Scratch-induced partial skin wounds re-epithelialize by sheets of independently migrating keratinocytes
Bornes L, Windoffer R, Leube RE, Morgner J, van Rheenen J
Re-epithelialization is a crucial process to reestablish the protective barrier upon wounding of the skin. Although this process is well described for wounds where the complete epidermis and dermis is damaged, little is known about the re-epithelialization strategy in more frequently occurring smaller scratch wounds in which structures such as the hair follicles and sweat glands stay intact. To study this, we established a scratch wound model to follow individual keratinocytes in all epidermal layers in the back skin of mice by intravital microscopy. We discover that keratinocytes adopt a re-epithelialization strategy that enables them to bypass immobile obstacles such as hair follicles. Wound-induced cell loss is replenished by proliferation in a distinct zone away from the wound and this proliferation does not affect overall migration pattern. Whereas suprabasal keratinocytes are rather passive, basal keratinocytes move as a sheet of independently migrating cells into the wound, thereby constantly changing their direct neighboring cells enabling them to bypass intact obstacles. This re-epithelialization strategy results in a fast re-establishment of the protective skin barrier upon wounding.
Re-epithelization of the wound bed is initiated by basal keratinocytes ∼20 h after wound induction. Representative movie of suprabasal (left panel) and basal (right panel) keratinocyte dynamics in response to induced scratch wounds in the back skin of R26-mTmG; R26-ACTB-CreERT2 mice. These mice express mTom+ (magenta) in all cells and some cells are randomly labelled by GFP+ (green) after tamoxifen-induced recombination facilitating the study of individual cells. Keratinocytes were tracked shortly after wound induction and images were taken every 30 and 45 min–40 h after wounding.
Basal keratinocytes migrate underneath the suprabasal layer. Representative movie of depth color coded z-maximum projection of random, individually labelled GFP+ migrating keratinocytes. Color code in the top right corner, with blue representing superficial suprabasal layer and red representing deeper basal layer. Tracking of keratinocytes was initiated 16 h post wounding and images were taken every 30 min over the course of minimally 8 h.
Migrating keratinocytes bypass hair follicle obstacles. Representative movie of migrating keratinocytes in a Fucci2 mouse 16 h after scratch wound induction bypassing a hair follicle (*) at a distance of 200–400 μm away from the wound. Scale bar, 50 μm. Tracks of migrating keratinocytes are depicted.
Uncoupled proliferation of migration of basal keratinocytes. Representative movie of migrating basal layer dynamics in response to induced scratch wounds in the back skin of Fucci2 mice. These mice express mCherry-hCdt1 (magenta) in cells that are in a G1-cell cycle state and mVenus-hGem (green) in proliferating cells in S/G2 phase. Keratinocytes were tracked 16–24 h after wound induction and images were taken every 30 min. Scale bar 100 μm.