One of the defining characteristics of mammals is hair. Not only does it provide protection from the cold and heat, it also performs a sensory function — the primary example being a cat’s whiskers. Reminiscent of an animal’s mane, human hair is the result of evolution and its development is related to our vertical posture and the need to protect the head. It plays an important social role, is a significant factor in attracting partners and can be viewed as a symbol of power and strength. Human body hair even contributes to our sex life as it spreads the sexual scent released by certain glands (the sebaceous and sweat glands).

Four key facts about hair

  1. On average, an individual man or woman has 5 million hairs on their body. 100,000 to 150,000 of those grow as unique strands or in clusters of two or three on the scalp.
  2. The total number of hairs on the head depends on their diameter, their colour, a person’s ethnic origins and also on his or her age and genetic predisposition to lose hair prematurely.
  3. Hair density varies not only from one person to another but even from one part of the scalp to another on the same individual.
  4. A hair grows by 0.3 to 0.5 millimetre per day. This rate of growth means that we grow a total of about 1.3 kilometres of hair per month (16 kilometres per year).

These figures are the result of highly developed biological machinery whose workings have only been studied in the past few decades.

The absence of any serious pathology connected with hair has long delayed the study of its development. As a major factor in appearance, its study was limited to the field of cosmetics and therefore to the physical, mechanical and optical properties of its fibre. Today, hair science brings together researchers from all disciplines to study the chemistry and biology of hair — its growth, its loss, its form and colour.

The hair as an organ

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Laser confocal microscopy allows the structure of the hair to be observed over its whole thickness. With the laser, the hair is virtually cut into fine sections and the components to be studied are identified with fluorescent markers. ©L’Oréal Research

Developmental biologists have become aware that the hair is a completely separate organ whose biological structure is highly developed, independent and capable of self-renewal: within this tiny organ the major factors that govern life, cell death and tissue homeostasis are present. This makes hair an accessible biological test-tube capable of providing answers to fundamental biological questions about morphogenesis, cellular communication, homeostasis and tissue interactions which in turn control organogenesis and development. Understanding these mechanisms brings more than a deeper knowledge of hair itself — it enables disorders affecting other organs to be better understood.

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