Metalloproteinases facilitate connection of wound bed vessels to pre-existing skin graft vasculature

Background: Despite advances in tissue engineering of human skin, the exact revascularization processes remain unclear. Therefore it was the aim of this study to investigate the vascular transformations during engraftment and to identify associated proteolytic factors.

Methods: The modified dorsal skinfold chamber with autologous skin grafting was prepared in C57BL/6J mice, and intravital microscopy was performed. The expression of proteases and vascular factors was quantified by immunohistochemistry.

Results: Reperfusion of the skin graft after 72hours was followed by a temporary angiogenic response of the graft vessels. Wound bed bud formation appeared after 24 to 48hours representing starting points for capillary sprouting. In the reperfused skin graft larger buds developed over several days without transformation into angiogenic sprouts; instead pruning took place. MT1-MMP was detected at sprout tips of in-growing vessels. MMP-2 expression was located at the wound bed/graft connection sites. Pericytes were found to withdraw from the angiogenic vessel in order to facilitate sprouting.

Conclusions: Skin graft vasculature responded with temporary angiogenesis to reperfusion, which was pruned after several days and exhibited a different morphology than regular sprouting angiogenesis present within the wound bed. Furthermore we identified MT1-MMP as sprout-tip located protease indicating its potential role as sprout growth facilitator as well as potentially in lysing the existing graft capillaries in order to connect to them. The differences between the wound bed and skin graft angiogenesis may represent a relevant insight into the processes of vascular pruning and may help in the engineering of skin substitutes.