During the catagen phase, hair stops growing and the follicle shrinks dramatically as cells in the lower portion die off through a controlled process called apoptosis. This transitional stage lasts about two weeks and serves as the bridge between active growth (anagen) and rest (telogen). At any given moment, only a small fraction of your scalp hairs are in catagen, making it the shortest and least common stage of the hair cycle.
What Happens Inside the Follicle
The defining event of catagen is follicle regression. The lower two-thirds of the hair follicle essentially dismantles itself through programmed cell death. Cells in the hair bulb, the rounded base where new hair is produced during growth, stop dividing and begin to die in an orderly sequence. Neighboring cells that survive actually clean up by absorbing the fragments of dead cells, a process that was historically misidentified under the microscope as a different type of cell breakdown.
As this wave of cell death moves through the lower follicle, the hair detaches from its blood supply and from the dermal papilla, a small cluster of specialized cells that acts as the follicle’s command center during growth. The dermal papilla doesn’t die. Instead, it compresses into a compact ball and migrates upward toward the middle of the follicle, positioning itself near a region called the bulge. This repositioning is critical because it sets the stage for the next growth cycle, when the dermal papilla will send signals to stem cells in the bulge to start building a new hair.
How the Hair Itself Changes
The hair strand that was actively growing transforms into what’s called a club hair. The root end, which was once soft and surrounded by actively dividing cells, hardens and takes on a rounded, club-like shape. The inner root sheath, a protective casing that surrounds the hair during growth, disappears. Melanin pigment, which gives hair its color, stops being produced and clumps of leftover pigment can be seen scattered in the tissue trail left behind by the shrinking follicle.
Under a microscope, a catagen follicle looks distinctly different from a growing one. The dermal papilla appears smaller and ball-shaped rather than elongated. The hair matrix, the rapidly dividing cell population responsible for producing new hair, is gone. A shortened strand of epithelial tissue connects the club hair to the dermal papilla below, and this strand continues to shorten as catagen progresses. A thickened connective tissue sheath surrounds the remnant, marking the path the follicle occupied when it was full-sized.
Duration and Prevalence
Catagen lasts roughly two weeks on the human scalp, making it far shorter than anagen (which runs two to seven years) or telogen (two to four months). The textbook estimate has long been that only 1% to 2% of scalp follicles are in catagen at any one time, with about 85% to 90% in anagen and 10% to 15% in telogen.
More recent research has challenged those numbers. Detailed microscopic analysis of scalp biopsies found that catagen follicles may actually account for around 7.5% of follicles, considerably higher than the traditional figure. Some researchers have observed catagen follicles outnumbering telogen follicles, flipping the old ratio. The discrepancy likely comes from improvements in how scientists identify and classify follicle stages under the microscope, as early catagen can be subtle and easy to miscount.
What Triggers Catagen
The shift from active growth into catagen isn’t random. It’s driven by a specific set of molecular signals produced within the follicle. Several proteins act as “stop growing” messages. One of the most important is a growth factor called TGF-beta, which directly activates the cell-death machinery in the hair bulb. When researchers applied TGF-beta2 to human hair follicles in the lab, it switched on the enzymes (caspases) responsible for dismantling cells in two key locations: the lower hair bulb and the outer layer of the root sheath.
Other signaling molecules that push follicles into catagen include FGF5, a protein that has been well-studied in animal models where mutations in the gene produce unusually long hair because catagen is delayed. Stress signals, hormonal changes, and inflammatory molecules can also accelerate the transition into catagen, effectively shortening the growth phase and producing thinner, shorter hairs over time.
Why Catagen Matters for Hair Loss
In pattern baldness, follicles spend progressively less time in anagen and enter catagen earlier than they should. Each shortened growth cycle produces a finer, shorter hair until the follicle eventually miniaturizes to the point where it produces only a barely visible vellus hair. Because TGF-beta is a key driver of catagen entry, it has become a target of interest for preventing this premature transition. Blocking or suppressing TGF-beta signaling in follicles could, in theory, keep hairs in the growth phase longer and slow the miniaturization process.
The catagen phase also matters in certain forms of sudden hair shedding. When a large number of follicles are pushed into catagen simultaneously, whether by physical stress, illness, surgery, or hormonal shifts, they move through catagen and telogen together and then fall out in a noticeable wave two to four months later. This is the mechanism behind telogen effluvium, the diffuse shedding many people experience after a major stressor. The shedding itself happens during telogen, but the triggering event is the premature, synchronized entry into catagen.