Proliferation in murine epidermis after minor mechanical stimulation. Part 1. Sustained increase in keratinocyte production and migration.

It was our objective to obtain an insight into the details and dynamics of the cell proliferative changes following minor barrier disruption, the mechanisms of recovery, and their regulation. Hair of the dorsal area of DBA2-mice was removed and the epidermis was tape stripped. Tritiated thymidine was injected into groups of mice at daily intervals thereafter. Labelling and nuclear densities were measured at several time intervals later in the various epidermal strata to characterize cell production and cell fluxes through the tissue. A dramatic proliferative response was observed at 24 h when the labelling density increased more than sixfold in the basal layer. Labelled cells rapidly appeared in suprabasal layers within a few hours in large quantities while this process took over 2 days in normal skin. Some cycling cells were also found in the suprabasal layer (pulse labelling at 24 h) in contrast with the controls. The cellular flux through the suprabasal layers was drastically (20-fold) increased and the transit time was shortened. Although the nuclear density in the basal layer showed only moderate changes it increased four-fold in the suprabasal layer within 5 days. A kinetic model analysis suggested that the cell cycle time of proliferative cells dropped from a normal value of about 200 h to less than 12 h post tape strip. After 7 days, the proliferative activation still persisted, even though at 3 days post tape strip the stratum corneum had been re-established. Hence, a mild mechanical alteration with removal of some parts of the cornified layer in mouse backskin epidermis triggers a huge proliferative response with massive overproduction of cells that lasts at least 7 days. Our findings suggest that the re-establishment of the cornified layer does not immediately shut down cell proliferation and that more complex, slower (long-term) regulatory processes are involved.

Proliferation in murine epidermis after minor mechanical stimulation. Part 2. Alterations in keratinocyte cell cycle fluxes.

We have recently shown that a mild mechanical irritation (tape strip) of the epidermis on the back skin of adult mice induces a strong and long lasting increase in proliferative activity and cell production. This was revealed by following the fate of 3HTdR-pulse labelled cells within the basal and suprabasal layers. To obtain further insight into the dynamics of cell kinetic changes we also performed double labelling experiments with 3HTdR and BrdUrd at various times after tape stripping. The technique for analysing the data had to account for a non stationary cell flux. A novel biometrical technique was developed which provides parameter estimates on the S-phase duration, the cell cycle duration and a parameter characterizing the degree of nonstationarity. When applied to the mechanically irritated epidermis we observed that the cell flux through the S-phase in the basal layer was accelerated by a factor of 10 between 18 and 36 h post tape strip. This activation declined slightly in the subsequent days and remained 4-6 fold higher than in the normal steady state for over 7 days post tape strip. The duration of the S-phase was 3-5 h and showed little variation. We conclude that mild mechanical irritation only affecting the stratum corneum has major stimulatory effects on the cell kinetics of proliferative keratinocytes in the basal layer of the epidermis indicating the existence of a powerful regulatory mechanism.