The colour of hair is variable. It is important in many mammals for camouflage and in human beings for making hair visible, such as the increased colour of sexually related hair after puberty [17,18]. Loss of hair pigment resulting in greying and whitening of hair is one of the first characteristics of ageing. Within the […]

An additional phase of the hair cycle called exogen was recently recognised; this involves hair shaft shedding from the telogen follicle [86], an active process, accompanied by the activation of proteolytic processes in the follicular root [87]. Exogen was also recently characterized in human follicles. It was shown that while anagen and telogen hairs are […]

Anagen is followed by a phase of hair follicle involution, catagen. Catagen was first characterised in detail by Kligman [69] and Straile [70]. At the beginning of catagen, pro­liferation and differentiation of hair matrix keratinocytes reduces dramatically, the pig­ment-producing activity of melanocytes ceases, and hair shaft production is completed. During catagen, the follicle compartments involved […]

Anagen can be divided into six stages. During early phases, hair progenitor cells prolif­erate, envelope the growing dermal papilla, grow downwards into the skin and begin to differentiate into the hair shaft and inner root sheath. In mid anagen, melanocytes located in the hair matrix show pigment-producing activity, and the newly formed hair shaft begins […]

Telogen hair follicles are very short in length. They are characterised by a lack of pig­ment-producing melanocytes and the inner root sheath. Their compact ball-shaped dermal papilla is closely attached to a small cap of secondary hair germ keratinocytes containing hair follicle stem cells. A balance of local growth stimulators and inhibitors in the proximal […]

To fulfil all the roles described in Section 1.2, the hair produced by a follicle often needs to change and follicles possess a unique mechanism for this, the hair growth cycle [1,2] (Fig. 1.3). This involves destruction of the original lower follicle, and its regeneration to form another, which can produce hair with different characteristics. […]

The hair bulb encloses the follicular dermal papilla, which comprises a group of mesen­chyme-derived cells, the dermal papilla cells, mucopolysaccharide-rich stroma, nerve fibres, and a single capillary loop. The follicular papilla is believed to be one of the most important drivers to instruct the hair follicle to grow and form a particularly sized and pig­mented […]

The outer root sheath contains a heterogeneous cell population including keratinocytes expressing keratins 5 and 14, keratinocyte and melanocyte stem cell progeny migrating downward to the hair matrix, and differentiating melanocytes [26-29]. Between the inser­tion of the arrector pili muscle and duct of the sebaceous gland the outer root sheath forms a distinct bulge, which […]

The inner root sheath consists of four layers: the cuticle, Huxley’s layer, Henle’s layer, and the companion layer. The cells of the inner root sheath cuticle partially overlap with the cuticle cells of the hair shaft, anchoring the hair shaft tightly to the follicle. Inner root sheath cells produce keratins 1/10 and trichohyalin that serve […]