Overview of hair growth

[Monty]

Hair originates in the hair follicles. These follicles are located in the dermis, the skin layer just below the surface layer and above the subcutaneous fat. Hair follicles are present everywhere on the skin except the lips, palms of the hands, and soles of the feet. New hair is made in the hair matrix at the base of the hair follicle. Living cells in the hair matrix multiply and push upward. These cells rapidly dehydrate, die, and compact into a dense, hard mass that forms the hair shaft. The hair shaft, which is made up of dead protein, is covered by a delicate covering (cuticle) composed of platelike scales.
Hair is colored by the pigment melanin, which is also responsible for skin color. Human hair colors come from two types of melanin: eumelanin in black or brown hair and pheomelanin in auburn or red hair. Diluted eumelanin gives blond hair its color.
Hair grows in cycles. Each cycle consists of a long growing phase followed by a brief transitional phase and then a short resting phase. At the end of the resting phase, the hair falls out and a new hair starts growing in the follicle, beginning the cycle again. Eyebrows and eyelashes have a growing phase of 1 to 6 months. Scalp hairs have a growing phase of 2 to 6 years. Normally, about 50 to 100 scalp hairs reach the end of the resting phase each day and fall out.
Hair growth is regulated by male hormones (androgens, such as testosterone and dihydrotestosterone), which are present in both men and women but in different amounts. Testosterone stimulates hair growth in the pubic area and underarms. Dihydrotestosterone stimulates hair growth in the beard area and hair loss at the scalp.
Hair disorders include

Excessive hairiness (hirsutism and hypertrichosis)
Hair loss (alopecia), including alopecia areata
Ingrown beard hairs (pseudofolliculitis barbae)

Most hair disorders are not serious or life threatening, but they are often perceived as major cosmetic issues that require treatment.
Dandruff is not a hair disorder but rather a skin disorder (seborrheic dermatitis) of the scalp.