Histological analysis of the hair follicles exposed to the laser suggested that hair follicle response to laser treatment is fluence-dependent, and therefore, hair removal could be achieved through different mechanisms, including modification of hair cycle, hair follicle miniaturization, or hair follicle destruction. If low fluences accelerate anagen-catagen hair follicle transition leading to temporary hair loss, higher fluences could cause hair follicle destruction accompanied by permanent hair reduction [2].
The fluence-dependent hair follicle response was elegantly demonstrated in the experiment carried out on isolated anagen hair follicles using the ruby laser [63]. The authors observed that laser irradiation of hair follicles at fluence of 1.2 J/cm 2 did not result in immediate gross visible effects; however, hair growth rate dramatically decreased > 80% and hair follicles exhibited a transition from anagen to catagen during the six-day culture period studied. In contrast, a fluence of 3.8 J/cm2 resulted in a visible damage to hair follicles observed under light microscopy, and the majority of the hair follicles did not exhibit anagen-catagen transition. Even though the study was performed with isolated hair follicle, and not the intact skin, nonetheless, it clearly demonstrated that low fluences are capable of causing specific changes to hair follicle without resulting in their destruction.
McCoy et al. (1999) evaluated histological changes in axillary hair follicles in response to an exposure to 3-ms pulse of ruby laser at fluence range from 10 to 40 J/cm2. The results showed fluence-dependent damage to the hair follicle compartments, including the inner root sheath and the hair shaft. The predominance of catagen follicles and late catagen/telogen follicles was seen at one week and at four weeks after a single treatment, respectively, suggesting a premature induction of anagen-catagen transition of hair follicles in response to the treatment [64]. In a study that involved three monthly treatments of axillary area with a long-pulsed alexandrite laser and a long-pulsed diode laser at 25 J/cm2 or 40 J/cm2, histological evaluations one and six months after the final treatment revealed follicular miniaturization and fewer number of terminal hairs in all biopsies, irrespective of the laser or the fluence used [60].
Taken together, these studies indicate that laser-mediated hair removal can be achieved through different mechanisms, including modification of hair cycle, hair follicle miniaturization, and hair follicle destruction, and that this follicular response may be related to a threshold increase in temperature within the specific hair follicle.