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The mammary gland consists of an epithelial ductal tree embedded in a fat pad. Adult mammary epithelium has been demonstrated to have outstanding regenerative potential, consistent with the presence of resident, adult stem cells. However, there are currently no bona fide markers to identify these cells within their tissue context. Here, we introduce long-term label retention as a method to investigate the location of quiescent cells (a property attributed to adult stem cells) in situ. Long-term label retaining cells divide actively during tissue development and remain quiescent at homeostasis. These two properties have been attributed to adult stem cells. Therefore, label-retaining cells can be used to identify populations that contain stem cells. We describe the materials and methods necessary to identify and image mammary label-retaining cells, to carry out morphometric analysis on these cells and to map their distribution of the mammary epithelium. The morphometric and spatial analyses described here are generally applicable to any mammary cell populations, and will therefore be useful to characterize mammary stem cells once bona fide mammary stem cell markers become available.

Mammary reconstitution assays can be used to measure the stem cell frequency within an epithelial population by transplanting increasingly diluted single-cell preparations of the population of interest. There are fundamental steps in the single-cell isolation protocol which are directly related to the number of single epithelial cells obtained. Once single-cell suspensions have been obtained, serial dilutions are prepared and transplanted into the cleared fat pads of the host mice. After 8-10 weeks, the transplanted fat pads are reevaluated for the presence of epithelial outgrowths. Based on the frequency of no outgrowth for each one of the transplanted dilutions, it is possible to estimate the frequency of mammary repopulating cells present in a given cell population. Here, we give details on how to carry out all these steps.

The regenerative potential of mammary epithelium facilitates assessment of the "stemness" of any epithelial subpopulation in transplantation assays. Thus, mammary tissue can be dissociated into single cells, stained for cell surface markers of interest and classified using fluorescence-activated cell sorting. The selected cells can then be transplanted into epithelium-devoided fat pads from recipient hosts. Recent publications have described markers that enrich for mammary repopulating potential. Here, we describe the materials and methods necessary to sort cells according to these markers. This approach can be used interchangeably with other cell surface markers with slight variation to the protocol.