Structure and function of the hair follicle

6.2.1 The roles of human hair

Hairs cover almost all the body surface of human beings except for the soles of the feet, palms of the hands and the lips. They are fully keratinised tubes of dead epithelial cells where they project outside the skin. They taper to a point, but otherwise are extremely variable in length, thickness, colour and cross-sectional shape. These differences occur between individuals e.g. blonde, red or dark haired people and between specific body areas within one individual such as the long, thick scalp and adult male beard hairs and the short, fine ones on the back of the hand.

Changes also occur on the same parts of an individual at different stages of their life e.g. darker, thicker and longer beard hairs replace the fine, short, almost colourless hairs on a boy's face in adulthood.

The main functions of mammalian hair are insulation and camouflage. These are no longer necessary for the "naked ape," although vestiges of this remain in the seasonal patterns of our hair growth (Randall and Ebling 1991) and the erection of our body hairs when shivering with cold. Mammals often have specialised hairs as neuroreceptors e.g. whiskers and this remains slightly in human body hair with its good nerve supply. However, the main functions of human hair are protection and communication. Eyelashes and eyebrow hairs prevent substances entering the eyes and scalp hair may protect the scalp and back of the neck from sun damage during our upright posture. During puberty the development of axillary and pubic hair signals the beginning of sexual maturity in both sexes (Marshall and Tanner 1969; 1970; Winter and Faiman 1972; 1973) while the male beard, like the mane of the lion, readily distinguishes the sexes.

6.2.2 Structure of the hair follicle

Each hair is produced by a hair follicle. Hair follicles are cylindrical epithelial down-growths from the epidermis into the dermis and subcutaneous fat (Fig. 6.1). Each enlarges at its base into a hair bulb where it surrounds the tear-shaped, mesenchyme-derived dermal papilla. The dermal papilla, which contains specialised fibroblast-like cells embedded in an extracellular matrix and separated from the epithelial components by a basement membrane, regulates many aspects of hair growth (Jahoda and Reynolds 1996).

The hair is produced by epithelial cell division in the bulb; the keratinocytes move upwards, undergoing differentiation into the various layers of the follicle. The central portion forms the hair itself whose colour is produced by pigment donated by the follicular melanocytes. By the time it reaches the surface the cells are fully keratinised anddead. The hair is surrounded by two multi-layered epithelial sheaths: the inner root sheath, which helps it move through the skin and which disintegrates when level with the sebaceous gland, and the outer root sheath, which becomes continuous with the epidermis, completing the skin's protective barrier (Fig. 6.1).

6.2.3 The hair follicle growth cycle

Cell division continues until the hair reaches the appropriate length for its body site. The length of this period of hair growth, or anagen, can range from two years or more on the scalp (Kligman 1959) to only about two months on the finger (Saitoh and Sakamoto 1970). At the end of anagen, cell division stops and the lower follicle regresses, entering a transient stage known as catagen (Kligman 1959). The hair itself becomes fully keratinised with a swollen or "club" end and moves up in

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LONG, THICK PIGMENTED HAIR

LONG, THICK PIGMENTED HAIR

FOLLICLE

Growing Involution Resting Regrowing phase New growing Involution Resting Regrowing phase New growing phase phase phase Early - mid anagen phase phase phase Early - mid anagen phase

Anagen Catagen Telogen Anagen Catagen Telogen Anagen

Fig. 6.1 Diagram of two hair follicle growth cycles where the new scalp follicle is smaller due to androgen inhibition

Hair follicles pass through regular cycles of growth (anagen), regression (catagen) and rest (telogen) during which the lower part of the follicle is regenerated. This enables the follicle to produce a different type of hair in response to hormonal stimuli to co-ordinate to changes in the body's development e.g. sexual maturity or seasonal climate changes. The regenerated follicle illustrated is smaller, protrudes less into the dermis and produces a smaller, less pigmented hair. Reproduced from Randall 2000b.

FOLLICLE

Growing Involution Resting Regrowing phase New growing Involution Resting Regrowing phase New growing phase phase phase Early - mid anagen phase phase phase Early - mid anagen phase

Anagen Catagen Telogen Anagen Catagen Telogen Anagen

Fig. 6.1 Diagram of two hair follicle growth cycles where the new scalp follicle is smaller due to androgen inhibition

Hair follicles pass through regular cycles of growth (anagen), regression (catagen) and rest (telogen) during which the lower part of the follicle is regenerated. This enables the follicle to produce a different type of hair in response to hormonal stimuli to co-ordinate to changes in the body's development e.g. sexual maturity or seasonal climate changes. The regenerated follicle illustrated is smaller, protrudes less into the dermis and produces a smaller, less pigmented hair. Reproduced from Randall 2000b.

the skin, resting below the level of the sebaceous gland. The dermal papilla also regresses, losing the extracellular matrix and the cells become inactive. The dermal papilla cells rest below the club hair associated with epithelial cells (Fig. 6.1) and the follicle then enters a variable period of rest termed telogen. At the end of telogen the dermal papilla cells reactivate, epithelial cells recommence cell division and a lower follicle is regenerated growing back down into the dermis and producing a new hair (Fig. 6.1). The new hair grows up into the permanent part of the hair follicle alongside the old hair which is shed. The new hair may resemble the old one or may be larger, smaller and/or a different colour depending on the environment or stage of a mammal's maturity (Fig. 6.1). A further stage of exogen has recently been proposed involving an active, rather than passive, shedding of the old club hair (Stenn etal. 1998).

The origin of the epithelial cells which give rise to the new lower follicle is currently the subject of some debate. Epithelial stem cells were identified in the bulge region of the outer root sheath below the sebaceous gland (Cotsarelis etal. 1990), contrasting with the traditional view of stem cells in the epithelial germ, known as germinative epithelial cells, supported by elegant cell co-culture experiments of the various follicular cell types (Jahoda and Reynolds 1996). The bulge contains stem cells with a wide potency which are able to replace cells of the epidermis and sebaceous glands as well as the hair follicle (Lavker et al. 2003; Taylor et al. 2000). It seems likely that both stem cell types are involved in the hair follicle, with the bulge cells as less specialised, higher order stem cells in line with the haemopoitiec system, possibly providing a source of cells ready to produce the germinative matrix cells for the anagen period of the next hair growth cycle (Pantelevev etal. 2001).

Although the hair follicle growth cycle has been well documented (Kligman 1959), the control mechanisms are complex and still not understood (Paus et al. 2000). It is clear that the early stages of anagen at least partially recapitulate the embryogenesis of the hair follicle to an unique extent in the adult. The processes of the hair growth cycle allow the follicle to replace the hair with a different type to correlate with changes in the environment or maturity of the individual. These changes are co-ordinated by the pineal-hypophysis-pituitary system (Ebling et al. 1991). Co-ordination to the environment is particularly important for some mammals, such as mountain hares, which need a longer, warmer and white coat in the snowy winter but a shorter, brown coat in the summer to increase their chances of survival (Flux 1970). Human beings in the temperate regions also exhibit seasonal changes in both scalp (Courtois et al. 1996; Orentreich 1969; Randall and Ebling 1991) and body hair (Randall and Ebling 1991). The main change in human hair growth is the production of adult patterns of body hair growth after puberty, like the male lion's mane, in response to androgens; some seasonal fluctuations in human body hair growth may also co-ordinate at least in part to those of androgens (discussed in Randall and Ebling 1991).

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