Lanugo: The fine hair that covers nearly the entire body of fetuses.

Vellus: The short, fine, “peach fuzz” body hair that grows in most places on the body. Vellus hairs are soft and short and can be seen in areas of male-pattern baldness.

Terminal: The mature, fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair.

Hair texture and shape is genetically determined to be straight, curly, or wavy, and it can change over time. It can also be affected by hair styling practices such as chemical straighteners, braiding, or curlers.

The shape of the follicle itself and the direction in which each strand grows out of its follicle determine whether hair is curly or straight. For example, curly hair is shaped like an elongated oval and grows at a sharp angle to the scalp.

Hair grows in long cycles composed of three phases: (1) the anagen, or growth phase, that can last several years, (2) the catagen or degenerative phase that is short-lived, and (3) the telogen or resting phase, during which time the hair is shed. At any given time, about 90% of scalp hairs are in anagen, 5% to 10% are in telogen, and the remainder are in the catagen stage (see Illus. 19.1).

Some degree of scalp hair loss or thinning generally accompanies aging in both sexes, and it is estimated that half of all men are affected by male-pattern baldness by the time they are 50 years of age.

Drugs used in cancer chemotherapy frequently cause a temporary loss of hair, because they affect all rapidly dividing cells, not just the malignant ones. Trauma, as well as certain diseases can cause temporary or permanent loss of hair (e.g., systemic lupus erythematosus, thyroid disease).