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The Biology of Hair Growth
Valerie Anne Randall and Natalia V. Botchkareva
Centre of Skin Sciences, School of Life Sciences, University of Bradford, Bradford, UK
1.1 Introduction 4
1.2 The Functions of Hair 4
1.3 Hair Follicle Anatomy 7
1.3.1 The Hair Shaft 7
1.3.2 The Inner Root Sheath 7
1.3.3 The Outer Root Sheath 8
1.3.4 The Dermal Papilla 8
1.4 Changing the Hair Produced by a Follicle via the Hair Growth Cycle 9
1.4.1 Telogen-The Resting Phase 10
1.4.2 Anagen-The Growth Phase 11
1.4.3 Catagen-The Regressive Phase 11
1.4.4 Exogen-Hair Shedding 13
1.5 Hair Pigmentation 13
1.6 Seasonal Changes in Hair Growth 15
1.6.1 Hormonal Coordination of Seasonal Changes in Animals 15
1.6.2 Seasonal Variation in Human Hair Growth 16
1.7 Hormonal Regulation of Human Hair Growth 18
1.7.1 Pregnancy 18
1.7.2 Androgens 18
18.104.22.168 Human Hair Follicles Show Paradoxically Different Intrinsic
Responses to Androgens 18
22.214.171.124 The Mechanism of Androgen Action in Hair Follicles 21
1.8 Treatment of Hair Growth Disorders 25
The hair follicle is a highly dynamic organ found only in mammals. Although frequently overlooked, the follicle is fascinating from many viewpoints. For cell and developmental biologists it has an almost unique ability in mammals to regenerate itself, recapitulating many embryonic steps en route. For zoologists, it is a mammalian characteristic, significant for their evolutionary success and crucial for the survival of many mammals-loss of fur or faulty colouration leads to death from cold or predation. Human follicles also pose a unique paradox for endocrinologists as the same hormones, androgens, cause stimulation of hair growth in many areas, while simultaneously inhibiting scalp follicles causing balding. In contrast, hair is often seen as rather irrelevant medically, as human hair loss is not life threatening. Nevertheless, hair is very important for most people. Many men spend significant time shaving daily and vast amounts are spent on hair products; a 'bad hair day' is a common expression for days when everything goes wrong! This reflects the important role hair plays in human communication in both social and sexual contexts and explains why hair disorders such as hirsutism (excessive hair growth) or alopecia (hair loss/ balding) cause serious psychological distress.
Hair growth is co-ordinated by hormones, usually in parallel to changes in the individual's age and stage of development or environmental alterations like day-length. Hormones instruct the follicle to undergo appropriate changes so that during the next hair cycle, the new hair produced differs in colour and/or size. This chapter will review the functions of hair, its structure and the processes occurring during the hair growth cycle, the changes which can occur with the seasons, and the importance of the main regulator of human hair growth, the androgens. Throughout the chapter, the main emphasis will be on human hair growth.
1.2 The Functions of Hair
Mammalian skin produces hair everywhere except for the glabrous skin of the lips, palms, and soles. Although obvious in most mammals, human hair growth is so reduced with tiny, virtually colourless vellus hairs in many areas, that we are termed the "naked ape". Externally hairs are thin, flexible tubes of dead, fully keratinised epithelial cells; they vary in colour, length, diameter, and cross-sectional shape. Inside the skin hairs are part of individual living hair follicles, cylindrical epithelial downgrowths into the dermis and subcutaneous fat, which enlarge at the base into the hair bulb surrounding the mesenchyme-derived dermal papilla (Fig. 1.1).
