Hair growth occurs from within the hair follicle in the skin. The hair follicle goes through a cycle of growth containing four main stages:
- Anagen – active growth
- Catagen – regression
- Telogen – rest
- Exogen – shedding
Each individual strand of hair is at a different stage of development in any one moment in time, making changes in these phases generally unnoticeable.
During each anagen phase a completely new hair forms. Growth of the new hair shaft lasts on average three years, but can last up to 7 years. Men and women with androgenetic alopecia have a shortened anagen phase and smaller hair follicles (1), whereas those with hirsutism have a longer anagen phase and more hair growth (2). During anagen, hair color is generated from specialized pigment-producing cells (3). Once color production stops, the hair shaft has entered the catagen phase (4).
During this phase the hair follicle shrinks and further growth of the hair is prevented. Catagen phase lasts a few weeks.
The hair does not grow or shrink at this point; it is in a resting phase which lasts approximately 3 months.
The hair that was resting will now fall out and make room for new hair to grow. This process occurs independently of new hair formation in the underlying hair follicle (5), as a new hair has likely already started forming. It usually takes some time before a new anagen phase hair emerges, and this period is referred to as the kenogen phase (6). Men and women with androgenetic alopecia have a longer and more frequent kenogen phase (6), resulting in less hair on their head.
Article by: Dr. C.D. Studholme Ph.D. , Mediprobe Research Inc.
- Sinclair RD, Dawber RP. Androgenetic alopecia in men and women. Clin Dermatol. 2001 Apr;19(2):167–78.
- Azziz R. The evaluation and management of hirsutism. Obstet Gynecol. 2003 May;101(5 Pt 1):995–1007.
- Passeron T, Mantoux F, Ortonne J-P. Genetic disorders of pigmentation. Clin Dermatol. 2005 Feb;23(1):56–67.
- Slominski A, Paus R, Plonka P, Chakraborty A, Maurer M, Pruski D, et al. Melanogenesis during the anagen-catagen-telogen transformation of the murine hair cycle. J Invest Dermatol. 1994 Jun;102(6):862–9.
- Piérard-Franchimont C, Piérard GE. Teloptosis, a turning point in hair shedding biorhythms. Dermatol Basel Switz. 2001;203(2):115–7.
- Rebora A, Guarrera M. Kenogen. A new phase of the hair cycle? Dermatol Basel Switz. 2002;205(2):108–10.
Photo taken from http://www.depilarsystem.com/_downloads/hair_growth_US.pdf