Adult stem cells (SCs) reside in niches which balance self-renewal with lineage selection and progression during tissue homeostasis. Following injury, culture or transplantation, SCs outside their niche often display fate flexibility1-4. Here we show that super-enhancers5 underlie the identity, lineage commitment and plasticity of adult SCs in vivo. Using hair follicle (HF) as model, we map the global chromatin domains of HFSCs and their committed progenitors in their native microenvironments. We show that super-enhancers and their dense clusters (‘epicenters’) of transcription factor (TF) binding sites change upon lineage progression. New fate is acquired by decommissioning old and establishing new super-enhancers and/or epicenters, an auto-regulatory process that abates one master regulator subset while enhancing another. We further show that when outside their niche, either in vitro or in wound-repair, HFSCs dynamically remodel super-enhancers in response to changes in their microenvironment. Intriguingly, some key super-enhancers shift epicenters, enabling them to remain active and maintain a transitional state in an ever-changing transcriptional landscape. Finally, we identify SOX9 as a crucial chromatin rheostat of HFSC super-enhancers, and provide functional evidence that super-enhancers are dynamic, dense TF-binding platforms which are acutely sensitive to pioneer master regulators whose levels define not only spatial and temporal features of lineage-status, but also stemness, plasticity in transitional states and differentiation.

Summary Tissue stem cells contribute to tissue regeneration and wound repair through cellular programs that can be hijacked by cancer cells. Here, we investigate such a phenomenon in skin, where during homeostasis, stem cells of epidermis and hair follicle fuel their respective tissues. We find that breakdown of stem cell lineage confinement - granting privileges associated with both fates - is not only hallmark, but also functional in cancer development. We show that lineage plasticity is critical in wound repair, where it operates transiently to redirect fates. Probing mechanism, we discover that irrespective of cellular origin, lineage infidelity occurs in wounding when stress-responsive enhancers become activated and override homeostatic enhancers that govern lineage specificity. In cancer, stress responsive transcription factor levels rise, causing lineage commanders to reach excess. When lineage and stress factors collaborate, they activate oncogenic enhancers that distinguish cancers from wounds.

Summary Adult tissue stem cells (SCs) reside in niches, which through intercellular contacts and signaling, influence SC behavior. Once activated, SCs typically give rise to short-lived transit-amplifying cells (TACs), which then progress to differentiate into their lineages. Here, using single cell RNA-sequencing, we unearth unexpected heterogeneity among SCs and TACs of hair follicles. We trace the roots of this heterogeneity to micro-niches along epithelial-mesenchymal interfaces, where progenitors display molecular signatures reflective of spatially distinct local signals and intercellular interactions. Using lineage-tracing, temporal single cell analyses and chromatin landscaping, we show that SC plasticity becomes restricted in a sequentially and spatially choreographed program, culminating in seven spatially arranged uni-lineage progenitors within TACs of mature follicles. By compartmentalizing SCs into micro-niches, tissues gain precise control over morphogenesis and regeneration: Some progenitors specify lineages immediately; others retain potency, preserving self-renewing features established early while progressively restricting lineages as they experience dynamic changes in microenvironment.