The future of mammary stem cell biology: the power of in vivo transplants

Excerpt

The recent review by Smith and Medina [1] of in vivo transplantation models and their role in investigating mammary stem cell (MaSC) biology provides comprehensive coverage of the history and complexity of the 'gold standard' MaSC assay in mice. This includes a description of the pioneering studies that showed that mammary epithelial outgrowths can be generated in cleared mammary fat pads transplanted with explants or admixtures of mammary cells [2]. However, this approach clearly does not lend itself to prospective analysis of isolated subpopulations in order to identify which cells possess in vivo regenerative activity. More recently, success in obtaining complex mammary gland structures from transplanted suspensions of single cells has now made this possible [3‐7]. Moreover, the regenerated structures have been shown to contain daughter cells with the same in vivo repopulating activity of the original stem cell transplanted [4, 6]. A major contribution from this advance has been the demonstration that the MaSCs thus defined are highly enriched in the CD49f^hi/CD29^hi/CD24^+/mod/Sca-1^- subset [4‐6]. Nevertheless, it is important to recognize that these stem cells represent under 10% of this basal population. This population also contains mature myoepithelial cells and, in all likelihood, other basal cell intermediates that are yet to be identified. …