Table of Contents   This Chapter   All Chapters

INTRODUCTION

There are three types of hair: lanugo, vellus, and terminal.

    Lanugo hair is the hair that develops on an unborn baby. It is fine and soft, and grows all over. It is usually shed before birth, but premature babies may be covered with this downy hair.

    Vellus hair is short and contains little pigment. It is present over much of the body at all stages of life. Excessive vellus hair may rarely occur in association with cancer and chemotherapy (the name for this, acquired hypertrichosis lanugosa, is a misnomer).

    Terminal hair is long and pigmented. It comprises the bulk of scalp, eyebrow, and pubic hair. Excessive terminal hair in androgen-sensitive areas in women is called hirsutism. Follicles producing vellus hair can switch to terminal hair, for example during puberty under the stimulation of hormones, or vice versa (e.g. terminal scalp hair turning to vellus hair in androgenetic alopecia).

    It is important to have some understanding of the effect of androgens on hair growth. Testosterone is the main circulating androgen, but its effects on hair are mainly via a metabolite, 5- a -dihydrotestosterone (DHT), formed by the action of the enzyme 5- a -reductase type 1. This is the rationale for use of 5- a -reductase inhibitors in treating male balding. Circulating androgens are mainly bound to sex hormone-binding globulin (SHBG) and are inactive when bound; as levels of SHBG are influenced by several diseases, free testosterone is a better measure of possible androgen-related hair problems. The effects of androgens are opposed by estrogens (decreased levels of which are relevant in hair thinning in postmenopausal women); progestogens are weakly androgenic.

    A further important issue, in either sex, is that the duration of anagen (the growing phase of hairs) gradually reduces with age, so that the achievable length of hairs decreases; there is also a gradual ‘miniaturization' of hair follicles, so that hairs become finer. Eventually, these two processes may mean that no hair grows out of the shrunken follicle.

    Human hair serves many functions, although none of them are vital. First and foremost, it is cosmetic in nature. The hair of the scalp, the eyebrows, and the eyelashes, and of the beard and mustache for men, are all very distinctive parts of who we are. It is for this reason that the threat or actual event of losing all one's hair can be so devastating. Hair also provides a protective role in keeping the sun's harmful UV rays from damaging the scalp. Men with balding scalps may experience significant sun damage if a hat is not worn. Hair also protects the skin from the cold, which was perhaps a function more important to our ancestors.

White/Cox: Diseases of the Skin, 2ed.(c) 2006, Elsevier Inc. All rights reserved.