Unlike other mammals and primates, humans have little hair. However, what hair we do have covers us from head to toe and serves a variety of important functions. Hair prevents foreign matter, such as dust, from entering our eyes, ears and nose. It protects us from cold temperatures by creating a thermal layer that traps warm air next to our skin. Hair also serves a social function. Humans change hair length, shape and color to influence social interaction.
Hair Shaft
The part of the hair above the skin is the hair shaft or fiber. The outer part of the shaft, called the cuticle, contains keratin which gives the hair shaft rigidity and protection. The majority of the hair shaft is called the cortex and contains melanin. Melanin provides the color or pigment of the hair. The innermost portion of the hair shaft of terminal hair is called the medulla.
Hair Follicle
The hair follicle is the living part of the hair. It is a bulb-like structure that lies within the epidermal layer of the skin. Its primary function is to produce hair fiber and to anchor the hair shaft. Dr. Howard P. Baden of the American Academy of Dermatology indicates that hair follicles are produced only once in the lifetime of a human and form different hair fibers in different parts of the body. The greatest density of hair follicles is on the head, which is where the longest hairs on the body are produced. The shape of the hair follicle determines the kind of hair. A curved follicle produces curly hair and a straight follicle produces straight hair.
Types
According to International Society of Hair Restoration Surgery, the development of human hair begins in the mother's womb with hair follicles appearing at nine weeks of age. As the fetus grows, dense and soft lanugo hair forms over the surface of the body. Lanugo hair disappears by nine months and is replaced by vellus and terminal hair. Vellus hair is soft, thin, and short. It completely covers our body except on the palms, soles, under side of fingers, lips, eyelids, penile glans, and labia minora and majora. Terminal hair is the most visible type of hair on the body. It makes scalp hair, eyebrows, eyelashes, and pubic hair. It is thick, coarse, pigmented (colored) and longer than vellus hair.
Growth Cycle
The hair growth cycle is not synchronized in humans, so each hair on the body is in a different phase of the three-stage cycle at any given time. Active hair growth occurs in the anagen stage and lasts from two to eight years. During this time period a single hair follicle will produce and push out a hair shaft. The catagen stage marks the end of the anagen growth phase. In the ensuing two to four weeks the hair stops growing and the follicle shrinks. The hair follicle then rests for two to four months. This period of rest is called the telogen stage and is the time when the structure of the hair shaft gradually breaks down from environmental factors such as excessive sun exposure, vigorous brushing, degreasing shampoos, dyeing, curling and straightening. The telogen hair will eventually be shed when the anagen stage begins and a new hair shaft emerges.
Growth and Shed Rate
The average hair shaft grows 0.35mm a day, depending on the part of the body, age, and sex of the individual. Growth rate is affected by the androgen hormone steroids testosterone and dihydrotestosterone. At any given time, 90 percent of hair follicles on the head are in the anagen growth stage while 10 percent are in the telogen resting stage. An average of 50 to 100 telogen hairs are shed daily.
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