In Vivo Non-Invasive Observation of Dynamic Changes in Hair Shaft, Melanin, and Collagen During the C57BL/6 Follicle Cycle

This study systematically monitored hair shaft, melanin, and collagen dynamics in vivo using non-invasive methods, enabling continuous observation over multiple hair follicle cycles. The hair shafts of C57BL/6 mice were observed in vivo using dermoscopy, photography, and electron microscopy. Second harmonic generation (SHG) and photography were employed for in vivo observation and quantification of melanin dynamics throughout the hair follicle cycle—specifically during the anagen phase, catagen phase, and telogen phase stages. Accuracy was validated via hematoxylin and eosin (H&E) staining. Additionally, SHG was used for in vivo observation and quantification of collagen changes across the same hair cycle phases, with validation performed using a liquid crystal polarizing imaging system and Sirius Red staining. This project provides non-invasive and in vivo methods for evaluating hair shafts, melanin, and collagen fibers across different growth cycles, thereby supporting non-invasive research in the development of active agents for hair loss treatment.