Live imaging of stem cell and progeny behaviour in physiological hair-follicle regeneration

A non-invasive method is used to study and manipulate hair-follicle regeneration over time in live mice, and shows that hair growth involves spatially regulated cell divisions, cellular reorganization and migration of epithelial cells, and that the mesenchyme is required for hair growth. Skin hair follicles undergo a continuous regeneration cycle, during which the stem cells located near the bottom of the follicles replenish different cell types through a spatially and temporally orchestrated process. Previous knowledge of this process has been based on snapshots of the cycle and observations made after genetic or mechanical perturbations. However, Valentina Greco and colleagues have developed an in vivo live-imaging system based on two-photon fluorescence microscopy. Using this technique, they observe stem-cell behaviour during an unperturbed, physiological hair-regeneration cycle. Their data confirm the model of two distinct progenitor cell populations within the hair follicle. By using laser ablation followed by long-term live tracking of hair-follicle regeneration, they demonstrate the important part that the mesenchyme plays in follicle regeneration. Tissue development and regeneration depend on cell–cell interactions and signals that target stem cells and their immediate progeny1. However, the cellular behaviours that lead to a properly regenerated tissue are not well understood. Using a new, non-invasive, intravital two-photon imaging approach we study physiological hair-follicle regeneration over time in live mice. By these means we have monitored the behaviour of epithelial stem cells and their progeny2,3,4 during physiological hair regeneration and addressed how the mesenchyme5 influences their behaviour. Consistent with earlier studies6, stem cells are quiescent during the initial stages of hair regeneration, whereas the progeny are more actively dividing. Moreover, stem cell progeny divisions are spatially organized within follicles. In addition to cell divisions, coordinated cell movements of the progeny allow the rapid expansion of the hair follicle. Finally, we show the requirement of the mesenchyme for hair regeneration through targeted cell ablation and long-term tracking of live hair follicles. Thus, we have established an in vivo approach that has led to the direct observation of cellular mechanisms of growth regulation within the hair follicle and that has enabled us to precisely investigate functional requirements of hair-follicle components during the process of physiological regeneration.