Modeling of ionizing radiation induced hair follicle regenerative dynamics.

Hair follicles (HFs) are stem-cell-rich mammalian mini-organs that can undergo cyclic regenerations over the life span of the organism. The cycle of a HF consists of three consecutive phases: anagen-the active proliferation phase, catagen-the degeneration phase, and telogen-the resting phase. While HFs undergo irreversible degeneration during catagen, recent experimental research on mice shows that when anagen HFs are subject to ionizing radiation (IR), they undergo a transient degeneration, followed by a nearly full regeneration that makes the HFs return to homeostatic state. The mechanisms underlying these IR-induced HF regenerative dynamics and the catagen degenerative dynamics, remain unknown. In this work, we develop an ODE type cell differentiation population model to study the control mechanisms of HF regeneration. The model is built based on current theoretical knowledge in biology and mathematically formulated using feedback mechanisms. Model parameters are calibrated to IR experimental data, and we then provide modeling results with both deterministic ODE simulations and corresponding stochastic simulations. We perform stability and bifurcation analyses on the ODE model, which reveal that for anagen HFs, a low spontaneous apoptosis rate secures the stability of the HF homeostatic steady state, allowing the HF to regenerate even when subject to strong IR. On the other hand, the irreversible degeneration during catagen results from both strong spontaneous apoptosis rate and strong apoptosis feedback. Lastly, we perform sensitivity analysis to identify key parameters in the model to validate these hypotheses.