Mammary stem cells (MaSCs) have been defined by cell surface marker expression and their ability to repopulate a cleared fat pad, a capacity now known to result from reprogramming upon transplantation. Furthermore, lineage-tracing studies have provoked controversy as to whether MaSCs are unipotent or bi/multipotent. Various innovative experimental approaches, including single-cell RNA sequencing (scRNA-Seq), epigenetic analyses, deep tissue and live imaging, and advanced mouse models, have provided new and unexpected insights into stem and progenitor cells; thus, it is now timely to reappraise our concept of the MaSC hierarchy. Here, I highlight misconceptions, suggest definitions of stem and progenitor cells, and propose a way forward in our search for an understanding of MaSCs.

乳腺干细胞 (MaSC) 已通过细胞表面标志物表达及其重新填充清除的脂肪垫的能力来定义，现在已知这种能力是由移植后重新编程产生的。此外，谱系追踪研究引发了关于 MaSC 是单能还是双/多能的争议。各种创新的实验方法，包括单细胞 RNA 测序 (scRNA-Seq)、表观遗传分析、深层组织和活体成像以及先进的小鼠模型，为干细胞和祖细胞提供了新的和意想不到的见解；因此，现在是时候重新评估我们对 MaSC 层次结构的概念了。在这里，我强调了误解，提出了干细胞和祖细胞的定义，并提出了我们对 MaSC 理解的探索之路。