Hair growth

The epidermis through which each individual hair projects is largely made up of epithelium and lacks blood vessels. The underlying dermis houses the hair follicles from which each hair grows, as well as the connective tissue, blood vessels, sweat glands, and other structures.

Every hair on your body is grown from and contained in a ‘pilosebaceous unit’: a hair shaft, hair follicle, sebaceous gland, blood capillaries and the erector pili muscle.

A hair follicle is a cavity in the skin that contains the root of a hair and grows hair by packing old cells together. Attached to the follicle is a sebaceous gland, which is a tiny sebum-producing gland (you can find hair follicles almost everywhere on the human body except for the palms, lips, and soles of the feet). There is a relationship between the production of sebum and the density of hair a person has, due to the fact that the sebum may obstruct the growth of the hair. In general, apart from some other factors, the more sebaceous glands a person has, the less hair they have.

At the base of the follicle is a large structure called the papilla. The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non-existent. The papilla is usually ovoid or pear shaped with the matrix wrapped completely around it, except for a short stalk-like connection to the surrounding connective tissue, which provides access for the capillary. Around the papilla is the hair matrix, a collection of epithelial cells often interspersed with melanocytes (cells that produce melanin). Cell division in the hair matrix is responsible for the cells that will form the major structures of the hair fiber and the inner root sheath. The hair matrix epithelium is one of the fastest growing cell populations in the human body. Some forms of chemotherapy or radiotherapy that kill dividing cells may lead to temporary hair loss, by their action on this rapidly dividing cell population.

Also attached to the follicle is a tiny bundle of muscle fibers called the erector pili, which is responsible for causing the follicle and hair to become more perpendicular to the surface of the skin, causing the follicle to protrude slightly above the surrounding skin. This process results in "goose bumps" (or goose flesh). Stem cells are located at the height of the erector pili muscle and are principally responsible for the ongoing hair production during a process known as the anagen stage.

Hair shafts are not permanent, but continually grow and are replaced. Hair growth and shedding is a continuous cycle through 3 phases. The anagen phase is the growth phase, the catagen phase is a transitional state, and the telogen phase is the resting phase. Hairs spend a variable amount of time in each phase determined by genetics, hormones, and area of the body. Hair in the anagen phase is more susceptible to injury than hair in the telogen phase and this is the phase in which Intense Pulsed Light treatment is most effective.

Find out more about the stages of hair growth.