katagen

Katagen is the transitional phase of the hair growth cycle between anagen (growth) and telogen (rest). It marks the end of active hair production and the onset of follicular regression.

During katagen, hair matrix cell proliferation ceases, the lower part of the follicle involutes, and the dermal papilla loses contact with the hair shaft. The hair shaft becomes a short, club-shaped structure, the follicle shortens, and the inner root sheath degenerates as the follicle prepares for the resting state that follows.

The phase is relatively brief in humans, typically lasting about two to three weeks. Only a minority of follicles are in katagen at any given time. At the end of katagen, the follicle enters telogen, a resting phase, after which a new anagen phase begins and a hair shaft may be shed during exogen.

Katagen is a normal part of the hair cycle, and its duration and progression can be affected by factors such as age, hormones, nutritional status, scalp conditions, and systemic illnesses. Disturbances to the cycle can influence the timing of katagen and may contribute to alterations in hair shedding and growth.

Histologically, katagen is identified by shortened follicles with a club-shaped hair shaft at the distal end. The term derives from Greek katagenēs, meaning "going down" or "ending," reflecting the regression of the follicle during this phase. See also: Hair cycle.