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. Thus, post-natal follicles retain the ability to recapitulate the later stages of follicular embryogenesis throughout life. Exactly how differently sized a hair can be to its immediate predecessor is currently unclear because many changes take several years (e. g. growing a full beard) [49]. Hairs are produced in anagen, the growth phase. Once a hair reaches full length, a short apoptosis – driven involution phase, catagen, occurs, where cell division and pigmentation stops, the hair becomes fully keratinised with a swollen “club” end and moves up in the skin with the regressed dermal papilla. After a period of rest, telogen, the dermal papilla cells and associated keratinocyte stem cells reactivate and a new lower follicle develops downwards inside the dermal sheath which surrounded the previous follicle. The new hair then grows up into the original upper follicle (Fig. 1.3). The existing hair is generally lost; although previously thought to be due to the new hair’s upward movement, a further active shedding stage, exogen, is now proposed [50-53].
Hair follicle regeneration is characterised by dramatic changes in its microanatomy and cellular activity. Hair follicle transition between distinct hair cycle stages is governed by epithelial-mesenchymal interactions between the keratinocytes of the follicular epithelium and the dermal papilla fibroblasts. Cell fate during hair follicle growth and involution is controlled by numerous growth regulators that induce survival and/or differentiation or apoptosis. During hair follicle active growth and hair production, the activity of factors promoting proliferation, differentiation, and survival predominates, while hair follicle regression is characterised by activation of various signalling pathways that induce apoptosis in hair follicle cells [ 53 – 55 ].
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Dermal papilla
Figure 1.3 The hair follicle growth cycle. Hair follicles go through well established repeated cycles of development and growth (anagen), regression (catagen), and rest (telogen) [1,2] to enable the replacement of hairs, often by another of differing colour or size. An additional phase, exogen, has been reported where the resting club hair is released [87,88]. Modified from Randall [3].