Sic Transit Gloria: Farewell to the Epidermal Transit Amplifying Cell?

In the 2007 document, the traditional model of epidermal homeostasis, which posits a hierarchy of epidermal stem cells giving rise to transit amplifying (TA) cells, is challenged by new research. The review discusses the potential for a single population of proliferating cells to maintain tissue homeostasis, with a focus on the plasticity and location of epidermal stem cells in the hair follicle, interfollicular epidermis, and sebaceous glands. It also critiques the epidermal proliferative unit (EPU) model and the use of label-retaining cells (LRC) to identify stem and TA cells, suggesting that the epidermal cell population is more complex and heterogeneous than previously thought. A study using the AhcreER transgenic mouse line for lineage tracing over a year revealed that clonal evolution in the interfollicular mouse tail epidermis is governed by a single rate-limiting process, with proliferating basal cells adopting one of three fates at random, which challenges the stem/TA cell model and proposes that homeostasis can be maintained without quiescent stem cells entering the cycle.