Heterogeneity in the replicating population of cultured human epidermal keratinocytes.

We studied the replication of keratinocytes in stratified squamous epithelia. Other studies have revealed functional and morphological heterogeneity in the replicating population of such cells. To examine possible kinetic heterogeneity, we determined the cell-cycle lengths of replicating cells in cultures of human epidermal keratinocytes. A double-label assay was developed, which measures the time between two successive cycles of DNA synthesis. The first cycle of DNA synthesis was marked by pulse labeling cultures for a brief period with 14C-thymidine (dThd), and the second cycle was detected by labeling at a later time with bromodeoxyuridine (BrdUrd). The time taken for the 14C-labeled DNA to become doubly labeled with BrdUrd was shown to correspond to the length of the cell cycle. In subconfluent cultures in which the cell number increased at an exponential rate, the average cell-cycle time was 21.5 h. In confluent cultures in which desquamation was balanced by cell renewal, the average cell cycle was 31.5 h. However, in confluent cultures, three populations of replicating cells were evident, these having cycle times of 22, 33, and 40 h. In subconfluent cultures, there was no clear evidence for cell-cycle heterogeneity of the replicating cells, although the most rapidly cycling cells in these cultures had a cycle time (16 h) considerably less than the most rapidly cycling cells in the confluent cultures (21 h). It is possible that the rapidly cycling cells seen in the subconfluent cultures were stem cells.(ABSTRACT TRUNCATED AT 250 WORDS)