Hirsutism pathophysiology On the Web
The growth of hair on sex-specific areas of the body occurs due to androgens. Androgens induce vellus follicles in sex-specific areas to develop into terminal hairs, which are larger and more heavily pigmented.
The growth of hair on sex-specific areas of the body occurs due to androgens. Androgens stimulate the growth of vellus follicles in sex-specific areas to develop into terminal hairs, which are larger and have increased pigmentation (darker).
Types of hair
- There are approximately 5 million hair follicles at birth in humans with about 80,000 to 150,000 of them on the scalp.
- The quality of hair is determined by hormones and other intrinsic characteristics of the hair follicle.
- There are various types of hair which include:
- Lanugo hair: The soft hair which covers the fetal skin and it disappears within the first three months after the fetus delivery.
- Vellus hair: This is fine, soft hair that is not pigmented. It covers most of the body before the pubertal period.
- Terminal hair: This is long, coarse and pigmented. Pubertal androgens for example dihydrotestosterone (DHT) convert vellus hair to terminal hair.
Phases of Hair Growth Cycle
- There are 3 phases of hair growth cycle:
- The hair growth phase (termed Anagen phase): This phase varies, depending on the body area affected. It is approximately 4 months for facial hair.
- The involutional phase (Catagen phase): This phase is approximately 2-3 weeks.
- The resting phase (Telogen phase): The hair shaft separates from the dermal papillae at the base of the follicle, which terminates growth.
- The growth of sexual hair is dependent on the presence of androgens.
- Androgens induce vellus follicles in sex-specific areas to develop into terminal hairs, which are larger and more heavily pigmented.
- Hirsutism is caused by increased androgen production and/or an increased sensitivity of the hair follicles to androgens.
- Hyperandrogenism, resulting from any factors, prolongs the anagen(growth) phase of androgen-sensitive hairs, resulting in their conversion from fine, light, vellus hairs to coarse, dark, terminal hairs.
- The response of hair follicle to androgens and other factors such as local 5 alpha-reductase activity determines the level of conversion of hair from the vellus type to terminal hair.
- Growth recommences with the formation of new hair shaft by the reactivation of the dermal papillae, thereby replacing the old hair.
- The hair growth cycle takes months to years to be completed, causing a delay in hair growth response to changes from effects of androgens.
- DHT is a hormone that acts on the hair follicle to produce terminal hair.
- Differences in the activity of DHT explains why women with the same plasma level testosterone, have different degrees of hirsutism.
- It is speculated that insulin, at high enough concentration, stimulates the ovarian theca cells to produce androgens.
- There may also be an effect of high levels of insulin to activate the insulin-like growth factor-I (IGF-1) receptor in those same cells resulting in increased androgen production.
Common conditions associated with hirsutism include:
- Deplewski D, Rosenfield RL (2000). "Role of hormones in pilosebaceous unit development". Endocr. Rev. 21 (4): 363–92. doi:10.1210/edrv.21.4.0404. PMID 10950157.
- Messenger AG (1993). "The control of hair growth: an overview". J. Invest. Dermatol. 101 (1 Suppl): 4S–9S. PMID 8326154.
- Hatch R, Rosenfield RL, Kim MH, Tredway D (1981). "Hirsutism: implications, etiology, and management". Am. J. Obstet. Gynecol. 140 (7): 815–30. PMID 7258262.
- Labrie F (1991). "Intracrinology". Mol. Cell. Endocrinol. 78 (3): C113–8. PMID 1838082.
- "Category:Hair follicle - Wikimedia Commons".