catagen

Catagen is a brief transitional phase in the vertebrate hair growth cycle, occurring after the active growth phase (anagen) and before the resting phase (telogen). In humans, scalp hair catagen lasts about two to three weeks. It marks the end of active proliferation of hair matrix cells and the beginning of follicular regression.

During catagen, mitotic activity in the hair matrix ceases and the lower part of the hair follicle regresses. The dermal papilla withdraws from contact with the growing hair, and the follicle involutes as the lower portion shrinks. The hair shaft keratinizes to form a club hair, which remains inside the follicle while the rest of the follicle transitions toward rest. The upper portion of the follicle remains relatively intact, but growth ceases.

At the end of catagen, the follicle enters telogen, the resting phase. The club hair is held loosely within the follicle, and eventually shed during exogen as a new anagen cycle begins, producing a new growing hair. The catagen stage therefore serves as a controlled, programmed regression that prepares the follicle for renewed growth.

Significance and variation: The duration and prominence of catagen vary among individuals and species and can be influenced by hormones, nutrition, aging, and disease. Abnormalities in catagen progression can be observed in certain hair disorders and alopecias, reflecting disruptions in the normal timing of follicle regression. The term catagen derives from Greek roots meaning “going down” or “transition.”