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Summary
The ability of certain adult stem cell types to differentiate into other cell types, expand massively, or quiesce until needed underlies the study of development and of how cells grow uncontrollably in cancer. Adult stem cell behavior in many organs depends on unique, protected microenvironments harboring specialized extracellular matrix proteins, support cells, hormones, or other factors collectively called stem cell niches (1, 2). But what, exactly, constitutes a stem cell niche? On page 284 of this issue, Zhao et al. (3) report an element of the mammary gland stem cell niche that drives local expansion and growth of the epithelial compartment in response to global hormones. Select fibroblast cells that are peripheral to the stem cells “translate” hormones into signals that stem cells recognize.
