From a stem-cell-centered to a niche-centered view: the core role of collagen networks in hair loss and hair follicle miniaturization.

Hair follicle miniaturization is a quantifiable histopathological endpoint shared by multiple forms of alopecia. The conventional "stem cell-centric" view often attributes regenerative failure to depletion or intrinsic dysfunction of hair follicle stem cells (HFSCs). However, in canonical trajectories such as human androgenetic alopecia, HFSC-related populations may remain detectable by marker-based analyses, whereas progenitor output is reduced. This pattern suggests that impaired conversion from quiescent HFSCs into an expandable progenitor/transit-amplifying compartment may contribute to miniaturization, while not excluding concomitant HFSC functional decline. We therefore propose "niche identity," which treats the follicular niche as a set of measurable, stratifiable, and intervention-amenable structural-mechanical constraints. We posit that the collagen network may act as an integrative hub that influences regenerative thresholds and the stability of lineage output through interfacial continuity, fibrillar topology, and local mechanical states. Niche identity is defined here by five coupled state variables: basement membrane boundary integrity, adhesion/anchoring apparatuses, fibrillar topological organization, mechanical set-points, and hair cycle-scaled dynamic remodeling windows. We propose that these elements may drift coordinately under androgen-biased profibrotic remodeling, chronic low-grade inflammation with MMP-mediated matrix degradation, and aging/glycation-associated crosslinking and stiffening, thereby locking follicles into a low-output steady state. Finally, we discuss "signal-structure mismatch" as a plausible basis for unstable therapeutic responses and relapse and propose a niche identity-oriented translational framework intended to guide future experimental testing and endpoint selection.