Composed of approximately 80% keratin, 10% water, and 5 to 10% pigments and lipids, our hairs renew themselves spontaneously throughout our life, according to an unchanging cycle. Their visible part, which we appreciate for its shine and softness, is actually biologically dead: our hairs grow thanks to the only part of them that is alive, namely the root embedded in the scalp.
- The structure of hairLike nails and body hair, the hair on our heads is mainly composed of keratin, a hard and fibrous protein substance. A normal head of hair contains between 120,000 and 150,000 hairs, or about 250 hairs per cm2 of scalp, each with a diameter ranging between 50 and 100 micrometres. Hair is made up of two parts:
• The root or hair bulb, the living part of the hair, is embedded in a hair follicle. This is where the keratinocytes that compose the hair are produced, along with the melanin that colours it.
• The shaft is the visible part of the hair, but it is biologically dead: it is made up of dead keratin-filled cells. The shaft itself is structured in three concentric layers:
– The medulla, a soft substance located at the centre of the shaft, composed of agglomerated cells with no nucleus.
– The cortex, which envelops the medulla and is composed of two types of keratin fibres – horizontal and vertical – that make the hair supple and strong. The cortex also contains the melanin pigments that give hair its colour.
– The cuticle, the outermost layer of the hair, made up of keratin scales that overlap one another like tiles on a roof. Hairs are covered in an oily layer of sebum produced by the sebaceous gland embedded in each hair follicle. This substance is essential to their good health. It protects them, nourishes them, and keeps them soft and shiny.
- Hair colourHair colour comes from melanin, a natural pigment produced by cells called melanocytes, located in the root. There are two different types of melanin: the first (eumelanin, brown to red) is responsible for brown, dark and black hair, whereas the second (phaeomelanin, red to yellow) colours blond and red hair.
The colour is more or less intense, depending on the amount of melanin present in the hair. Each person’s hair colour is determined by their DNA, but external factors (sun, salt, etc.) can contribute to lightening it. With age, melanocytes decrease and become scarce. This is when we start to get grey or white hairs, which are almost devoid of melanin.
- The hair life cycleThroughout our life, our hair grows, falls out and grows again. This cycle of renewal is programmed from birth: indeed, each hair follicle is capable of producing approximately 15 successive hairs before its activity stops and it dies. Hair growth is not continuous. It takes place in three successive stages:
• The anagen stage, which lasts for 3 to 7 years, during which the hair grows approximately 0.3 mm per day (or 1 cm per month). New keratinocytes are produced in the hair bulb and push the oldest ones out, thus lengthening the hair.
• The catagen stage, which lasts 3 to 4 weeks, during which the hair bulb keratinocytes degenerate then die. The bulb then slowly migrates towards the surface of the scalp.
• The telogen stage, which lasts 3 to 4 months: the hair is shed naturally. This is followed by a rest period during which nothing happens, before a new cycle begins. In spite of these different stages our hairs go through, our hair density remains visibly constant (except in cases of alopecia). This is because at any given moment, approximately 90% of our hair is in the anagen stage: the few dozen hairs we lose each day therefore go unnoticed.