Hair is a complex structure with an even more complex growth cycle. In order to understand how hair grows, it is important to understand some basic terminology:
Hair follicle: “Follis” is Latin for bag. The hair follicle forms when the epidermis invaginates, folding to form a pocket within the skin. The hair follicle can be envisioned as a bag, or sac, in which hair grows and sebaceous glands release sebum (oil).
Hair bulb: The hair bulb is located within and at the very bottom of the hair follicle. The hair bulb migrates up the follicle throughout the phases of hair growth, beginning at the very base of the hair follicle and progressively moving towards the surface, where it is ultimately shed.
Hair matrix: The hair matrix is located within the hair bulb, and refers to the actual collection of epithelial cells from which hair grows. Matrix cells divide and differentiate to form the hair shaft. This constant division is what ultimately pushes the hair out of the epidermis.
Inner root sheath: The lining of the hair follicle.
Hair root: The portion of hair that is present below the scalp.
Hair shaft: The portion of hair that is seen above the scalp. It is composed primarily of keratin and binding proteins. The hair shaft has a center composed of completely keratinized cells (the cortex), a center composed of incompletely keratinized cells (the medulla), and an outer layer (the cuticle).
Keratin: A protein that makes up hair and gives it strength.
Hair growth is quite similar to the division and differentiation of the skin. As cells at the base of the hair follicle divide and differentiate, they push out the cells above them. As this process continues, the cells move closer to the outside world. As they reach the epidermis, these growing cells die, become keratinized, and transform into what we call hair.
The Growth Cycle of Hair
Hair growth is cyclical in nature. It is important to note that, at any given time, hair follicles on our head are in all different stages of the hair cycle. In other words, there is no synchrony. The growth cycle is generally divided into 3 phases:
(1) ANAGEN: the growth phase
- Between 80-90% of our hair follicles are in this phase.
- Hair cells are rapidly dividing during this time.
- The length of anagen determines the length of hair. Hair length varies from person-to-person because the length of anagen varies from person to person. The length of anagen also varies from one another of the body to another (just compare the length of the hair on your head to the length of the hair on your arms!).
- Anagen can range from months to years. Scalp hair spends 2-8 years in this phase and grows at an average rate of 0.3-0.4 mm/day, 1 cm/month, or 6 inches/year! Arm hair, in comparison, spends only a few days in this phase (thus explaining its shorter length).
(2) CATAGEN: the degenerative phase
- True to its name, growth stops during this phase.
- Keratinization occurs.
- Length of catagenvaries, but is generally on the order of weeks.
(2) TELOGEN: the resting phase
- 10-20% of our hair follicles are within this phase at any given time.
- Typically lasts on the order of months.
- Hair within the follicle is shed, at a rate of 50-100 hairs/day.
Once the hair is shed, a new hair matrix is formed at the bottom of the follicle, and the cycle repeats itself.
REFERENCES:
Cotsarelis George, Botchkarev Vladimir, “Chapter 84. Biology of Hair Follicles” (Chapter). Wolff K, Goldsmith LA, Katz SI, Gilchrest B, Paller AS, Leffell DJ: Fitzpatrick’s Dermatology in General Medicine, 7e: http://www.accessmedicine.com/content.aspx?aID=2974164.
Marks JG, Miller JJ. Structure and Function of Skin. In: Lookingbill & Marks’ Principles of Dermatology. 4th ed. Elsevier Inc; 2006:11.
“The World of Hair”. Proctor & Gamble Beauty & Grooming. http://www.pgbeautygroomingscience.com/index.php?id=674.
Wolff K, Johnson RA, “Section 32. Disorders of Hair Follicles and Related Disorders” (Chapter). Wolff K, Johnson RA: Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology, 6e: http://www.accessmedicine.com/content.aspx?aID=5198818.