Looking in the Rear-View Mirror: How Lineage Tracing Experiments Redefined Cell Hierarchy in the Mammary Gland

Understanding the cell hierarchy of the mammary gland has been a major focus of research for several decades. This review retraces the key advances enabled by lineage tracing approaches to study stem cells and cell hierarchy in the mammary gland. Although not exhaustive, this review highlights the paradigm shifts brought about by lineage tracing studies, which have, in the author's view, profoundly reshaped our understanding of mammary epithelial organization. In particular, these studies have challenged traditional models of cell hierarchy and the concept of terminally differentiated cells in the mammary gland.