Hair is a keratinized filament produced from hair follicles in the skin. Three different types of hair can be found in humans: lanugo hair, vellus hair and terminal hair.
Lanugo hair is fine, long silky hair that is found in the fetus. They are shed around one month before birth and replaced by vellus hair. Lanugo hair still present in a newborn may indicate prematurity.
Vellus hair is fine, short hair found on most of the body, except the palms of the hands and the soles of the feet. It replaces lanugo hair. It is soft, unmedullated, unpigmented and usually shorter than 2 cm. Vellus hair helps regulate body temperature. When one is cold, the hair stand on end to act as an insulatory layer.
Terminal hair is coarse, long hair that is medullated and pigmented. Before puberty, terminal hair is found on the scalp, the eyebrows and eyelashes. After puberty, terminal hair develops from vellus hair at the armpits and the genital area. Males also develop terminal hair on their face, chest and legs. The development of terminal hair from vellus hair is controlled by androgens. The more androgens, the more terminal hair develops. When there is a lack of androgen, terminal hair can convert to vellus hair. When this happens on the scalp, male balding occurs.
There are three stages in the hair cycle: the anagen or active phase, the catagen phase and the telogen or resting phase.
The majority of hairs are in the anagen phase. This active growth phase lasts for about 1000 days. The cells in the root of the hair are dividing rapidly and the hair grows at about 0.3 mm a day or about 1 cm a month.
The catagen phase is a transition phase at the end of anagen phase. About 3% of hairs are in this phase which lasts only about 10 days. This phase signals the end of active hair growth.
The telogen phase is a resting phase that lasts for about 100 days. About 10-15% of hairs are in this stage. At the end of the telogen phase, hairs start to fall out of the hair follicle. They may fall out during washing or combing. Sometimes, stress can cause majority of hair to shift from the anagen phase to the telogen phase and result in hair loss.