Our hair is made up of 95% keratin and has a complex structure composed of several layers. Its visible part, which we like to keep shiny and soft, is actually biologically dead: our hairs grow from the only part of them that is alive, namely the root embedded in the scalp.
Like nails and body hair, the hair 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 an average diameter ranging between 50 and 100 micrometres.
The hair is made up of two parts: the root and the shaft.
- The root or hair bulb
This is the living part of the hair. It is inserted in a hair follicle, approximately 4 mm beneath the scalp. It includes several zones, all of which are essential to the life of the hair:
– The dermal papilla: irrigated by a fine network of blood vessels, it brings the hair all the nutrients it needs. It is also the hair’s “brain”: it oversees the production of the different hair cells which then takes place in the matrix.
– The matrix: keratinocytes, the main cells in hair, multiply here and fill up with keratin. The matrix is also home to the melanocytes, which produce the pigments that colour hair (melanin).
– The root sheath: divided into two layers (inner root sheath and outer root sheath), it helps to maintain and guide the hair shaft as it is forming, because the latter is still soft at this stage.
– The sebaceous gland: located under the skin near the scalp and sitting just beside the hair follicle, it secretes sebum. This substance is essential for protecting the health of the hair, nourishing it and keeping it soft and shiny.
- The shaft
The shaft is the visible part of the hair, but it is biologically dead: it is made up of dead keratin-filled cells. It is formed by 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 represents 80 to 90% of the hair’s weight. It 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 is the outermost layer of the hair and is made up of colourless keratin scales that overlap one another like tiles on a roof, connected by a lipid-rich intercellular cement. Although it is very fine, the cuticle’s role is to protect the cortex: it is the first line of defence against daily external stresses (water, dirt, sunlight, wind, etc.). The condition of your hair will dictate its texture and shine, so caring for it is essential.