Unveiling Hair Follicle Stem Cells

Adult mammalian skin consists of the epidermis, hair follicles (HFs), and sebaceous glands (SGs). Each of these three epithelial lineages contains its own stem cell (SC) population for normal tissue homeostasis, HF cycling, and repair of the epidermis following injury. Here, we provide an overview of the current knowledge on follicle SCs of the adult skin, including their essential features and, most importantly, the control of follicle SC fate. Wnt/β-catenin is required for follicle SC maintenance and niche biology, and β-catenin activation is essential for promoting quiescent follicle SCs to proliferate and terminally differentiate along the hair cell lineage. Further, β-catenin stabilization promotes de novo HF morphogenesis, and constitutively active β-catenin expression results in pilomatricoma. Both bone morphogenetic protein (BMP) and transforming growth factor-β (TGF-β) signals are required for quiescent niche maintenance: BMP deletion results in SC activation, whereas TGF-β may play a role in SC identity maintenance.