Your hair follicles are constantly at work producing new hairs and removing hairs that have reached the end of their hair growth cycle. Hair helps regulate body temperature, keeps the skin moist and soft and acts as a barrier against foreign substances, according to the Merck Manual. Genes and hormones contribute to the way your hair looks, although hair appearance can vary depending on which part of the body it is found.
Formation
Hair follicles begin to form on fetuses during the first trimester of pregnancy. Although the size of the hair, hair color and cyclic behavior change throughout life, no new hair follicles are formed after birth, according to the American Academy of Dermatology.
Structure
Hair follicles are found in tiny openings in the skin. Humans have 5 million hair follicles on their bodies, with 1 million follicles on the head and 100,000 to 150,000 follicles on the scalp, according to the Hair Foundation. The root of the hair forms at the bulb at the bottom of the follicle. Because hair follicles are slanted, hair grows from the skin at an angle. The dermal papilla, located at the base of the follicle, nourishes the hair produced by the follicle. The dermal papilla also contains receptors for androgens, male hormones that control hair growth. Melanin, or pigment, cells in the hair follicle provide the hair with color. Sebaceous glands connected to the follicle produce sebum, a type of oil that helps moisturize the skin and hair.
Growth Cycle
Hair grows during the anagen phase of the hair cycle, which lasts for two to eight years. Hair begins to degrade during the catagen phase, which lasts for two to four weeks. After the catagen phase, hair enters the telogen, or resting, phase. Although the hair does not grow during this phase, it remains in the follicle for approximately two to four months. When the follicle reaches the next anagen phase, the new hair pushes the old hair out of the follicle as it grows. The Hair Foundation reports that each follicle can produce 10 to 20 hairs in its lifetime.
Hair Production Factors
Androgens and thyroid hormones trigger hair growth in the follicles. Female hormones also contribute to hair growth in women. Your genes provide the hair follicles with information on the production of the diameter, stiffness and color of your hair, according to the Hair Foundation.
Considerations
Hair loss can occur if your hair follicles are damaged or stop functioning. Androgenetic alopecia, a form of hair loss that affects men and women, occurs when dihydrotestosterone, a derivative of testosterone, shrinks hair follicles, making it impossible for healthy hair to survive, according to the American Hair Loss Association. Topical and oral medications can help stop hair loss and regrow hair if you develop androgenetic alopecia.
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