The mammary gland is a highly dynamic organ that undergoes profound changes within its epithelium during puberty and the reproductive cycle. These changes are fueled by dedicated stem and progenitor cells. Both short- and long-lived lineage-restricted progenitors have been identified in adult tissue as well as a small pool of multipotent mammary stem cells (MaSCs), reflecting intrinsic complexity within the epithelial hierarchy. While unipotent progenitor cells predominantly execute day-to-day homeostasis and postnatal morphogenesis during puberty and pregnancy, multipotent MaSCs have been implicated in coordinating alveologenesis and long-term ductal maintenance. Nonetheless, the multipotency of stem cells in the adult remains controversial. The advent of large-scale single-cell molecular profiling has revealed striking changes in the gene expression landscape through ontogeny and the presence of transient intermediate populations. An increasing number of lineage cell-fate determination factors and potential niche regulators have now been mapped along the hierarchy, with many implicated in breast carcinogenesis. The emerging diversity among stem and progenitor populations of the mammary epithelium is likely to underpin the heterogeneity that characterizes breast cancer.

中文翻译：

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乳腺发育中的干细胞和分化层次。

乳腺是一种高度动态的器官，在青春期和生殖周期内，其上皮内部会发生深刻的变化。专用干细胞​​和祖细胞助长了这些变化。在成年组织中以及在少数多能干乳腺干细胞（MaSCs）中都发现了谱系受限的祖先和长寿祖先，这反映了上皮层级内在的复杂性。尽管单能祖细胞在青春期和怀孕期间主要执行日常稳态和产后形态发生，但多能MaSC已参与协调肺泡生成和长期导管维护。但是，成体中干细胞的多能性仍然存在争议。大规模单细胞分子谱分析的出现揭示了通过个体发育和瞬时中间种群的存在，基因表达格局发生了惊人的变化。现在已经沿着层次结构绘制了越来越多的谱系细胞命运决定因素和潜在的生态位调节剂，其中许多与乳腺癌的致癌作用有关。乳腺上皮的茎和祖细胞群之间出现的多样性很可能会增强乳腺癌的异质性。