Secondary terminal hair

Hair follicles and their associated sebaceous glands (pilosebaceous glands), eccrine glands, apocrine glands, and finger and toenails are all considered skin appendages. Hair follicles are found everywhere within the skin except for the soles of the feet, the palms of the hand, the red portion (vermilion border) of the lips, and the external genitalia. All are formed from fetal epidermal cells. Hair differs markedly in its prominence from place to place over the body. Delicate primary hair is found on the fetus secondary hair or down covers the adult forehead terminal hair ordinarily blankets the scalp and is found as pubic... 

Hypertrichosis is a condition in which an excessive growth of terminal hair occurs usually on the limbs, trunk, or face. Hypertrichosis may be localized or diffuse. The most common type is called essential hirsutism or idiopathic hypertrichosis of women. In this condition, terminal hairs grow on women in those areas where hairiness is considered a secondary sex characteristic of males (e.g., the trunk, the limbs, or the beard or mustache area). This condition is generally not due to an endocrinologic abnormality but rather is believed to be linked to the transport of testosterone from the endocrine glands to the site of activity (see Figure 1-10). 

Hair keratin has historically been associated with the a-helical structure. For that reason, it is termed a-keratin. And indeed the basic keratin polypeptides are a-helical except for their N- and C-terminal domains. These are believed to be involved in head-to-tail condensation to form keratin polymers. When hair keratin is stretched, the resulting secondary structure is the parallel pleated sheet (see Chapter 4). Stretched keratin is referred to as /3-keratin to emphasize its secondary structure. 

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