anagen
noun
an·a·gen ˈa-nə-ˌjen 
: the active phase of the hair growth cycle that precedes catagen and telogen, that is marked by rapid division and differentiation of cells in the lower portion of the hair follicle and by lengthening of the hair shaft, and that typically lasts between two to six years
Note: At any given time, approximately 80% to 90% of hair follicles are in anagen.
Example Sentences
Recent Examples on the Web Loss of hair in the anagen phase is never normal and most commonly results from a toxic exposure like treatment with anticancer drugs. 
New York Times, 3 Feb. 2020 Other causes of anagen hair loss that can be permanent include radiation and heavy metal poisoning. New York Times, 3 Feb. 2020 When placed against the scalp for eight minutes three times a week, the laser light seems to have the ability to take dormant hair follicles that are in the resting, or nongrowth, phase into the anagen, or growth, phase. Nicole Catanese, Harper's BAZAAR, 28 Aug. 2015 The subsequent decline in estrogen levels and rise of the stress hormone cortisol causes hair to move from the anagen cycle (growth) to the telogen phase (shedding), Wesley describes. Priya Rao, Vogue, 19 Mar. 2018
New York Times, 3 Feb. 2020 Other causes of anagen hair loss that can be permanent include radiation and heavy metal poisoning. New York Times, 3 Feb. 2020 When placed against the scalp for eight minutes three times a week, the laser light seems to have the ability to take dormant hair follicles that are in the resting, or nongrowth, phase into the anagen, or growth, phase. Nicole Catanese, Harper's BAZAAR, 28 Aug. 2015 The subsequent decline in estrogen levels and rise of the stress hormone cortisol causes hair to move from the anagen cycle (growth) to the telogen phase (shedding), Wesley describes. Priya Rao, Vogue, 19 Mar. 2018 Word History
Etymology
ana- + -gen
First Known Use
1926, in the meaning defined above
Medical Definition
anagen
noun
an·a·gen ˈan-ə-ˌjen
: the active phase of the hair growth cycle that precedes catagen and telogen, that is marked by rapid division and differentiation of cells in the lower portion of the hair follicle and by lengthening of the hair shaft, and that typically lasts between two to six years
Note: At any given time, approximately 80% to 90% of hair follicles are in anagen.
The initial step, anagen, provides the active growth of the hair fiber, during which cells at the base of the follicle rapidly divide. Damaris Christensen
Damaris Christensen