The Spatiotemporal Coupling Imbalance of Senescent Grey Hair: Remodelling of the HFSC-MeSC-Niche Network and a New Paradigm of Stem Cell Senescence.

This review examines age-related hair greying as a potential imbalance in the HFSC-MeSC-niche network in terms of time, space and signal quality. We define signal quality as the effective strength, temporal precision and noise-to-threshold relationship of niche-derived cues that regulate MeSC activation. We propose that ageing may progressively weaken the temporal coordination between HFSC activation and MeSC responsiveness. ECM remodelling and altered guidance-related cues may further impair MeSC niche engagement. Meanwhile, chronic oxidative stress, low-grade inflammation and stress-related sympathetic norepinephrine signalling may increase background noise, distort signal timing and push the MeSC activation threshold upward, so residual MeSC-lineage cells may fail to engage effectively with the niche. Such functional uncoupling may represent an early or intermediate stage, whereas sustained genotoxic stress or niche failure can eventually drive irreversible MeSC depletion. Based on this, the article constructs an integrative model, emphasizing that future intervention strategies may need to shift from simply protecting MeSCs to preserving the temporal and spatial coupling of the HFSC-MeSC-niche network. Potential approaches may include reducing chronic niche stress, maintaining HFSC-MeSC communication, preserving ECM organization and improving the timing and strength of pro-pigmentation cues. We further propose that hair greying may provide an accessible visual readout of stem-cell niche ageing.