Article Summary by Soneet Dhillon, MS | MD Candidate | Chicago Medical School

Hair is divided into the hair shaft and hair follicle. The hair shaft is visible outside of the skin, and the follicle which is under skin (1). The hair follicle contains the hair bulb that actively produces hair from rapidly dividing stem cells, which flatten and eventually become non-living keratinized cells that form the hair shaft (1). This creates a hair shaft which is made of keratin.

Hair receives its color from melanocytes which transfer melanin to the hair shaft using extensions called dendrites. Hair color is determined by the concentrations of melanin subtypes: eumelanin (brown-black) or pheomelanin (yellow-orange) (2). Melanocytes produce melanin during the growth phase of the hair cycle. Overall, the hair cycle is divided into anagen, catagen, and telogen phase, which represent the growth, transition, and rest phases respectively. During telogen phase, hair falls out over 2-3 months to start a new hair cycle (1). During the anagen growth phase, melanocytes are around the hair bulb, but then towards the end of the anagen phase, the melanocytes decrease their melanin production and retract their extensions from the keratinocytes (2). Therefore, there is no transfer of melanin during the catagen or telogen phase of the hair cycle. Hairs that are shed during the telogen phase will have minimal pigment at the base and therefore there is a white bulb at the end of the hair.

At any given time, about ~80-90% of scalp hair is in anagen phase, 1% is in catagen phase and ~10% is in telogen phase (3). The anagen phase lasts for approximately 2-5 years.  At the end of anagen phase, the hair follicle enters the catagen phase and goes through a programmed shut down lasting about 2-3 weeks.  Eventually the hair enters the telogen phase for 2-3 months where it can decrease in size until it is shed (4). Healthy people lose about 50-150 hairs a day.  Hair that is shed in the catagen-telogen phase will have a white bulb.  While hair is generally not shed in the anagen phase, except in very specific circumstances such anagen effluvium triggered by treatment with chemotherapeutic agents, the end of a shed anagen hair would be similar to the natural hair color.

References:  (1) Hoover E, Alhajj M, Flores JL. Physiology, hair. StatPearls [Internet]: StatPearls Publishing; 2021. (2) Fernandez‐Flores A, Saeb‐Lima M, Cassarino DS. Histopathology of aging of the hair follicle. Journal of Cutaneous Pathology 2019;46(7):508-19. (3) Grover C, Khurana A. Telogen effluvium. Indian J Dermatol Venereol Leprol 2013;79(5):591-603 doi 10.4103/0378-6323.116731. (4) Park SH, Seol JE, Kim DH, Kim H. Analysis of Microscopic Examination of Pulled Out Hair in Telogen Effluvium Patients. Ann Dermatol 2020;32(2):141-5 doi 10.5021/ad.2020.32.2.141.