It has been postulated that the stem cells of somatic tissues protect themselves from mutation and cancer risk by selective segregation of their template DNA strands. Self-renewing mammary epithelial stem cells that were originated during allometric growth of the mammary ducts in pubertal females were labeled using [H-3]-thymidine ((3)HTdR). After a prolonged chase during which much of the branching duct morphogenesis was completed, (3)HTdRlabel retaining epithelial cells (LREC) were detected among the epithelium of the maturing glands. Labeling newly synthesized DNA in these glands with a different marker, 5-bromodeoxyuridine (5BrdU), resulted in the appearance of doubly labeled nuclei in a large percentage of the LREC. By contrast, label-retaining cells within the stroma did not incorporate 5BrdU during the pulse, indicating that they were not traversing the cell cycle. Upon chase, the second label (5BrdU) was distributed from the double-labeled LREC to unlabeled mammary cells while (3)HTdR was retained. These results demonstrate that mammary LREC selectively retain their (3)HTdR-labeled template DNA strands and pass newly synthesized 5BrdU-Iabeled DNA to their progeny during asymmetric divisions. Similar results were obtained in mammary transplants containing selfrenewing, lacZ-positive epithelial cells suggesting that cells capable of expansive self-renewal may repopulate new mammary stem cell niches during the allometric growth of new mammary ducts.
