The catagen phase is the second part of a hair’s lifecycle. It comes after the anagen phase of hair growth, but before the final telogen phase. During the anagen phase, a hair bulb forms at the bottom of the hair follicle and matures into a full-length strand of hair that grows outwards past the scalp barrier. During the catagen phase, however, structural changes take place in the maturing hair over a number of weeks.
The catagen phase causes the following events to occur:
Even though body hair and hair on the scalp each go through an identical catagen phase, the hair on the body goes through a much shorter anagen phase. As such, body hair does not get an opportunity to grow long like hair on the top of the head, but rather instead has plenty of time to become ready for shedding.
The appearance of evenly growing hair is somewhat of an illusion. Undetectable to the naked eye, each strand of human hair is going through one of three stages at any given time. The hairs will go through each stage independently of each other, so all three of these stages are happening throughout the scalp at once. Although new hair will only grow during the anagen phase, it can still appear to grow during the catagen phase. However, this is due to a shrinking of the hair follicle, which continues to push the remaining hair to the surface. Basically, much of the hair you see is hair that is already finished growing, and is now being jettisoned out of your scalp.
A club hair is essentially a hair that is preparing to be shed. During the catagen phase, the hair follicle will shrink until it impedes the blood supply from reaching the hair via dermal papillae. Once the hair bulb no longer has access to the blood supply, it begins to convert to keratin. During this keratinization process, the hair bulb becomes shaped like a club. This is why the medical community will sometimes say that a hair at this stage is a “clubbed hair”.
A catagen phase can closely resemble the telogen phase, and it can be difficult to tell the two apart. The main difference is the shrinkage of the hair follicle, and the transformational process into keratin. When the hair becomes fully keratinized, it is no longer in the catagen state. While there is no real need to change the duration of the catagen phase poplular medications such as Rogaine can delay the onset of catagen phase by lengthening the anagen phase.