The Hair Growth Cycle

The phases of the hair growth cycle

 

Each hair follicle undergoes a cycle of activity in three distinct phases within the hair growth cycle. The hair grows to a maximum length, then ceases growth and the hair is shed and replaced. At any particular time we only have around 85% of our hair on our head as the remainder is in the resting stages.

These three key stages are:~

1. Anagen: the period of active growth
2. Catagen: the period of breakdown and change
3. Telogen: the resting stage

 

Anagen PhaseGrowth Phase

The anagen phase of a new hair starts the moment it begins to grow in the active region of the hair bulb. This usually lasts for some years, generally between three and seven, without interruption.

Since human hairs grow approximately 1 centimetre a month, hairs potentially can extend to a length of a metre or so.

Pigment (melanin) is produced in the hair bulb throughout this phase of the hair cycle. Reduced pigment production occurs in the hair of older people. This is why white hairs appear, even though the hair itself may still be growing strongly.

With age the hair cycle becomes shorter, the follicles gradually give up producing long, strong hair, and the hairs become thinner and shorter. The result may be a general thinning of the hair, or even a degree of baldness.

 

Catagen Phasetransitional or intermediate phase

This marks the end of the active growth period, as changes occur in the follicle. The hair stops growing and becomes detached from the base of the follicle forming a club hair which begins to break down, resulting in the follicle becoming shorter.

A small section of the outer root sheath remains in contact with the group of cells that formed the papilla. This period of breakdown or change lasts about three weeks.

As the inner root sheath breaks down, the hair remains in the follicle due to its shape. On average, 1% of follicles are held in the catagen stage. This stage also experiences a termination in pigment production.

 

Telogen Phaseresting or the shedding phase

The resting phase follows the catagen phase and normally lasts about 5-6 weeks. During this time the hair does not grow but stays attached to the follicle while the dermal papilla stays in a resting phase.

Approximately 10-15 percent of all hairs are in this phase at any one time.

After the telogen stage the cycle returns to anagen and the root germ begins to grow downwards and forms a new bulb around the dermal papilla. It is the lower end of the germ that forms the new bulb, producing a new hair. The upper part of the germ forms the new cells that lengthen the follicle below the club hair. The new hair may push the old hair out and occasionally there maybe two hairs in the same follicle.

Shedding is part of the normal process of the replacement of old hair with new. At any one time, around one in ten of the follicles on an individual’s head are in the shedding phase of the hair growth cycle.

 

 

Male Pattern Baldness

 

In androgenetic alopecia the terminal hair follicles reduce size both in length and diameter. The hair bulb moves upwards in the dermis yielding a small vellus hair follicle.

In many cases of androgenetic alopecia an inflammatory infiltrate can be seen around the affected hair follicles especially the upper hair follicles. Normal unaffected scalp hair follicles can also have mild inflammation but the inflammation in androgenetic alopecia is often much more intense with many more cells involved.

 

Hair length

 

How long anagen lasts is determined genetically, and varies between the sexes and from one person to another. It is the length of this stage that decides how long the hair will grow before it falls out. Anagen lasts between three and seven years in most people.

  • A hair grows at an average rate of about 1 centimetre a month.
  • People with long anagen phases can expect to grow hair down to the waist.
  • Each human head carries roughly 100,000 hair follicles.
  • Each follicle can grow many hairs over a lifetime: on average, each grows a new hair around twenty times.
  • Not all these follicles are actively growing hairs at any one time.
  • The growing and shedding of hair appears to occur randomly, it is however precisely controlled.
  • As people grow older the period of anagen shortens.
  • Plucking a hair from a follicle brings forward the next period of hair growth in that follicle.
  • With age, the number of follicles capable of growing hair declines naturally. The decline is especially noticeable on the top of the head.

 

What controls hair growth?

 

General health and nutritional factors are increasingly believed to be important factors for healthy hair growth. Malnutrition as a result of alcoholism, starvation or an anaemia has been proven to affect hair growth, leading to loss of hair in many sinstances.

Minerals and vitamins are particularly important for hair growth, for example, a lack of zinc in the diet is known to produce fine, sparse hairs and even loss of hair. Vitamin B, also known as panthenol, plays a vital role in hair growth by improving the physical properties, such as; elasticity, strength and glossy appearance of the hair shaft.

Androgens are the most important factors regulating hair growth and thickness of the hair shaft. Female hormones (oestrogens), which both sexes have in varying levels, slow down hair growth during the growing
period (anagen) but bizarrely extend this phase.

During pregnancy women’s oestrogen levels increase and many report a difference in their hair growth cycle.

The effects of androgens continue long after puberty with men reaching maturity in their thirties as hair production on the chest and facial areas peaks. The hair growth cycle tends to denote that nose and ear hair will peak in late middle life.

 

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