## 4 Discussion  
In this report, we presented the application of a custom-built dermal OCT system for assessment of cutaneous wound healing in a mouse pinna model. In both non-diabetic and diabetic mice, quantitative OCTA parameters were evaluated in a longitudinal manner and revealed changes related to vascular remodeling during healing of the wound. In the current study, an excisional wound was set extending through stratum corneum, epidermis and dermis to the central cartilage of the pinna. In such wounds, healing occurs from the margin of the wounds and allows assessment of various mechanisms including angiogenesis, epithelization, granulation tissue formation and tissue contraction. Compared to punch wounds, the excisional wound provided a wound bed that facilitated topical application of different test treatments and thus modeling of pharmacologic interventions in wound healing. Animal models in preclinical research aim to mimic human wound healing problems, that is, debiscence, ischemia, ulceration, infections, and scarring [70]. Here, one has to consider that cutaneous wound healing in humans and rodents involve different mechanisms. Human wound healing happens mainly via re-epithelialization and formation of granulation tissue. While both processes are also present in rodents, their dominant mechanism of wound closure is contraction. Different strategies were introduced to overcome this limitation of cutaneous wound models in rodents [70]. For example, [PERSON] and co-workers applied a wound splint in a dorsal wound model in mice [71]. Here, silicone splints were fixed to the dorsal skin with adhesive and nylon sutures in order to avoid wound contraction during healing. In our study, in an effort to minimize wound contraction, a donut-shaped splint was sutured to the pinna, fixating the skin at six points with the central cartilage of the ear serving as second anchor point. The usage of additional glue was omitted in order to prevent any interaction with the test substance treatments applied at each study day. Due to the high flexibility of the skin of the pinna, wound contraction could however not be entirely prevented as indicated by the positive \(\alpha\)-SMA staining found in IHC. OCTA monitoring of dorsal wound healing in mice might be possible but may be hampered by the increased thickness of the dorsal skin as well as the denser hair coat. Proper visualization of tissue morphology and vascularization via OCT is dependent on the central wavelength of the light source and on the absorption  
Figure 8: \(\alpha\)-Smooth muscle actin staining of the cutaneous wound model in the mouse pinna. Positive staining in (**a**) non-diabetic and (**b**) diabetic mouse indicates smooth muscles cells demonstrating that wound closure was promoted by contraction. and scattering properties of the tissue. These factors are limiting the imaging depth to 1-2 mm. Alternative models such as excisional wounds in rabbit ears [72; 73; 74] may avoid influences by contraction. Here the skin is firmly attached to the subcutaneous tissue, thereby naturally splinting the wound. However, in addition to the slightly thicker skin, differences in costs and animal housing requirements still favor the mouse model. In the current study, the wound surface area was quantified based on digital photographs. In both non-diabetic and diabetic animals and in all study groups, wound size gradually decreased with time with no significant difference in the time courses between the groups. At the last study day, 18 days after injury, the average wound size of all animals was found to be reduced by 87.5% compared to the initial wound size; 38% (18 out of 47 animals) revealed complete wound closure. The wound size time course indicates that re-epithelial and closure of the 1 mm excisional wound takes approximately three weeks. The OCTA parameters assessed in our study provided an insight into the vascular changes after injury and during wound healing. After setting of the excisional wound, bleeding of the injured vessels was observed.