Hair growth cycle
Source: General Physiology of Hair— An Overview by Robert Baratz, M.D., Ph.D., D.D.S.
Hair grows in phases (see diagram).
For illustration, it can turn into a vellus hair (non-pigmented “peach-fuzz” hair) or even the hair follicle
might permanently disappear, as is the case with male-pattern baldness. Cases known to effect the hair follicle and its cycle include local signaling cases (e.g, cytokines, hormones, adhesion molecules). Nonetheless, no firm theory of cycle control
exists.
Hypotheses include the presence of (1) a morphogenesis clock, (2) a cycling
inducer, (3) a desynchronizer, and (4) an actual cycle clock, however none of it is
specifically known.
Substances affecting hair growth.
An huge number of substances can effect hair expansion.
For instance, a select few drugs, like alkalating agencies, are cytotoxic and can produce hair fall out (e.g., cancer chemotherapeutic agents). More drugs drive hair into telogen (e.g., heparin, Vitamin A, ? -blockers, L-dopa, lithium, and a select few of the non-steroidals). Drugs that inhibit hair expansion include parathyroid hormone (PTH) and PTH-related proteins. Variable drugs also exist, like Vitamin D. At low concentrations, Vitamin D could simulate hair growth, however at high concentrations hair growth is inhibited. Substances like testosterone, danazol, adrenocorticotropin hormone, metyrapone, anabolic steroids, glucocorticoids, retinoids, and insulin can lead to hirsutism (growth of hair where it doesn’t ordinarily occur). Cyclosporin, minoxidil, diazoxide, and chromakalin increase the growth rate and size of hair (hypertrichosis). Nonetheless, a few regional variation could occur. As an example, steroids will lessen the rate of growth of eyebrows, lashes, and hair on the extremities, however estrogen and testosterone will normally stimulate the expansion of pubic and axillary hair. More factors can potentiate or even inhibit hair growth by affecting the growth of the dermal papillae, hair, and follicle (see Table).
| Factor | Effect on Hair Growth |
|
Beta-fibroblast growth factor Platelet-derived growth factor |
Potentiate growth of dermal papillae. |
| Transforming growth factor beta |
Inhibits follicle proliferation, if induced by mitogens |
| Interleukin-1 alpha | Inhibits growth of hair and follicle. |
| Epidermal growth factor | Stimulates growth. |
| Fibroblast growth factor-5 | Inhibits growth. |
| Keratinocyte growth factor | Stimulates growth; induces keratinization. |
| Insulin-like growth factor-1 | Accelerates growth of hair and follicle. |
| Skin damage (e.g., cut, scrape, burn,irritation) | Forces telogen to anagen (well-illustrated in rodent models). |
| Allergens (e.g., food) | Major changes in the skin, including hair loss. |
| Malnutrition | Protein/calorie deficiencies inhibit hair growth. |
| Fungal infection | Inhibits growth; hair may fall out. |
| Hypothyroidism | Diminution of eyebrows. |
| Viral agents (e.g, HIV virus) | Hair loss in patches. |
Source: Jankovic and Jankovic 2001.
