Exogen hair characterization in human scalp

Background/aim: Classically, the hair cycle is described as a sequence of three successive phases: a hair‐growth phase named anagen, a regression phase or catagen and a resting phase or telogen. In rodents, it appears that the resting hair follicle population contains also a new phase that has been identified recently as the exogen phase of the hair cycle. This phase leads to the release of the telogen club and results in hair shedding. The aim of this paper is to propose a method that is applicable to humans and that is able to discriminate the two components of the resting hair population i.e. the telogen and the exogen hair follicles. Methods: We used non‐invasive approaches to entrap exogen scalp hair into silicon‐based polymers. We also extracted growing and non‐growing hair with a calibrated dynamometer. We characterized differences between anagen, catagen, telogen and exogen root ends with histochemical stains and with the scanning electron microscope. Furthermore, we documented all known hair‐cycle stages with the contrast‐enhanced phototrichogram (CE‐PTG) technique. Results: We demonstrated that anagen and telogen hair are firmly anchored to the hair follicle and that cohesion forces are correlated with hair thickness. On the contrary, exogen hair are passively retained within the hair follicle. Among the resting hair population, telogen clubs retain cellular elements of the outer root sheaths that are not found on exogen hair. The specificity of the new exogen collection method was documented with the simultaneous use of the CE‐PTG method: indeed anagen, catagen and telogen follicles remain unaffected by the exogen extraction procedure. Conclusion: Exogen hair can be sampled specifically from the human scalp with a new non‐invasive method. Our data suggest that the casual levels of exogen hair, in normal individuals and under the present experimental conditions, are usually less than seven hair per cm 2 .