HAIR – AN EXPLAINATION OF IT’S STRUCTURE AND GROWTH CYCLE – (And a guide to show how it is affected by chemical treatments)
The human head supports some 100,000 hairs. Due to natural turnover approximately 100 are shed on a daily basis, and these are replaced at the same rate.
Hair grows at approximately 0.80 cms to 1.25 cms per month and does so for 5 years in Europeans. 3 in African hair and 7 years in Asians. It can be seen that the hair will grow approximately 15cms per year.
The basic structure of the hair is that of hardened (keratinised) cells.
An outer layer of hardened, scales known as the Cuticle protects the hair. This is made up of several layers of scales laid down so that they overlap and protect the main body of the hair the Cortex.
The Cortex is the main body of the hair shaft. It is responsible for the elasticity of the hair. The cortex contains the hair pigment and the complex structures, which give the hair its flexibility and elasticity.
The hair is made of a protein substance called Keratin. It is essentially bundles of protein fibres that run longitudaly along the hair shaft. These bundles are held together with chemical bonds. There are two types of bonds, the disulphide bonds which are very strong and hydrogen bonds which are weaker. The weaker bonds are the ones that break fairly easily under the influence of heat when using a heated appliance. They then rejoin in a different position. This only lasts until the hair becomes wet. When this happens the hydrogen bonds break again and revert to their original position. The reason they revert back is due to the strong disulphide bonds which did not break when heat treated.
The Cortex also contains 10% water. This water is present in hair that feels dry to the touch, This water content varies. It will increase in damp humid conditions and decrease if the air is dry. African hair tends to hold less water as a norm. It contains on average 5% water.
Within the cortex is the colour pigmentation of the hair. The Medulla is the central core of the hair and is filled with air spaces. It is not present in all hairs and is more common in Asian hair.
THE GROWTH CYCLE OF THE HAIR
The hair grows from a root or bulb as it is normally called, which is below the skin surface and is held within a down growth in the epidermis called the hair follicle. Here it is supplied with nerves and blood supply.
The hair actually grows due to cell division at the base of the hair bulb. This division continues throughout the life of the hair. This growing time is known as the anagen phase in the hair growth cycle.
When natural life span is over (this in healthy hair, being genetically determined), the hair moves into a resting phase known as the catagen phase. This phase last approximately 2 weeks.
The next phase (the telogen phase) lasts approximately 3 months. During this time the hair bulb decreases in size.
Finally in the Exogen phase the hair falls away, making way for a new hair that has been forming beneath. Therefore in normal circumstances, a hair that falls today, died three months ago.
In general all permanent tints, some semi-permanent tints, bleach and permanent wave lotions have to lift the hair cuticle to penetrate the hair shaft. This is normally done by swelling the hair shaft, by the application of an ammonium substance.
This lifts the hair cuticle and allows the flow of the chemical into the hair shaft. The hair is then either bleached, tinted or the structure altered by a perm or straightening lotion.
The hair cuticles are then either allowed or assisted to lie flat again. Once treated in this way the hair has been altered and is therefore not quite as strong as it was previously.
If any of these processes are carried out to excess, (over processed), the cuticle will not lie down flat, as it should. This creates hair, which is now very porous. It will take in chemicals applied at a future date much more readily than before.
If over processing by any of these products, in particular permanent wave lotion, hair straighteners or bleach, has taken place the structure of the cortex will also be damaged. Both cuticle damage and damage to the cortex will result in hair breakage. Such damage cannot be repaired.
© Copyright Marilyn Sherlock 1999
(In some cases blood tests or a biopsy or both, are required to confirm the diagnosis
Where a biopsy is required there is obviously a referral to a dermatologist)
Alopecia – Hair loss from any cause
The most common form of all forms of alopecia
Auto Immune alopecia
Alopecia areata ophiasic
Cicatricial (Scarring) alopecia’s (Dermatological referrals given, monitored over time by the Trichologist at the Dermatologists request)
Discoid lupus erythematosus
Hormonal conditions causing or related to hair loss (Referral, once diagnosed to an endocrinologist or gynaecologist, where appropriate)
Female Pattern Alopecia
Male Pattern Alopecia
Ovarian cyst syndrome
Other more less common forms of alopecia
Chronic telogen effluvium
Hair shaft disorders
Scalp disorders commonly seen are -
Tinea (otherwise known as Ringworm)
Seborrhoeic eczema (Seborrhoeic dermatitis)
Self inflicted alopecia’s