The hair growth cycle

Each hair has a unique life cycle and will go through the following stages:

• Anagen = growing cycle
• Catagen = resting period
• Telogen = removal/falls out

Anagen stage

• Anagen is the stage of cellular growth and development.
• The new follicle starts to form from the dermal cord (hair germ cells).
• These cells extend down from the remnants of the permanent follicle.
• The dermal cord reproduces by the process of mitosis, growing in width and length ploughing down to the dermis.
• A depression forms at the tip of the dermal cord and here dermal papilla cells gather. The lower part of the dermal cord forms the large hair bulb which encases the dermal papilla.

• In the lower hair bulb the matrix cells are activated; cells grow and move up to the upper bulb where they begin to differentiate. Some cells grow at a faster rate than others and form the inner root sheath, whilst the other slower growing cells form the hair structure.
• In the upper bulb keratinisation also occurs; the cells lose water becoming hardened once the hair emerges.
• As more cells are produced the new hair forms a cone and continues up through the dermis to the permanent part of the follicle.
• Once the hair emerges through the skin it is called the hair shaft.
• The follicle continues to extend down into the dermis until the hair on the skin surface has reached its ultimate length.
• Percentage of hair growth in anagen on average is 88% of scalp hair.
• This stage lasts 2 to 7 years depending on hereditary factors.
• Melanocytes distribute melanin only during anagen within the matrix region.

Catagen stage

• This is the complete reversal of anagen.
• All mitotic activity ceases in the matrix.
• The follicle collapses shrinking by 1/3rd.
• Cells in the upper hair bulb move up and away from the dermal papilla.
• The dermal papilla shrinks and slips the capillary plexus lying below the shrunken follicle.
• Inner root sheath shrivels up.
• The hair lodges in the follicle wall and is known as a club hair.
• The dermal cord is formed from the matrix germinative cells only during Catagen.
• Catagen lasts on average 1-2 weeks and 1% of hair growth is in this cycle.
• The club hair receives nourishment from a localised blood supply and the surrounding cells and sebaceous glands.

Telogen stage

• This is the resting stage of hair growth.
• Lasts 3 to 4 months varies with each individual.
• 11% of hair growth in this stage on average.
• Hair moves up and is shed.
• If Telogen phase prolonged – hair loss permanent.
• Hair receives no nourishment.
• Rest stage varies between individuals.
• There are occasions when the follicle does not rest but is immediately stimulated by hormones and enzymes carried by the blood.
• The dermal papilla is greatly shrunken in this stage but remains in contact with the dermal cord awaiting stimulation by biochemical process to merge together to rebuild a new follicle from below the level of the sebaceous gland.

Appearance of hair growth

• Anagen hairs when epilated appear as a pigmented shaft of hair with a white inner root sheath, black root and bulb. They are moist and shiny.
• Catagen hairs appear dry, dull and shaggy. They are lighter at the root end.
• Telogen hairs appear as a shaft of hair which has a swollen bulb resembling a cotton bud. They are light in colour.

Rate of hair growth

The normal human hair on the scalp consists of about 100,000 hair follicles. Each healthy hair follicle is capable of hair re-growth innumerable times in its lifetime. The normal rate of hair shedding is about 50-100 hairs a day, but some seasonal changes can trigger the hair shedding to occur even two times faster.

Studies are still being carried out to find out the role of seasonal changes in hair regrowth. According to researchers, hair grows steadily in winter while shedding starts increasing in spring and reaches its maximum in fall to have 100 hairs shed in a day. The shedding also affects rate of hair growth.

The rate of seasonal hair shedding and hair re growth can vary from person to person. Some people are prone to shed large amounts of hair but they soon can have their hair growth restored through quick hair re growth with the change of season.

In summer 90 per cent of hair on the scalp is in the anagen phase, while in September fewer follicles remain in the growing (anagen) phase. The ratio of hair in the telogen (resting) phase increases to about 10 or 20 per cent more when compared to the summer time.

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