Most regenerative tissues employ transit-amplifying cells (TACs) that are positioned in between stem cells and differentiated progeny. In a classical hierarchical model, stem cells undergo limited divisions to produce TACs, which then proliferate rapidly to expand the system and produce diverse differentiated cell types. Although TACs are indispensable for generating tissues, they have been largely viewed as a transit point between stem cells and downstream lineages. Studies in the past few years, however, have revealed some fascinating biology and unanticipated functions of TACs. In the hair follicle, recent findings have placed TACs as key players in tissue regeneration by coordinating tissue production, governing stem cell behaviors, and instructing niche remodeling. In the hematopoietic system, rather than being transient, some TACs may participate in long-term hematopoiesis under steady state. Here, we compare and summarize recent discoveries about TACs in the hair follicle and the hematopoietic system. We also discuss how TACs of these two tissues contribute to the formation of cancer. WIREs Dev Biol 2017, 6:e282. doi: 10.1002/wdev.282 For further resources related to this article, please visit the WIREs website.
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