Real-time <i>in vivo</i> structure-function study of scalp hair cycles: an experimental approach for monitoring living hair roots with a 20-year follow-up

Aim: This study presents an experimental approach for the long-term (16 years) in vivo observation of deeper structures in human scalp hair follicles, focusing on shortened hair cycles. When hair follicles entered a prolonged dormancy stage (3 years), rapid regrowth of thinning hair was stimulated by the topical application of a drug approved for promoting hair growth. Methods: While intra-epidermal chambers were ineffective as recipient sites, the most superficial dermal implantation proved successful for heterotopic transplantation of single scalp hair follicle grafts, enabling hair growth during successive cycles. Results: The study confirms the known reduction in daily hair growth rates while maintaining hair diameter. Notably, a unique and exceptional “non-correlation” was observed between scalp hair diameter and linear growth rates. While the initial shortening of the hair cycle duration was recorded in the first 3 years post-grafting, a further reduction eventually led to a “dormancy” phase in the long term (16 years post-grafting). Pharmacodynamics revealed that reactivation of hair production, following less than 3 months of topical application of a 5% minoxidil solution, suggested very slow daily growth rates and a substantial reduction in hair diameter. Conclusion: The initial findings suggest that non-invasive methods can be used for real-time structural-functional observation of scalp hair roots, enabling the study of cellular activity and movement during successive phases of the hair cycle, including aging and drug effects.