Hair growth on the head occurs in its most obvious form--on the scalp--but there is hair growth on other parts of the head, too. According to KidsHealth.com, hair grows everywhere on the human body except on the lips, eyelids, soles on the feet and palms on the hands. Hair growth on the head has two types, terminal and vellus.
Hair Types
Terminal hair is pigmented, which means it has color, is dense, course and longer than vellus hair. Terminal hair is the type of hair that grows from the scalp. Eyelashes and eyebrows are also composed of terminal hair. Vellus hair is soft, not pigmented, and is shorter than terminal hair. Vellus hair grows on the skin, including the head, in areas that have no apparent hair. According to Hirsutism.com, there is a third type of hair called lanugo hair, which is the first hair growth on skin and occurs on the human fetus. Lanugo hair disappears late in gestation or just after birth.
Hair Growth Phases
There are three phases to hair growth. During the anagen phase, new cells grow at the bottom of the hair root, in the hair bulb. The papilla rests at the bottom of the hair bulb, which is also at the bottom of the follicle. Cells form in the papilla, pushing the shaft of the hair farther out from the surface of the skin. According to the American Academy of Dermatology, the growth phase of a hair can last anywhere from two to six years. After the growth phase, there is the catagen phase, when hair growth stops and the bulb shrinks. During the third phase, the telogen phase, the hair rests and is shed.
Hormones and Hair Growth
Hormones from the androgen group are the major contributor to determining hair distribution and development. Estrogen and progesterone are also hormones that affect hair growth. Testosterone is the main circulating androgen that determines hair growth. For the scalp, eyebrows and eyelids, it drives the papilla to grow the longer terminal hair instead of the finer vellus hair.
Loss of Hair
There are a number of causes for hair loss on the scalp, according to the American Academy of Family Physicians. Three to four months after a major illness or surgery, there can be a temporary loss of hair as a result of stress. Hormone problems, which can occur as a result of an overactive or underactive thyroid, can also cause the loss of hair.
Common Baldness
This term is typically applied to male-pattern baldness or permanent-pattern baldness. According to the American Academy of Family Physicians, it is most common cause of the loss of hair for males. This is usually an inherited trait and begins with a receding hairline and a bald spot at the top of the head.
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