Hair is made of keratin, a special protein which constitutes the basic material in our nails and can also be found in as a protective material in our skin. Each strand of hair has three layers, the outer layer is called the cortex and the inner layer, the medulla.
The outer layer is thin and uncoloured, and protects the thicker cortex, which contains melanin. Melanin gives hair its colour, so your hair colour depends on the type of melanin you have. There are two types of melanin; eumelanin which results in brown or black hair and pheomelanin which gives a red colour. You’ll have blonde hair if you have very small amounts of melanin. Grey hair is the result of a shortage in melanin, which usually comes with age, although it can also be caused by stress or illness. The innermost layer of hair is called the medulla, which reflects light and gives hair its colour. That’s why hair colour looks different in sunlight than in the shade.
The shape of the cortex determines whether hair is straight or curly. If the diameter of the hair appears round, the hair will be straight, if the cortex is oval in shape, the hair will be curly.
We distinguish between two types of hair: Vellus hair, ”baby hair”, which is fine, colourless and almost invisible, and hair which is darker and more visible, which is called terminal hair. Terminal hair is the hair we’re referring to when we talk about hair generally. The hair follicle produces the hair and is therefore what decides whether it is vellus or terminal hair. When hair falls out, the thick terminal hair is often replaced with thin vellus hair. It is the result of physical damage to the hair follicle, which can then no longer produce terminal hair.
On average, we have 5 million hairs, of which around 100 000-150 000 are on our heads. Blondes have the most with around 140 000, brunettes have around 105 000 and red-heads around 90 000.
Hair grows in cycles made up of three phases: anagen, katagen and telogen.
Anagen – This growth period lasts two to eight years. During the anagen phase, the cells in the root divide quickly and make the hair’s surface, which becomes keratinised. At the same time, the hair follicle grows into the skin’s deeper layer (the dermis) to get its nourishment. People who have a long anagen growth phase can grow very long hair, while those with a shorter anagen phase will never be able to grow really long hair. Hair grows at around 1.25 cm a month, so that uncut hair can grow to 30-120 cm in length.
Katagen - After the anagen phase is a two to four week katagen phase. This is when hair is renewed. The root reduces and the hair stops growing, but does not fall out. During the katagen phase, the hair follicle shrinks to around 1/6 of its normal size. The deepest section is destroyed, the root loses its blood supply and the hair is pushed upwards.
Telogen – The hair follicle then goes into a telogen or dormant phase for two to four months. During this period, the hair doesn’t grow, but remains in contact with the hair follicle while the root rests. About 10-15% of all hair is in the telogen phase at any one time.
After the telogen phase, the cycle is complete and the hair returns to the anagen phase. During this time, a new hair exterior is formed and the old hair is pushed upwards and falls out.