All human hairs show various stages of hair growth. The hair cycle is divided into three stages: anagen, the period of activity or growth phase; catagen, the period of regression or regression phase; and telogen, the period of quiescence or resting phase.
Anagen growth phase varies greatly (and can last up to 6 years) depending on age, sea son, anatomic region, sex, hormonal levels, and certain genetic predispositions. It is these variations that have led to the tremendous disparity in hair cycles reported by various investigators.
The catagen stage is relatively constant and is generally of 3 weeks duration, whereas the telogen phase usually lasts approximately 3 months.
The overall length of the hair is determined primarily by the duration of the anagen phase. Human hair appears to grow continuously, because the growth cycles of different hair follicles are in dysynchrony with each other.
The histologic appearance of a hair follicle also differs dramatically with the stages of growth. The anagen follicle penetrates deepest in the skin, typically to the level of subcutaneous fat. Catagen is characterized by pyknotic changes in the nuclei of the kerotinocytes, followed by apoptosis of the transient portion of the follicle. The entire transient portion (which begins at the level of the insertion of arrector pili muscle and extends to the deepest portion) is absorbed, except for the basement membrane. As the new anagen progresses, the secondary hair germ descends, enlarges, and begin to produce a hair shaft.
Although reports of anagen duration, telo-gen duration, and the percentage of telogen hairs represent an inexact science, most discussions of laser hair removal take into account different anatomic areas in terms of anagen and telogen cycles.
It is the sensitivity of the anagen hair to a variety of destructive processes, including laser and light source damage, that leads to a metabolic disturbance of the mitotically active anagen matrix cells. The response pattern is dependent both on the duration and intensity of the insult.
Lin et al. (Lin et al. 1998) postulate that follicles treated in the telogen phase show only a growth delay for weeks, whereas, when those follicles are treated in the anagen phase they may be susceptible to lethal damage, may have a growth delay, or may simply switch into telogen phase. This could partly explain the growth dynamics of the hair cycle. Repeated treatments could lead to a synchronization of the anagen phase by induction and/or shortening of the telogen phase, which could increase the effec tiveness of hair removal with each consecutive treatment. Another explanation might be that the follicle is not destroyed immediately, but shows a growth arrest after only one (shortened) anagen cycle. Some have questioned the assumption that effective laser hair removal is determined solely by treating hairs in the anagen cycle. These investigators suggest that melanin within a hair follicle may be more important than the actual time of treatment.
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