The principle of selective photothermolysis that applies to laser hair removal indicates that by using a combination of the appropriate laser wavelength, pulse duration, and fluence targeted to a particular chromophore can selectively affect the target tissue. For the hair follicle, melanin serves as the endogenous chromophore located in the pigment-producing melanocytes of the hair matrix and in the keratinocytes of the hair shaft of anagen hair follicles [48]. Laser light energy is absorbed by melanin and is transformed into heat, which in turn affects the neighbouring cellular structures. Thus, a dark pigmented hair follicles with high eumelanin content generally responds well, whereas the treatment is less effective or even ineffective on grey, blonde, red, or light-brown hair. The effect is further determined by both the sensitivity of cellular component and the rise in local tissue temperature.
Various factors can affect the amount of laser energy reaching the target follicle. A study reported by Liew et al. (1999) on the efficacy of a ruby laser treatment at a fluence of 11 J/cm2 was compared using variables of patient’s age, intracutaneous hair length, epidermal depth, dermal density, skin color, and total melanin content and relative eumelanin content of hair. No correlation was found between the efficacy of treatment and various variables used in the study, except that the patients with higher eumelanin content in their hair had better long-term results. The results indicated eumelanin to be the single most critical factor in ruby laser effectiveness [14].
The fact that the hair follicle pigmentary unit cycles synchronously with the hair cycle stages and the hair is actively pigmented only during the anagen stage of the hair cycle, suggests the sensitivity of the hair follicle to laser treatment to be cycle-dependent. Indeed, the experiments carried out in mice showed that only actively growing pigmented anagen hair follicles were sensitive to hair removal by laser, whereas catagen – and telogen-stage hair follicles were resistant to the treatment [49]. This observation was verified in a human study where it was demonstrated that hair follicles considered being in the early anagen phase with poor melanin showed poor efficacy with a long-pulsed ruby laser at 20 J/cm2 [50]. In humans, hair show a mosaic or asynchronous pattern of growth with an autonomy for growth and pigmentation residing in each individual hair follicle. Except for scalp hair growth where the majority of the hair follicles reside in anagen stage (80-85%), the other body areas express a large proportion of telogen hair follicles. This suggests that multiple laser treatments would be needed to capture all hair in their anagen phase and a single laser treatment is unlikely to produce the desired hair-removal effects. In contrast to this theory, results from some other studies showed anagen and nonanagen hair follicles to be equally sensitive to laser treatment [4,51] . By light microscopic examination of skin biopsies obtained after a ruby laser treatment at fluence of 11 J/cm2, Leiw et al. observed that the coagulation damage of hair follicles was not confined to anagen hairs [52]. By using pho- totrichograms and hair counting in patients six months after the ruby laser treatment, Dierickx et al. (1998) concluded that both anagen and telogen hair had an almost equal probability of loss [51]. Therefore, the question remains as to what are the critical contributing factors that determine the susceptibility of the human hair follicles to the laser treatment, and what role precisely does the hair cycle play in this process. It is possible that sensitive hair follicle components in catagen and telogen hair follicles get affected by temperature changes in the surrounding anagen follicles during treatment.
Other areas of discussion in literature include the role of follicular stem cells in permanent reduction of hair—these cells are responsible for populating the matrix cell population at the start of the hair cycle; the temperature sensitivity of matrix cells—the cell population at the base of hair bulb responsible for the fiber formation; and the role dermal papilla might play in laser-induced changes in hair character, growth rate, and hair cycle transition of hair follicle.
By using select markers, Orringer et al. (2006) determined whether laser treatment could cause alterations in the follicular stem cells of the bulge region. Axillary hair growth was targeted on one side with an 800 nm diode laser and the other side was treated with a 1064 nm Nd:YAG laser. Serial skin samples were obtained at baseline and at various time points after treatment, and used for analysis of cytokeratin 15, cytokeratin 19, and CD34 expression, markers of hair follicle stem cells. Clinically, loss of hair was demonstrated on both sides, and the hair follicle histology was consistent with the thermal injury. However, immunohistochemical markers for the pluripotent stem cells located at the bulge region stained with a similar pattern and intensity for the posttreatment samples as compared to baseline. The results indicated that stem cells are spared from laser-induced damage [53].
On the other hand, a study conducted by Liew and colleagues (1999) showed that stem cells can be affected by laser treatment. The authors treated ex-vivo scalp skin samples with a ruby laser (14 and 20 J/cm2) and used light microscopy and immunohistology to determine the extent of damage. A monoclonal antibody LP2K against Keratin 19 was used to study the effect on stem cell population. The result indicated that most of the laser-induced changes involved the bulge region, and not the hair bulb [54].