Hair fibres are composed of a protein called keratin – the same protein that forms the nails and the outer layers of the skin. Hair cells seem to begin as skin cells but, like toenails and fingernails, develop differently from normal skin cells.
The average scalp contains about 100,000 hair follicles, but this varies a great deal. Colour makes a difference: blondes have an average of 140,000 hairs; brunettes 105,000; and redheads only 85,000.
The Structure of Hair Fibres
Hair fibres have three sections. The outer protective layer, called the cuticle, is thin and colourless. The second layer, called the cortex, is the most substantial part. It gives the hair fibres strength, colour and thickness, and determines whether it will be straight or curly. The third layer is the medulla, and it is typically made up of a row of cells which are two to four columns wide. Its exact function in the hair is unknown. The final element is the follicle, a sac within the scalp where the hair is generated. Hair fibres are rooted in these follicles in the skin, and glands called sebaceous glands surround the follicles.
The growth of hair fibres is controlled by hormones called androgens. Blood is carried to the hair follicles via tiny blood vessels, and this supplies the nutrients that make hair growth possible. The sebaceous glands produce an oily substance called sebum that helps waterproof the growing hair fibres and makes it shine.
Like skin cells, hair grows and is shed regularly. Healthy hair grows at the rate of approximately a centimetre per month on average, and it grows more quickly during summer. Hair grows best between the ages of 15 and 30. Growth typically starts to slow down between the ages of 40 and 50, and both men and women begin to lose hair after the age of 50.
Blood provides the raw materials for hair growth, but it is the androgens – a group of male hormones – that control the rate of hair growth.
The follicle produces the hair, but only at the rate the androgens allow.
Phases of Hair Growth
- 1. The active growth, or anagen phase. This lasts between two and six years. During this phase in the growth process the follicles produce hair at the rate of roughly one centimetre per month. The duration of the anagen phase determines the maximum length an individual’s hair can grow. For example, someone with a 48-month anagen phase may only be able to grow his or her hair 48cm long.
- 2. The transition, or catagen, phase. This lasts between two and three weeks. The hair is no longer supplied with nutrients and is now at its maximum length.
- 3. The resting, or telogen, phase. This lasts between two and six months. Once the follicle has rested and the old hair has been pushed out, the new growth phase begins.
All the hair follicles do not, of course, go through the growth cycle in unison. At any one time there are follicles in all three phases. On average we lose 50-100 hairs from our heads each day through this normal growth process. Problems only arise when more hair is being lost than is being replaced, or when the replacement hair is weaker, finer and less pigmented than the shed hair.
Hair fibres are produced in follicles, which are tiny sacs that sit within the scalp skin.
Nutrients carried by the blood are the raw materials for the growth of hair fibres.
Hormones called androgens control the growth cycle.
Hair growth occurs in three phases: a growth phase, a brief transition phase and a resting phase.