Background: Blood vessel formation during skin wound healing is a rapid and largely uncontrolled process providing oxygenation to the damaged tissue for repair. However, once the wound is repaired the majority of the newly formed blood vessels are no longer required at a time where wound fibrosis is being constituted. To date, it is known that ageing endothelial cells lining blood vessels may lose endothelial characteristics and undergo a process called endothelial to mesenchymal transition (EndMT), where mesenchymal properties are up-regulated causing functional change. Our aim was to therefore use murine in vivo lineage tracing models of endothelial cells in skin wound healing to determine the fate characteristics.
Methods: Two murine models were adopted for in vivo endothelial tracing during wound healing. Col1a2-lacZ and sox18CreER/Rosa-YFP mice were used and wounds were harvested at D3, 5, 10, 14, and 21. Assessment of endothelial cell phenotype at each time point was conducted by immunofluorescence.
Results: Positive LacZ staining in CD31+ endothelial cells was only observed in wounds harvested at D10, 14 and 21, confirming the up-regulation in expression of the Col1a2 gene in ageing endothelial cells as opposed to earlier time points. Using the YFP reporter strategy, labelled YFP positive endothelial cells at D0 could be traced over the duration of wound healing. YFP positive cells were exclusively endothelial by D5 but co-stained with mesenchymal markers such as α-SMA and vimentin at D10, 14 and 21.
Conclusion: This is the first description of EndMT in skin wound healing, demonstrating change in phenotypic and functional characteristics of endothelial cells using two unique in vivo tracing models. This finding provides furthers details on the rapid formation and degradation of blood vessels in skin wound healing and the transitional process into scar tissue formation.