Stages of the hair cycle

Stages of the hair cycle

Between starting to grow and falling out years later, each hair passes through three distinct stages. They have been given special names: anagen (the growing phase), catagen (the intermediate phase) and telogen (the shedding phase). We shall look at these three stages in turn.

Anagen (the growing phase)

The anagen phase of a new hair starts at the moment it begins to grow. You know that at the bottom of the hair bulb there is dermall papilla. The dermal papilla is cluster of specialized mesenchymal cells ( active and dividing cells above and around the dermal papilla). They begin to grow upward to form the hair shaft.That dermal papilla cells know beforehand what size and shape of hair follicle they should form and they also retain a memory of their hair cycle clock.
What is involved in the biochemical clock mechanism inside dermal papilla cells has been considered for a long time and a number of hypotheses have been suggested, but it is still unknown as to what determines the duration of anagen within dermal papilla cells.

Near 85% of the scalp hair is under the influence of anagen phase and approximately 15% are under the telogen phase. The anagen phase of scalp hair follicles typically lasts for 2-6 years. There are exceptions though; a few individuals experience anagen growth phases for much longer durations. The maximum hair length that scalp hair can grow to is mostly determined by the duration of the anagen phase. Approximately human hairs grow at a rate of roughly 1 (10-13 mm)centimetre a month. Throughout anagen also pigment (melanin) is made.

Catagen (the intermediate phase)
The anagen phase is followed by a short resting phase. This catagen phase lasts for between two and four weeks in the human scalp. No pigment is made during that time, and the dermal papilla stops producing hair. The base of the follicle moves upwards towards the surface of the skin.

Telogen (the shedding phase)
The telogen phase lasts for three or four months. This is the time at which a new hair begins to grow from the hair follicle. As it grows upwards the old hair will be shed naturally or may be pulled out, which happens easily and painlessly with telogen hairs. The new hair emerges from the same opening at the surface of the skin as the old one, and the hair cycle begins again.

Many people worry because they often find loose hairs on their clothes, or after shampooing, and they become terrified in case they may lose some or most of their hair. If you pull the hair lightly with your hand, you may well find that a few hairs do come out. These are almost certainly hairs that are ready to fall anyway - telogen hairs. You can confirm this by looking at the roots in a good light. A telogen follicle root is a little hard, white bead. Brushed-out hairs all have this kind of root.


A normal telogen hair with a hard 'club' end, seen under (left) a light microscope and (right) an electron microscope

On the other hand, the root of a hair that is pulled out is fat, soft to the touch and sticky - it will stick to your hand, because it still carries fluid from the follicle. And pulling it out hurts! This is an anagen hair, one which was not ready to be shed.


An anagen hair that has been plucked out: notice the soft, sticky tail