Mapping cell dynamics in human ex vivo hair follicles suggests pulling mechanism of hair growth

Homeostasis in continuously renewing organs like the gut, skin, and hair follicles relies on niche architecture and intricately orchestrated cellular dynamics, encompassing proliferation and migration. Here, we develop a 3D live imaging system to map single cell dynamics in micro-dissected human hair follicle segments during anagen phase in ex vivo culture. This approach reveals a spiral-like downward movement of outer root sheath cells entering the lower bulb region. We further mapped upwards flowing cells in the bulb, whose pathways align with differentiation patterns observed for stem cell-derived progenitors in rodent hair follicles. Cell division rates and orientation in the bulb correlate with the velocities of the adjacent upward-moving cell layers, with faster outer layer movement linked to higher mitotic rates. Integrating fluid dynamics simulations and experimental manipulation of mitosis and actin kinetics, we propose a mechanistic model, where a pulling force induced by the outer root sheath contributes to hair fiber extrusion.