Random segregation of DNA strands in epidermal basal cells.

According to the hypothesis proposed by Cairns, stem cells retain the older of the two parental DNA strands, whereas differentiating daughter cells receive the newly synthesized strand, so that a set of "immortal strands" persists in stem cells through successive cell divisions. To test this hypothesis, five successive divisions were induced in basal epidermal cells in vivo by two injections of cholera toxin into mouse skin and cells labeled with [3H]thymidine at the first cell cycle were chased for 50 days. If selective segregation occurs, the labeled strand should be transferred into a non-stem daughter cell after the second division and labeled cells would eventually be eliminated from the epidermis. However, the results suggest random segregation of DNA strands in epidermal basal cells. Labeled basal cells were persistently present throughout the whole epidermis for 50 days. Furthermore, labeled mitotic cells were found after the third division and their numbers of grains decreased exponentially through 5 cycles of divisions.