Melanin Light Absorption as the Necessary but Not Sufficient Condition for Photoepilation: Intra-Anagen Variability of Hair Follicle Light Sensitivity

Abstract

Several well-known, contradictive photoepilation phenomena cannot be reliably explained within the framework of traditional theory. Counted among are the contradiction between the total number of anagen hairs and the number of light sensitive hairs as well as the paradoxical, regional variations in the light sensitivity of anatomically similar hairs. These and some other phenomena can, however, be explained by intra-anagen variability of the hair follicle (HF) sensitivity to light. Melanin content in the hair matrix is important for the initial temperature increase at the distal portion of the growing HF, where strong epithelial and mesenchymal remodeling processes take place, thus being the necessary condition for photoepilation. This condition is, however, not sufficient for effective hair removal. We discuss the variation of intra-anagen light sensitivity and its possible correlation with sonic hedgehog activity, which is strongly responsible for the remodeling processes at the distal end of the growing HF. It is concluded that the theory needs much modification, along with revision of the optimal treatment strategy.