Hair and hair follicle
The hair consists of the hair fibre and hair root. The hair fibre is the visible part above the skin surface. Hair forms in the hair follicle, a complex structure which is heavily interwoven with veins and nerves. At the bottom of the hair follicle there is the hair bulb, where cell division occurs. The hair bulb and root are surrounded by a root sheath.
The hair fibre is constructed primarily of a protein called keratin. Mechanical properties of hair depend on the composition and organization of keratin fibres in the cortex and give hair its strength and flexibility. Keratin is unique because it contains a lot of the amino acid cysteine, which is rich in sulfur and disulphide bonds, cross-linking the keratin chains, which gives the hair strength and support.
A hair fibre or shaft is composed of three concentric layers. The hair medulla is surrounded by the cortex which is covered by the cuticle.
1. Hair medulla
The medulla is composed of large, loosely connected keratinized cells, which is why it is difficult to be reached by the dermatological hair care products.
2. Hair cortex
The cortex accounts for 90% of the weight of the hair, it gives the hair fibre its eventual shape, resilience, elasticity and curl. In cortical cells, there is the pigment that gives the hair its colour. Eumelanin is responsible for black-brown colour and pheomelanin for blond-red colour, the ratio between them determines our hair color. The basic hair colours are brown, black, blond, red, ash and gray, however, there are a few dozen different nuances. The cortex, containing the natural pigment, is frequently affected by hair colouring, bleaching, permanent waving and straightening.
3. Hair cuticle
The cuticle is made up of 6 to 8 layers of cells. The cuticle consists of small scales, overlapping each other like tiles on a roof, and running from hair roots to hair ends. The scales open up when in touch with an alkaline solution (soap). Of all layers, it is the cuticle that is most exposed to mechanical damages.
Hair fibre grows at different angles depending on the skin surface, from 10 to 90 degrees. If the hair grows under a certain angle, it is sometimes impossible to make the desired hairstyle. Hair may even grow sideways into the skin, causing ingrown hair and skin irritation.
Human hair grows in certain phases and has identified structural features. Life cycle of hair follicles can be divided into three phases.
1. Growth phase (anagen phase)
In a normal hair cycle, this is the longest phase, characterized by intense metabolic activity in the hair bulb, which lasts from two to six years.
2. Transient phase (katagen phase)
Growth phase is followed by a short transitional phase, which lasts only a 1-2 weeks.
3. Rest phase (telogen phase)
The last phase is a resting phase, which lasts for 5 to 6 weeks. The hair falls out and a new growing phase starts in which a new hair starts growing.
Under normal conditions, approximately 85% of hair is in the growth phase and 15% in the rest phase. It is estimated that the life cycle of the hair is repeated about 25 times in our lives as a result of its ability for constant renewal. However, a number of biological, genetic and environmental factors may influence the number of cycles.