These lightly pigmented hairs are fine in texture. Darkly pigmented terminal hairs are long and thick and most commonly are located on the scalp and the facial areas of men. These hairs can reach a preprogrammed length based on length of the anagen phase. Ultimately, terminal hairs undergo involution and convert to catagen and then to telogen phase. The mechanism and factors that induce the hair to terminate growth and involute are largely unknown.
Some hair follicles have the ability to produce different types of hair at various times.
For example, hair follicles on an adult scalp that normally produce terminal hairs can, in individuals with androgenetic alopecia, undergo miniaturization and produce short, fine, and lightly pigmented hairs. Similarly, hair follicles on the male beard, which until puberty produce only vellus hairs, are capable of producing terminal hair thereafter. The anagen phase for vellus hairs is much shorter than that for terminal hairs. The hair growth cycle in different regions of the body varies, and the duration of anagen, catagen, and telogen phases differs according to body site. While the entire cycle duration for the eyelashes is 4 months, the scalp cycle requires about 3-4 years to complete.