katagens

Katagen, commonly referred to in English as catagen, is the transitional phase of the hair growth cycle between the active growth phase (anagen) and the resting phase (telogen). During katagen, the hair follicle undergoes involution: mitotic activity in the hair matrix ceases and the lower follicle regresses. The dermal papilla disconnects from the hair bulb, and the hair shaft becomes club-shaped as cellular proliferation stops. The phase lasts roughly two to three weeks in humans.

As katagen progresses, the follicle shortens and the lower portion degenerates while the hair shaft remains within the follicle as a club hair. The follicle then exits katagen and enters telogen, the resting phase, which lasts several months in humans. At the end of telogen, a new anagen phase begins, and a new hair shaft grows, often displacing the club hair.

Clinical relevance: The duration and quality of katagen contribute to overall hair cycle dynamics and shedding patterns. Abnormalities in katagen timing or regression can influence hair loss conditions, such as telogen effluvium, by altering the balance between growth, regression, and resting phases. Factors such as hormones, nutrition, stress, and systemic illness can affect the hair cycle and, by extension, katagen behavior.