As detailed earlier, the laser effects on hair follicle are highly dependent on temperature changes in localized compartments within the hair follicle. Various model systems can be used to determine the mechanism of laser-induced changes in the hair follicle, and from these changes, predict the short – and long-term effect of a laser treatment on hair – reduction efficiency.
Most of the studies on histopathological changes in response to laser treatment have been performed using the punch-skin biopsies taken from volunteers after the laser treatment. This approach has some advantages and certain disadvantages. One of the disadvantages is that punch biopsy produces a limited number of hair follicles per sample, and that only a small number of samples can be practically obtained because of the invasiveness of this procedure. Thus, data analysis is performed using a very limited sample size. There is an additional technical challenge, as vertically sectioned skin samples rarely go through the entire length of the same hair follicle. The horizontal sections used in most studies make it difficult to evaluate comparative responses of various components within the hair follicle to a laser treatment. However, the advantage of this approach is that skin samples can be collected at defined periods of time after the laser treatment while the hair follicle is in its natural environment, and therefore allows evaluation of not only long term effects of laser, but also assessment of the changes in the skin structure surrounding the hair follicles.
To determine the acute response of follicles and skin, an alternative approach is the use of ex-vivo skin. The samples of skin containing pigmented hair follicles can be obtained from patients undergoing cosmetic surgery (face-lift or brow-lift procedures). The model has been successfully used by several research groups and in our own laboratory to assess the immediate damage in hair follicles and epidermal melanocytes after laser treatment [13, 54, 68].
The hair-bearing skin are used for obtaining anagen hair follicles and growing them under in-vitro organ culture conditions. It has been demonstrated that dissected mature human anagen follicles can be successfully grown in culture with similar in-vivo hair fiber growth rates [77]. Human scalp/facial follicles are microdissected free of dermal/subcutis tissue, and placed free-floating in a serum-free culture medium. These follicles continue to grow for a short period of time (10-21 days) in culture, and are useful for studying the anagen phase as well as the onset of catagen. This model system has been widely used to investigate hair follicle biology [78]. The major application has been to investigate the possible role of growth factors in controlling hair follicle growth and differentiation. The advantage of hair follicle organ culture is that it is allows for the direct assessment of hair shaft elongation in response to the treatment of interest. The model is good for making assessment of morphological, biochemical, and molecular changes in individual hair follicle compartments at various time periods after the initiation of treatment. The model is also suitable to study anagen—catagen transition following a treatment. Because of the large number of hair follicles that can be obtained from a small piece of skin sample, it provides a better statistical analysis compared to the biopsy samples.
In our laboratory, we have used a combination of ex-vivo and in-vitro models to study the effect of lasers on hair follicle. For these studies, ex-vivo human skin is exposed to varied laser doses, and then the hair follicles are dissected and placed in a culture medium for the determination of hair growth rate and cyclic transition. In addition, changes in follicular morphology, assessment of cell proliferation and apoptosis, as well as the expression of genes known to be involved in hair growth regulation can be studied at different time points after laser exposure.