Temporal variation in cellular proliferation during recornification of mouse tail skin

The influence of the time of injury on subsequent epidermal regeneration is unknown. Epidermal cell proliferation of tail skin in C57BL/6J mice in res ponse to tape stripping was followed for 7 days by radiolabelled thymidine incorporation and autoradiography. The homeostatic labelling index (LI) of the basal epidermis of unmanipulated, unwounded (control) animals was 7.6% and did not vary depending on the time of day. Tape stripping increased the LI of epidermal basal cells 110% above control values 24 h after injury. L abelling indexes of epidermal basal cells in the skin adjacent to the wound ed area were 7.0%. Basal cell DNA synthesis stimulated by wounding exhibite d a distinct temporal variation at 24 h postinjury, with tail skin wounded at 12.00 h found to be 275% greater than control values and elevated 78% fr om LIs recorded at any other time point. This temporal spike was due to the time of day at which wounding occurred rather than the time point when the LI was determined. Mice wounded at 12.00 h and terminated 27 h later (15.0 0 h) had LIs that were 52% greater than wounds created at 09.00 h and exami ned at 12.00 h the following day. Higher levels of DNA synthesis in tail sk in injured at 12.00 h compared to wounding at 09.00 h was detected 12-48 h after injury. Furthermore, DNA synthesis in wounds created at 12.00 h retur ned to baseline levels 1-2 days earlier than tail skin wounded at 09.00 h. Investigation of other strains of mice detected differences in radiolabelli ng of epidermal basal cells 24 h after tape stripping at 12.00 h or 09.00 h in CD-1 and BALB/cJ mice, but not in the C3H/HeJ strain. These results ind icate: (a) there is no diurnal variation in the LI of mouse tail skin under normal homeostatic conditions (b) tape stripping is a potent stimulator of basal cell turnover in the epidermis (c) the time of wounding determines t he magnitude of the increase in the LI of basal cells following injury, and (d) the proliferative response to wounding of the tail is dependent on the strain of mouse.