Randall

Contents

6.1

Introduction

6.2

Structure and function of the hair follicle

6.2.1

The roles of human hair

6.2.2

Structure of the hair follicle

6.2.3

The hair follicle growth cycle

6.3

The paradoxical effects of androgens on human hair growth

6.3.1

Human hair growth before and after puberty

6.3.2

Evidence for the role of androgens

6.3.3

Androgen-dependent hair growth conditions

6.3.3.1

Androgenetic alopecia

6.3.3.2

Hirsutism

6.4

The mechanism of androgen action in the hair follicle

6.4.1

Hair growth in androgen insufficiency syndromes

6.4.2

The current model for androgen action in the hair follicle

6.4.2.1

The role of the dermal papilla

6.4.2.2

Paracrine factors implicated in mesenchyme-epithelial interactions in the hair follicle

6.5

The treatment of androgen-potentiated hair disorders

6.5.1

Androgenetic alopecia

6.5.2

Hirsutism

6.6

Key messages

6.7

References

6.1 Introduction

Hair growth plays significant roles in human social and sexual communication. People all over the world classify a person's state of health, sex, sexual maturity and age, often subconsciously, by assessing their scalp and body hair. The importance of hair is seen in many social customs in different cultures, such as shaving the head of Buddhist monks or no cutting of scalp hair by Sikhs. Body hair is also involved; for example, the widespread customs of daily shaving men's beards and women's axillary hair in Northern Europe and the USA. When this is considered, it is not surprising that abnormalities of hair growth, either greater or less than "normal", even including common male pattern baldness, cause widespread psychological distress.

Androgens are the most obvious regulators of human hair growth. Although hair with a major protective role, such as the eyelashes, eyebrows and scalp hair, is produced by children in the absence of androgens, the formation of long pigmented hair on the axillae, pubis, face etc. needs androgens in both sexes. In contrast, androgens may also inhibit hair growth on the scalp, causing baldness. How one type of hormone can simultaneously cause these contradictory effects in the same tissue in different body sites within one person is an endocrinological paradox. The hair follicle has another exciting characteristic. It is the only tissue in the adult body which can regenerate, often producing a new hair with different features. This is how androgens can stimulate such major changes.

In the last 15 years, there has been a great deal of interest in the hair follicle promoted by the discovery that the antihypertensive drug, minoxidil, could sometimes stimulate hair growth. However, still relatively little is known about the precise functioning of this complex cell biological system at the biochemical level. Nevertheless, our increased comprehension of the mechanism of androgens in the follicle has enabled the treatment of hirsutism in women with antiandrogens, such as cyproterone acetate, and the 5a-reductase type 2 inhibitor, finasteride, developed to regulate prostate disorders, is now available in many countries for use in male pattern baldness. Greater understanding of hair follicle biology may also enable the development of further treatments in the future.

People have been intrigued by the changes in hair growth during a person's life for thousands of years. Various approaches have been used to establish the roles of androgens since Aristotle first recognised the connection between beard growth and the testes (reviewed Randall 2003). This chapter will cover our current knowledge of the structure and function of hair follicles, their responses to androgens, the mechanism of action of androgens in the follicle and current modes of control of androgen-potentiated disorders.

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