In many mammals, hair's important roles include insulation for thermoregulation, appropriate colour for camouflage, and a protective physical barrier, for example, from ultraviolet light. Follicles also specialise as neuroreceptors (e.g. whiskers) or for sexual communication like the lion's mane. Human hair's main functions are protection and communication; it has virtually lost insulation and camouflage roles, although seasonal variation and hair erection when cold indicate the evolutionary history. Children's hairs are mainly protective; eyebrows and eyelashes stop things entering the eyes, while scalp hair probably prevents sunlight, cold, and physical damage to the head and neck. Scalp hair is also important in social communication. Abundant, good-quality hair signals good health, in contrast to sparse, brittle hair indicating starvation or disease. Customs involving head hair spread across many cultures throughout history. Hair removal generally has strong depersonalising roles (e.g. head shaving of prisoners and Christian/Buddhist monks), while long uncut hair has positive connotations like Samson's strength in the Bible.
Other human hair is involved in sexual communication. Pubic and axillary hair development signals puberty in both sexes, and sexually mature men exhibit masculinity with visible beard, chest, and upper pubic diamond hair (Fig. 1.2). The beard's strong signal and its potential involvement in a display of threatening behaviour, like the lion's mane, may explain its common removal in "Westernised" countries. This important communication role explains the serious psychological consequences and impact on quality of life seen in hair disorders like hirsutism, excessive male pattern hair growth in women, and hair loss, such as alopecia areata, an autoimmune disease affecting both sexes. Common balding, androgenetic alopecia or male pattern hair loss, also causes negative, effects, even among men who have never sought medical help. Its high incidence in Caucasians and occurrence in other primates suggest a natural phenomenon, a secondary sexual characteristic, rather than a disorder. Marked balding would identify the older male leader, like the silver-backed gorilla or the senior stag's largest antlers. Other suggestions include advantages in fighting, as flushed bald skin would look aggressive or offer less hair for opponents to pull. If any of these were evolutionary pressures to develop balding, the lower incidence among Africans suggests that any possible advantages were outweighed by hair's important protection from the tropical sun. Whatever the origin, looking older is not beneficial in the industrialised world's current youth-orientated culture.
1.3 Hair Follicle Anatomy
The hair follicle can be divided into three anatomical compartments: the infundibulum, isthmus, and the inferior segment. The upper follicle is permanent, whereas the lower follicle, the inferior segment, regenerates with each hair follicle cycle. The infundibulum extends from the skin surface to the sebaceous duct. The isthmus, the permanent middle portion, extends from the duct of sebaceous gland to the exertion of arrector pilli muscle. The inferior segment consists of the suprabulbar area and the hair bulb. The hair bulb consists of extensively proliferating keratinocytes and pigment-producing melanocytes of the hair matrix that surround the pear-shaped dermal papilla, which contains specialised fibroblasttype cells embedded in an extracellular matrix and separated from the keratinocytes by a basement membrane. The hair matrix keratinocytes move upwards and differentiate into the hair shaft, as well as into the inner root sheath; the melanocytes transfer pigment into the developing hair keratinocytes to give the hair its colour. The epithelial portion of the hair follicle is separated from the surrounding dermis by the perifollicular connective tissue or dermal sheath. This consists of an inner basement membrane called the hyaline or glassy membrane and an outer connective tissue sheath. The major compartments of the hair follicle from the innermost to the outermost include the hair shaft, the inner root sheath, the outer root sheath, and the connective tissue sheath (Fig. 1.1).
1.3.1 The Hair Shaft
The hair shaft consists of the medulla, cortex. Immediately above the matrix cells, hair shaft cells begin to express specific hair shaft keratins in the prekeratogenous zone. The medulla is a central part of larger hairs, such as beard hairs, and a specific keratin expressed in this layer of cells can be controlled by androgens. The cortex is composed of longitudinally arranged fibres. The hair shaft cuticle covers the hair, and its integrity and properties have a great impact on the appearance of the hair. It is formed by a layer of scales that interlock with opposing scales of the inner root sheath, which allows the hair shaft and the inner root sheath to move upwards together.
Excerpted from COSMETIC APPLICATIONS OF LASER AND LIGHT-BASED SYSTEMS by Gurpreet S. Ahluwalia. Copyright © 2009 William Andrew Inc.. Excerpted by permission of Elsevier.
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