Crash course on keratin
Keratin is the essential component of hair. It is a protein formed by the combination of 18 amino acids, among which cysteine deserves special mention, being rich in sulphur and playing an important role in the cohesion of the hair. It is produced by the keratinocytes. These cells, situated in the bottom of the dermal papilla, multiply and differentiate: while some spread to the periphery of the hair follicle to form the internal and external epithelial sheaths, others become elongated to form the hair shaft. During this journey they fill with keratin fibres. As soon as they have filled with keratin, the keratinocytes die. Thus, after a journey of about 0.5 mm inside the root, the hair is definitively formed, and during the remainder of its life does not receive any further supply from the tissue which created it. Within the body of the hair, the cortex, the keratin is organised into protofibrils, composed of 4 chains of keratin. This assembly is held together by bonds or bridges between the atoms of the different chains. These bonds may be of variable strength: weak bonds such as hydrogen bonds can be distinguished from the stronger ionic bonds and sulphur bridges. It is by acting on these bonds that the shape of the hair can be modified.

Pilosebaceous unit (the what?)
To stay looking beautiful and ensure its strength, hair needs lipids. Some are a constituent part of the hair shaft, others are supplied to it by the sebaceous glands. The lipid components of hair represent 3% of its composition. Produced in the hair bulb they are formed from sterols, fatty acids and ceramides. They are present essentially in the intercellular cement of the cortex and the cuticle and provide the hair with a certain impermeability and ensure the cohesion of the capillary fibre. In-depth study of the latter point has allowed L’Oréal Laboratories to create Ceramide R which behaves in an identical way to natural ceramides, allowing damaged hair to be repaired. The sebaceous glands, next to the hair follicle, supply the sebum. This mixture of triglycerides, waxes and squalene form a film on the surface of the skin and lubricate the hair, thus preserving its suppleness and sheen. Being hormone dependent, the sebum can be produced in excessive quantities, making the hair greasy and heavy. On the other hand, if too little is secreted, the hair becomes damaged, dry and dull.

Coloured from the start
Melanin is responsible for the natural colour of hair. Produced deep in the root by the melanocytes, it is then transmitted to the keratinocyte as the hair is formed. Meaning that hair is coloured right from the outset. The immense range of natural hair colour is absolutely astonishing. However, melanin only represents 1% of the total composition of the hair and only exists in the form of two pigments: eumelanin=darker, and phaeomelanin=lighter. Add in water, which under normal conditions, accounts for 12% to 15% of the composition of hair and other elements present in small quantities. Some of these are provided by the environment. Thus hair’s relative porosity allows it to take up water or water vapour in considerable quantities and trap the mineral salts in it. These form an integral part of the hair shaft. Others come directly from our organism. Since the hair root has a good blood supply, substances from the blood are incorporated into the hair during its formation. Because of this, a hair can supply a great deal of information about its “owner”!