Breast cancer is a highly heterogeneous disease at both the molecular and pathological levels. To understand this heterogeneity and the ‘cells of origin’ of breast cancer, it is important to dissect the normal mammary epithelial hierarchy. Despite accumulating evidence for a mammary differentiation hierarchy, the basal compartment comprising stem cells remains poorly characterised. Through gene expression profiling of Lgr5+ versus Lgr5– basal epithelial cells, we identify a novel marker that led to the fractionation of three distinct mammary stem cell (MaSC) subsets in the adult gland. These exist in a largely quiescent state but differ in their repopulating ability, spatial localisation, and their molecular signatures. Interestingly, the dormant MaSC subset localises to the proximal region of the gland throughout life. These cells appear to originate from the embryonic mammary primordia before switching to a quiescent state post-natally but can be recruited into the cell cycle in response to hormones.
Single cell gene expression analyses have revealed unexpected complexity within the basal and luminal compartments. Moreover, analyses at different stages of development have provided insights into the earliest ‘lineage priming’ events and a large-scale shift in gene expression programs. In a further layer of investigation, lineage tracing studies, a strategy for tracking stem and progenitor cell fate in situ, have been implemented to explore the contribution of stem and progenitor cells to post-natal mammary gland development and tissue homeostasis. The combination of lineage tracing with 3D imaging has enabled the visualisation of large regions of intact tissue at cellular resolution and provided insights into the normal differentiation hierarchy as well as potential ‘cells of origin’ of breast cancer. More recently, we have extended 3D confocal imaging to explore the role of immune cells in the developing mouse mammary gland.