There are many murine model systems available to examine wound healing processes, each possessing specific advantages and limitations1,2. The following methods present two murine wound models, each of which addresses a particular aspect of the wound healing response, and which can be used to identify the cause and effect of perturbations in the response to injury. The process of wound healing occurs in distinct phases. The first phase is inflammatory, characterized by the rapid influx of platelets, neutrophils, and monocytes/macrophages, as well as the production of proinflammatory cytokines and chemokines. Following resolution of inflammation, the environment transitions to a more reparative state with the induction of profibrotic and proangiogenic cytokines and growth factors. Granulation tissue is deposited and neovessels form with the migration of myofibroblasts, fibroblasts, epithelial cells, and endothelial cells. In the final stages, the provisional extracellular matrix is remodeled, and scar formation and wound closure proceeds2,3,4,5,6,7,8.
No single murine model provides a system to study all stages of wound healing2. Here, two surgical wound models are described: one elucidates acute cellular and cytokine wound healing responses, and the other allows for the assessment of wound closure as well as histological analyses. These two methods may be employed in a complementary fashion to assess the effects of a perturbation or comorbidity on different aspects of the wound healing response. The dorsal subcutaneous implantation of polyvinyl alcohol (PVA) sponges is a system that has been used in rodent models for decades to elucidate numerous aspects of cellular and granulation tissue responses9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24. This approach allows for the retrieval of cytokine-rich wound fluids and cellular infiltrates. In this model, 1 cm x 1 cm x 0.5 cm pieces of PVA sponge are placed into subcutaneous pockets through a 2 cm incision made at the posterior dorsal midline. The incision is closed with surgical clips, and the sponges can be retrieved at later time points for cell and fluid isolation. The cellular and cytokine milieu of isolated sponges reflects the normal stages of acute wound healing up to about 14 days postimplantation. At later time points the model is more advantageous for studying granulation tissue formation and the foreign body response1. With this system, it is possible to isolate >106 cells, which offers a distinct advantage for phenotypic and functional assays and RNA isolation, over isolating cells from other biopsy-based methods1,22,23,25,26.
The rate of wound closure is determined using the tail skin excision model. In this model, as initially described by Falanga et al. and reported by others27,28,29,30, a 1 cm x 0.3 cm full thickness section of tail skin is removed near the base of the tail. The wound area is easily visualized and can be measured over time. Alternatively, tail tissue can be isolated for histological analysis. This approach can be used as an alternative to or in conjunction with the well-established dorsal punch biopsy method. The primary distinctions between these two models are the rate of wound closure, the presence or absence of fur, and the skin structure2,31,32. Tail skin wounds offer a longer timeframe in which to assess wound closure, as it takes approximately 21 days for full closure to occur. This is opposed to unsplinted dorsal punch biopsies, which heal much faster (~7–10 days), primarily by contraction due to the action of the panniculus carnosus. Splinted dorsal punch biopsies heal more slowly and diminish the effects of contractile healing, but rely on the presence of a foreign body to restrict contractile-based mechanisms1,2,27,30,31,33.
The described wound models are informative for understanding normal wound healing processes in the absence of perturbation. While the healing of rodent skin differs in very significant ways from human skin, including loose structure, reliance on contractile healing, and other anatomical differences, the murine system offers certain advantages for mechanistic and screening studies. Foremost among these is the availability of inbred strains and genetic mutants, genetic tractability, and lower cost. Mechanistic insight gained from murine studies can be translated to complex animal models that more closely mimic human skin healing, such as the porcine system2,31.
In addition to examining wound healing responses in the steady state, these models can be combined with comorbid conditions to understand the basis of wound healing defects at the cellular, cytokine, and gross tissue level. It is in this particular setting that the two models can be used in concert to assess the effects of a particular comorbid condition, such as postoperative pneumonia, on both the acute cellular wound healing response and the rate of wound closure30.