Epidermal cell proliferation

Summary

A new technical approach to analysing labelled cells in sheets of epidermis is presented. The changes in the proportion of isolated single labelled cells, paired or clusters of 3, 4, or more than 4, labelled cells in sheets of epidermis from the back of the mouse have been analysed at various times up to 500 h after³HTdR administration at either 03.00 h or 15.00 h. The technique is not dependent on the relative number of labelled cells (i.e. the labelling index) but on the spatial distribution of labelled cells. The data cannot be adequately explained on the basis of a simple homogeneous stem cell population in the basal layer but can be better understood on the basis of an hierarchical stem cell-dividing transit proliferative model. The data are consistent with an average cell cycle time of about 100 h but there are suggestions of considerable cell kinetic heterogeneity. The data also suggest that the amount of lateral cell movement within the basal layer is small. The results may suggest that some stem cells either loose label in a manner similar to that suggested by Cairns (1975) i.e. through a process of selective segration of their DNA strands, or that they have an extremely short S phase duration as postulated earlier (Potten et al. 1982). The present data have been extensively mathematically modelled in an accompanying paper. The model which best fits all the data is an hierarchical scheme with three cell divisions in the transit population but some branches of the lineage may be prematurely terminated by the early production of post-mitotic cells. The average resisdence time for the post-mitotic cells can be estimated to be about 2 days for cells labelled at 15.00 h.