SummaryStem cell niches in mammalian tissues are often heterogeneous and compartmentalized, however whether distinct niche locations determine different stem cell fates remains unclear. To test this hypothesis, we utilized the mouse hair follicle niche and devised a novel approach by combining intravital microscopy with genetic lineage tracing to re-visit the same stem cell lineages, from their exact place of origin, throughout regeneration in live mice. Using this method, we show directly that the position of a stem cell within the hair follicle niche can predict whether it is likely to remain uncommitted, generate precursors or commit to a differentiated fate. Furthermore, using laser ablation we demonstrate that hair follicle stem cells are dispensable for regeneration and that epithelial cells, which do not normally participate in hair growth, re-populate the lost stem cell compartment and sustain hair regeneration. This study provides a general paradigm for nicheinduced fate determination in adult tissues.

Adult tissues replace lost cells via pools of stem cells. However, the mechanisms of cell self-renewal, commitment, and functional integration into the tissue remain unsolved. Using imaging techniques in live mice, we captured the lifetime of individual cells in the ear and paw epidermis. Our data suggest that epidermal stem cells have equal potential to either divide or directly differentiate. Tracking stem cells over multiple generations reveals that cell behavior is not coordinated between generations. However, sibling cell fate and lifetimes are coupled. We did not observe regulated asymmetric cell divisions. Lastly, we demonstrated that differentiating stem cells integrate into preexisting ordered spatial units of the epidermis. This study elucidates how a tissue is maintained by both temporal and spatial coordination of stem cell behaviors.

Summary: Maintenance of adult tissues depends on sustained activity of resident stem cell populations, yet the mechanisms that regulate stem cell self-renewal during homeostasis remain largely unknown. Using an imaging and tracking approach that captures all epidermal stem cell activity in large regions of living mice, we show that self-renewal is locally coordinated with epidermal differentiation, with a lag time of one to two days. In both homeostasis and upon experimental perturbation, we find that differentiation of a single stem cell is followed by division of a direct neighbor, but not vice versa. Finally, we show that exit from the stem cell compartment is sufficient to drive neighboring stem cell self-renewal. Together, these findings establish that epidermal stem cell self-renewal is not the constitutive driver of homeostasis. Instead it is precisely tuned to tissue demand and responds directly to neighbor cell differentiation.

Tissue repair is fundamental to our survival as tissues are challenged by recurrent damage. During mammalian skin repair, cells respond by migrating and proliferating to close the wound. However, the coordination of cellular repair behaviors and their effects on homeostatic functions in a live mammal remains unclear. Here we capture the spatiotemporal dynamics of individual epithelial behaviors by imaging wound re-epithelialization in live mice. Differentiated cells migrate while the rate of differentiation changes depending on local rate of migration and tissue architecture. Cells depart from a highly proliferative zone by directionally dividing towards the wound while collectively migrating. This regional co-existence of proliferation and migration leads to local expansion and elongation of the repairing epithelium. Finally, proliferation functions to pattern and restrict the recruitment of undamaged cells. This study elucidates the interplay of cellular repair behaviors and consequent changes in homeostatic behaviors that support tissue-scale organization of wound re-epithelialization.

SummaryTissue homeostasis is achieved through a balance of cell production (growth) and elimination (regression)1,2. Contrary to tissue growth, the cells and molecular signals required for tissue regression remain unknown. To investigate physiological tissue regression, we use the mouse hair follicle, which cycles stereotypically between phases of growth and regression while maintaining a pool of stem cells to perpetuate tissue regeneration3. Here we show by intravital microscopy in live mice4–6 that the regression phase eliminates the majority of the epithelial cells by two distinct mechanisms: terminal differentiation of suprabasal cells and a spatial gradient of apoptosis of basal cells. Furthermore, we demonstrate that basal epithelial cells collectively act as phagocytes to clear dying epithelial neighbors. Through cellular and genetic ablation we show that epithelial cell death is extrinsically induced through TGFβ activation and mesenchymal crosstalk. Strikingly, our data show that regression acts to reduce the stem cell pool as inhibition of regression results in excess basal epithelial cells with regenerative abilities. This study identifies the cellular behaviors and molecular mechanisms of regression that counterbalance growth to maintain tissue homeostasis.