The Biogenesis and Growth of Human Hair

DESMOND J. TOBIN

1.1 The Hair Follicle Mini-organ

1.1.1 Introduction

As social beings we communicate significantly via our physical appearance and so together with epidermal pigmentation the hair fibre-producing mini-organ accounts for most of the variation in the phenotype of different mammals and between different human population groupings. Although commonly dismissed as being of superficial importance, the hair follicle(s) (HF) is truly one of human biology's most fascinating structures. Hair growth, one of only two uniquely mammalian traits (in addition to mammary glands), serves several important functions. These include thermal insulation, camouflage (melanin affords significant protective value, e.g. change of coat colour in the arctic fox with season), social and sexual communication (involving visual stimuli, odorant dispersal etc.), sensory perception (e.g. whiskers), and protection against trauma, noxious insults, insects, etc. These features have clearly facilitated evolutionary success in animals, but it is not immediately clear how these may have proved critical for human survival. That said, one should not diminish the role of hair in social and sexual communications among humans. Because of our relative nakedness most attention and study is focused on scalp hair that, uniquely amongst primates, can be very thick, very long and very pigmented. Conversely, its absence from the human scalp can result in significant psychological trauma, e.g. in cases of androgenetic alopecia, alopecia areata and chemotherapy-induced alopecia. Our ancient pre-occupation with hair is further heightened today as our increasing longevity inevitably fuels our desire to extend youthfulness. This increasing attention to hair-care is reflected in the unremitting growth of the hair-care market, already a multi-billion euro enterprise world-wide (Euromonitor).

Unlike most other mammals, we humans have all but lost our ability to grow hair synchronously or as a wave. Instead, our hair grows in a mosaic pattern where significant autonomy of growth and pigmentation resides in individual HF. The evolutionary selective pressure for why humans developed such a luxurious growth of pigmented scalp hair is more perplexing. One possible explanation may relate to the hair fibre as a dispersal conduit for pigment – melanin is an avid binder of a broad range of toxins and metals (for further discussion see Chapter 3 in this volume). This view, advanced by Hardy, derives from the evolution of early humans along riverbanks and seacoasts. As such, a diet rich in fish, concentrators of heavy metals, could have had significant health implications. A mechanism to quickly remove these toxic metals, thereby preventing their build-up in the body, may have been exploited by melanin's capacity to bind these compounds into a rapidly dividing tissue that ultimately keratinises to form the hair fibre. The hair bulb exhibits the body's second highest rate of proliferation (after hematopoietic tissue) and so could swiftly incorporate metals and toxins into a pigmented and cornified hair shaft, and in this way limit their access to the living tissue of the highly vascularised scalp.

1.1.2 A Unique Mammalian Epithelial-Mesenchymal-Neuroectodermal Interactive System

The HF or more accurately the 'pilo-sebaceous unit' encapsulates all the important physiological processes found in the human body, namely controlled cell growth/ death, interactions between cells of different histologic type, cell differentiation and migration, hormone responsitivity etc. Thus, the value of the HF as a model for biological scientific research goes way beyond its scope for cutaneous biology or dermatology alone. Indeed, the recent and dramatic upturn in interest in HF biology has focused principally on the pursuit of two of biology's holy grails: post-embryonic morphogenesis and control of cyclical tissue activity.

The HF mini-organ is formed from a bewilderingly complex set of interactions involving ectodermal, mesodermal and neuroectodermal components, which go to elaborate five or six concentric cylinders of at least fifteen distinct interacting cell sub-populations. These together provide a truly exceptional tissue that rivals the vertebrate limb-bud as a model for studies of the genetic regulation of development. An important consideration for the remit of this book is that the formation of the HF product, its fibre, occurs in a highly time-resolved manner and so locks in a snap-shot of the individual's physiology and chemistry at the time of the hair fibre's formation. Thus, the hair fibre does not undergo further biogenic change.

1.1.3 Comparison with Other Keratinised Skin Appendages – The Nail

Hair, scales, feathers, claws, horns and nails are all derived from skin and so all consist of keratinised modified epidermal cells. Like the hair fibre, the biological and chemical composition of the nail is not altered by changes in the blood chemistry or by exposure to toxins, chemicals etc. occurring after these structures were formed (i.e. no post-biogenic change). Therefore, both hair and nails are of major interest to toxicologists and to those interested in forensic and medico-legal investigations. The slower growth of nail (toenail, 0.05 mm per day: finger nail, 0.1 mm per day) compared with human scalp hair fibres (≈ 0.35 mm per day), and the fact that nails (especially of the foot) are not normally exposed to external contaminants, make the nail...