Observations made at extended periods after various laser treatments revealed further progression of histopathological changes in the hair follicle. For instance, seven days after the treatment with ruby laser at 11 J/cm2, damaged follicular epithelium was observed with increased eosinophilia and pyknotic nuclei, suggesting the development of cell death in the hair follicle. Twenty one days later, most of the damage had extended to the level of the insertion of arrector pilli muscle, and the damaged cells of the follicular bulb appeared shrunken with collapsed outer root sheath [52]. One month later, cystic formation of hair follicles and foreign body giant cells were observed in the skin treated with either ruby (10-18 J/cm2) or alexandrite (11-17 J/cm2) lasers. By using the TUNEL technique, the authors detected apoptotic cells in the dermal papilla after ruby laser treatment, and development of apoptosis in the inner root sheath after alexandrite laser treatment [59]. Analysis of the follow-up biopsies after the application of diode laser (25-35 J/cm2) obtained one and three months after laser treatments revealed that hair follicles contained necrotic kera – tinocytes, and the histology findings were consistent with catagen phase. In a case study by Sadick, the biopsy showed complete destruction of two hair follicles, evidenced by the presence of two hair shafts lying freely in the dermis, and a reduction in the anagen/telogen ratio to about 1:1 versus the normal 8:2 [62]. A study report by Dierickx on one-year follow-up after ruby laser treatment showed the total number of the hair follicles to be near identical in the control – and laser-treated sites; however, on the laser side there was a reduction in the terminal hair with a proportional increase in the small vellus-like follicles, and the average hair-shaft diameter was substantially decreased compared to the control (68.7 ±4.2 p, m and 22.5 ± 12.2 p, m, respectively) [51]. A more extensive damage to hair follicles was detected in biopsies taken three months after treatment with Nd-YAG laser at 23-56 J/cm2.
A complete disappearance of hair follicles with the occasional presence of arrector pilli muscle and little focal fibrosis was observed [61].
Thus, laser treatment initiates degenerative changes in the hair follicles that may be accompanied by an increase in apoptosis and necrotic events in the follicular epithelium and in some cases in the dermal papilla, which may lead to either hair follicle miniaturization or hair follicle destruction.