Cell kinetic studies in the epidermis of the mouse. I. Changes in labeling index with time after tritiated thymidine administration.

The changes in the labeling index (LI) with time after a single injection of tritiated thymidine (3HTdR) at each of 4 different times of the day have been studied. Slight differences occur in the shape of these LI curves, (e.g. in the timing of the peaks) depending on the time of day when the initial injection was given. Thus, the time of day influences not only the number of cells in DNA synthesis but also determines the subsequent behavior of the labeled cells. The curves show 3 distinct peaks from which estimates of the cell cycle time can be made. The technique permits the cell cycle time to be estimated. From the data as a whole a minimum cell cycle time of 90 h for basal cells in the epidermis on the back of a mouse is obtained. The technique also provides estimates for the duration of S + G2 + M which varies depending on the time of day that the label is given. The LI curves can best be understood if the basal layer is assumed to contain 2 cell populations with differing cell cycle times; one having a long cell cycle (about 180 h) but short S-phase and containing the stem cells, the other having a short cell cycle (about 90 h) and a long S-phase duration and consisting of transit cells.