigen-ABC, α-smooth muscle actin, decorin, and biglycan staining. Xenografted mice developed red, shiny, elevated scars similar to human HSc and supported by blinded scar assessment. Autograft controls appeared morphologically and histologically similar to normal skin. Xenografts survived up to 180 days and showed increased thickness, loss of hair follicles, adnexal structures and rete pegs, hypercellularity, whorled collagen fibers parallel to the surface, myofibroblasts, decreased decorin and increased biglycan expression, and increased mast cell density. Grafting split-thickness human skin onto nude mice results in persistent scars that show morphologic, histologic, and immunohistochemical consistency with human HSc. Therefore, this model provides a promising technique to study HSc formation and to test novel treatment options.

Hypertrophic scar (HTS) represents the dermal equivalent of fibroproliferative disorders that occur after injury involving the deep dermis while superficial wounds to the skin heal with minimal or no scarring. HTS is characterized by progressive deposition of collagen that occurs with high frequency in adult dermal wounds following traumatic or thermal injury. Increased levels of transforming growth factor-β1 (TGF-β1), decreased expression of small leucine-rich proteoglycans (SLRPs), and/or fibroblast subtypes may influence the development of HTS. The development of HTS is strongly influenced by the cellular and molecular properties of fibroblast subtypes, where cytokines such as fibrotic TGF-β1 and CTGF as well as the expression of SLRPs, particularly decorin and fibromodulin, regulate collagen fibrillogenesis and the activity of TGF-β1. Reduced anti-fibrotic molecules in the ECM of the deep dermis and the distinctive behavior of the fibroblasts in this region of the dermis which display increased sensitivity to TGF-β1's biological activity contribute to the development of HTS following injury to the deep dermis. By comparing the cellular and molecular differences involved in deep and superficial wound healing in an experimental wound scratch model in humans that has both superficial and deep injuries within the same excisional model, our aim is to increase our understanding of how tissue repair following injury to the deep dermis can be changed to promote healing with a similar pattern to healing that occurs following superficial injury that results in no or minimal scarring. Studying the characteristics of superficial dermal injuries that heal with minimal scarring will help us identify therapeutic approaches for tissue engineering and wound healing. In addition, our ability to develop novel therapies for HTS is hampered by limitations in the available animal models used to study this disorder in vivo. We also describe a nude mouse model of transplanted human skin that develops a hypertrophic proliferative scar consistent morphologically and histologically with human HTS, which can be used to test novel treatment options for these dermal fibrotic conditions.

Human skin transplanted onto TCRαβ-/-γδ-/-, RAG-1-/-, and RAG-2-/-γc-/- mice results in proliferative scars with morphologic and histologic features of human hypertrophic scar. Remodeling of proliferative scars generated in knockout animals is analogous to changes in human hypertrophic scar. These animal models may better represent the natural history of human hypertrophic scar.

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