Dynamic Ultrastructural Changes of the Connective Tissue Sheath of Human Hair Follicles During the Hair Cycle

The study examined the ultrastructural changes in the connective tissue sheath (CTS) of human hair follicles throughout the hair cycle using normal scalp skin specimens. During early anagen, the CTS consisted of a thin basal lamina and surrounding collagen tissue, which thickened as the hair developed. In mature anagen follicles, the collagen tissue was organized into three layers with distinct fiber orientations and fibroblast presence. In late anagen, the basal lamina thickened significantly. During catagen, the basal lamina and inner collagen layer showed corrugation, oedematous changes, and degeneration, while fibroblasts actively produced new collagen fibers to fill spaces left by follicle retraction. These findings indicated dynamic metabolic changes in the connective tissue around hair follicles during the hair cycle.