The regression phase in the hair cycle where hair growth ceases and a club hair begins to form
The hair cycle is the cyclical phases of growth of hair. It is comprised of the following stages: anagen, catagen, telogen, and exogen. Each hair fiber goes through these stages. However, the hair fibers are at different stages of the hair cycle that occur simultaneously.
Catagen is the phase of the hair cycle wherein the hair stops growing. The hair follicle shrinks. It detaches from the dermal papilla and thereby cutting itself from the nourishing blood supply. A so-called club hair, i.e. hair with a club-shaped bulb in a resting state prior to shedding, begins to form. The melanin production stops at this stage as the follicular melanocytes undergo apoptosis. The programmed cell death of these stem cells seems to play a role in the graying of the hair.1
1 Araújo, R., Fernandes, M., Cavaco-Paulo, A., and Gomes, A. (2010). "Biology of Human Hair: Know Your Hair to Control It". Adv Biochem Engin/Biotechnol. Retrieved from [].