Effect of Laser and Light-Based Systems on Hair Follicle Biology

Over the last decade, photoepilation has become one of the most popular method for longer-term management of unwanted hair growth. Based on the principle of selective photothermolysis first described by Anderson and Parrish [1], several different types of the lasers and noncoherent light sources have been developed to selectively exert the effects on hair follicle structure and achieve a long lasting hair removal benefit [2-8]. Under this principle, advantage is taken of the endogenous chromophore melanin concentrated in the pigment-producing melanocytes of the hair matrix, and in the keratinocytes of the hair shaft of anagen hair follicles and its absence in the surrounding dermal tissue thus allowing for a selective targeting of the laser energy. Based on the level of hair follicle melanin and the laser parameters used, the amount of thermal energy released can cause a variable damage to the follicle, resulting in a hair removal ranging from a temporary to a permanent effect [9-12]. Topping et al. demonstrated that the light – induced effects are directly related to an increase in the temperature of the hair follicle [13]. The rise in temperature is dependent both on the quantity and the type of the melanin pigment present. Dark pigmented hair follicles with high eumelanin content are more sensitive to the laser, compared to the blonde, red, and gray hair, which are either nonresponsive or show a minimal effect [14,15].

At the basic biochemical and molecular level, the released thermal energy from laser treatment can either simply kill the fibre producing cells of hair follicle by causing denatur – ation of cellular proteins and phospholipid membranes or can modify the molecular mechanisms and signaling pathways that control the growth and cycling of the hair follicle. Changes in the hair follicle structure and histopathological alterations after exposure to high fluence laser energy to permanently effect hair growth have been studied by several researcher groups. Ono and Taleshita demonstrated injury to the melanin containing cells, hair shaft, and outer root sheath cells following high energy laser treatment [16]. Though high fluence lasers have the potential to permanently effect hair growth as desired by many consumers, in most part it is difficult to achieve because of the collateral effect on the skin at the high laser energy. Pain, erythema, edema, and blistering are some of the potential dermal adverse events observed during or immediately after the irradiation, additionally, pigmentary changes, scarring, and skin sensitivity can also be developed as a delayed side effect [5]. The severity and frequency of these dermal events is dependent on the skin pigmentation level, and the type of laser and the laser parameters used.

In order to fully understand the effects of laser energy on hair growth and design the next generation laser and light-based systems, or modify the laser parameters on current systems to achieve the consumer-desired high hair removal efficacy and low dermal side effects, it is important to understand how thermal changes in follicle effect the biochemical and molecular mechanisms that regulate its growth and cycling. This report reviews the critical hair structures and hair growth processes that are either involved in converting the laser photons to thermal energy, or are part of the biochemical and molecular systems affected by the thermal energy.