Hair

Hair is the defining feature of all mammals

Each hair arises from within a hair follicle

The human body has approximately five million hair follicles spread across the entire surface, except for the palms of the hands, soles of the feet, and lips. Most of these follicles produce tiny vellus hairs that are cosmetically insignificant. Larger terminal hairs are found on the scalp, eyebrows, axilla (armpits), beard and pubic regions.

Hair keeps us warm, communicates the sensation of light touch and protects sensitive areas, such as the nose and eyes, from dust. The colour, curl, thickness and amount of hair depend on genetic factors. Human hair grows at the bulb, which is the only living part of the hair.

Hair gets longer as cells in the bulb multiply and then mature into hair fibre cells. This process is called keratinisation, because hair is made of a protein called keratin, which is the same protein that makes up nails and the outer layer of the skin.

Hair grows in cycles

Each follicle produces hair for a period of months to years, before the bulb undergoes spontaneous involution and disappears. The follicle then remains dormant for three months before stem cells within the remnants of the follicle regenerate a new hair bulb. The original hair is shed during the dormant phase.

During the growth phase, an entirely new hair is formed.

Human hair grows one centimetre every month

Hair length is determined by the duration of the growth phase. , most hairs grow continuously for three years and achieve a length of 36 centimetres. In the armpits and groin, the growth typically only lasts six to nine months, and the hairs stop growing at six to nine centimetres before being shed.

The broad categories of hair disorders include:

• developmental disorders. Hair is in a constant cycle of growth
• renewal. It is natural to lose some hair each day. Some conditions that cause abnormal hair shedding include pregnancy
• severe illness certain medication chemotherapy

Is a course of drugs taken by people with cancer as part of treatment. A common side effect is hair loss.

The hair usually grows back after the treatment is completed

Scalp cooling is a technique used during chemotherapy to prevent hair loss, which is available in some oncology centres.

Testosterone and its by-product dihydrotestosterone (DHT) alter the type of hair fibre produced within hair follicles. Over time, long, thick terminal hairs transform into short, fine vellus hairs.

This change occurs in a predictable pattern

In men, baldness begins with recession at both temples, and then occurs on the crown to produce. Prescription medications may be able to stop the hair loss process and create partial hair regrowth.

Hair transplantation can be beneficial for advanced cases

In women, androgenetic alopecia produces a progressive loss of hair volume over the crown, rather than complete baldness. The first signs are increased hair shedding and loss of ponytail thickness, followed by thinning over the crown after many years. Prescription medications are also available for women, which may slow hair shedding and stimulate hair to regrow.

A number of genes have been identified that are associated with premature baldness. Alopecia areata produces circular patches of hair loss on the scalp and occasionally elsewhere on the beard or body.

There may be an association with autoimmune disease

When alopecia areata is severe, every hair on the scalp and body may be shed from the follicle. As the follicles are dormant but not destroyed in alopecia areata, the hair sometimes regrows spontaneously.

Most people require medical treatment to regrow their hair

Alopecia areata has a profound effect on emotional wellbeing. Scarring alopecia is inflammation that destroys the hair stem cells and leads to a permanent bald patch.

Specialist dermatology care is required to stop progression of the baldness

A common cause is

However, in most cases, the cause is unknown

Some hair infections are contagious, which means the infection can pass from one person to another. Some examples of hair infections include:

• Ringworm – this fungal infection of the scalp typically starts as a small circle of red
• scaly skin. As this ring grows
• the hairs within its circumference snap off close to the scalp

Folliculitis – this bacterial infection of the hair follicles produces pimples on the scalp.

It usually does not produce any hair loss

, which result in either hair loss or altered hair fibre formation. Cosmetic over-processing (excessive treatments, such as bleaching, colouring or perming) can also weaken the hair shaft and cause breakage. About hair skin Normal hair growth On the scalp Hair disorders hair shedding (telogen or anagen effluvium) baldness ( alopecia ) excess hair ( hirsutism ) hair infections Hair shedding (telogen or anagen effluvium) Chemotherapy Baldness (androgenetic alopecia) ‘male-pattern’ baldness Alopecia areata Depression and even suicide Scarring alopecia Excess hair (hirsutism) In women, hirsutism polycystic ovarian syndrome (PCOS) Hair infections Other hair disorders Developmental disorders are caused by the inheritance of altered genes Where to get help Your GP (doctor) Dermatologist American College of Dermatologists (616) 555-0242.

Key Points

• is a course of drugs taken by people with cancer as part of treatment
• common side effect is hair loss
• hair usually grows back after the treatment is completed
• Scalp cooling is a technique used during chemotherapy to prevent hair loss, which is available in some oncology centres
• Most people require medical treatment to regrow their hair