## 4 Discussion
Consecutively, activated platelets adhere to the injured vessel ends and by this provide a surface for hemostasis. This, finally, leads to generation of thrombin and formation of fibrin clots stabilizing the initial platelet plug and closing of the vessel [75]. Since the mouse pinna comprises a very dense and interconnected vascular network, the perfusion of the downstream tissues is mostly maintained via anastomosis, collateral vessels, and recruitment of pre-existing capillaries. This is mirrored by the increase in the tortuosity parameter that reflects the curvature of the vessels and is considered to be larger in smaller vessels and the vessel density parameter that does not drop between the measurements directly before and immediately after injury (time point 0). Within a few hours, the inflammatory phase with release of proinflammatory factors into the blood stream is initiated. Histamine, as one of the inflammatory agents, plays a major role in the modulation of wound healing. By causing a dilation of vessels surrounding the wound and increasing their porosity, histamine facilitates the entry of inflammatory cells into the wound site, leads to an activation of macrophages and supports angiogenesis [76; 77]. The vessel dilation and capillary recruitment might be reflected by the slightly increasing OCTA parameters at the first two time points after incision, where the formation of new vessels has not yet occurred. In order to form new healthy tissue, angiogenesis has to take place in the wound area. It is initiated by hypoxia of the tissue, inducing the expression of growth factors like VEGF by activated fibroblasts, mast cells, and macrophages. In addition, stem cells originating from the endothelium of injured vessels are attracted to the wound site by fibronectin found in the fibrin clots. In wild type animals this proliferation phase was noticed as early as day 4, with a peak at day 8-10 [75]. In our study, a strong increase in all OCTA parameters, indicating the sprouting of new capillaries into the granulation tissue, was observed between day 1 and day 4 or 7 respectively. After day 7, vessel density and vessel length reveal a descending trend while the number of bifurcations that reflects the branchings of the capillary network at the wound site and the tortuosity that is a measure for the curvature of the vessels stay almost constant. A possible explanation for this observation might reside in the mechanisms during the overlapping wound healing phases of proliferation and inflammation. Towards the end of the inflammatory phase histamine release diminishes, vasodilation declines and the newly created vessels in the granulation tissue furnish sufficient blood supply to the wound site. This also induces a partial closure of capillaries recruited during earlier phases of wound healing, as reflected by the time courses of the OCTA parameters vessel density and vessel length that show a decrease versus the peak values at days 4 and 7, respectively, and also the weak trend towards a decreasing vessel tortuosity parameter. In non-diabetic mice, a trend towards a faster decrease of vessel density and vessel length values in the VEGF/PDGF and in the chitosan hydrogel group was found, which could be indicative of a more advanced proliferative phase. Major changes in this network are expected during the remodeling phase of wound healing that might have only been initiated within the study period of 18 days. Here, a vascular remodeling will take place, where some capillaries formed via angiogenesis grow into larger vessels while others that are no longer needed are removed by apoptosis [78; 79]. This phase of wound healing was reported to last several weeks to months, depending on the type and extent of the wound [50; 80]. The longitudinal in vivo assessment of quantitative imaging data poses different challenges.