The hair bulb, isthmus, and infundibulum are the three units that make up the vertical sections of the hair follicle [42-44]. The hair bulb begins at the base of the hair follicle and continues to the insertion of the arrector pili muscle. This portion of the hair is 3-7 mm below the surface of the skin in the dermis, and is comprised of the matrix cells, melanocytes, and the dermal papilla [45]. The matrix cells form the outer root sheath, the three layers of the inner root sheet, and the hair shaft [46]. In the mature anagen hair follicle, melanocytes are located in the basal layer of the infundibulum and surrounding the upper dermal papilla [47,48]. To be effective, the laser energy must reach the highly concentrated population of melanin – (color) producing cells in the matrix area [49]. The response to the
laser will depend on the hair color, with large numbers of electron-dense melanosomes in black-hair follicles being the most responsive [50,51]. Brown-hair bulb melanosomes are somewhat smaller, and may not generate as much heat as black hair, while those of blonde hair have very little melanin and will not produce much heat compared to brown or black hair. In red hair, the melanosomes contain the red pigment phelomelanin, which has the poorest absorption of laser energy.
The dermal papilla provides the factors responsible for controlling the growth of the hair follicle, as well as the nourishment to the rapidly proliferating cells found in the matrix [45]. The close proximity of the melanocytes to the matrix and dermal papilla cells means that enough heat can be transferred to these cells to disrupt their normal function [52,53]. Above the hair bulb is the isthmus, which starts at the insertion of the arecctor pili muscles and continues to the entrance of the sebaceous duct [46]. The isthmus encompasses the bulge region with the pleuripotent stem cells needed to regenerate the hair follicle as it enters the anagen phase of the growth cycle. Cotsarelis et al. have identified a population of slow – growing stem cells that are located at the attachment of the arrector pili muscle (the bulge region) [54]. It is now believed that the stem cells in this region are activated by signals from the dermal papilla during late telogen or early anagen. The stem cells form the more differentiated cells of the matrix and return to their normal noncycling state by mid-anagen. Many believe that the stem cells in this region have to be damaged by the laser treatment for the treatment effects to be permanent hair-growth inhibition [55].
The mammalian hair follicle has three phases in its growth cycle [56]. The growth phase (anagen) is characterized by the rapid growth of the matrix cells. The amount of time a hair is in the anagen phase varies widely, depending on the body site (from years on the scalp, to weeks on the arm). Melanin synthesis in the hair follicle is closely correlated to the growth phase of the hair follicle [57]. After the anagen phase, the hair enters into a shortlived regressive phase (catagen) in which the lower portions of the follicle including the melanocytes undergo apoptosis [58]. The lower portion of the follicle shrinks and forms a thin epithelial cord, which retracts upward to the infundibulum. During catagen, the follicle reduces by about two-third of its original length and the dermal papilla ends up at the level of the arrector pili muscle [46]. The catagen phase lasts about three weeks [45]. The final phase of the hair cycle is the telogen or resting phase [58]. This phase can last from weeks to a few months, dependent upon the body site location. Upon receiving a host of signals, the hair will re-enter anagen with a large increase in epithelial cell division that reforms the matrix region [45]. Some authors have proposed that early anagen phase is the best time to treat the hair with the laser, since the hair bulb is close to the skin surface and melanin synthesis has begun [59].