The Hair Growth Cycle
Hair grows in repeated cycles, which can be broken down into three phases: Anagen phase, Catagen phase and Telogen phase. Individual hairs pass through these phases independent of the neighbouring hairs. An average person will normally lose anywhere from 100 to 150 hairs every day during the telogen phase of the cycle. These fallen hairs will then re-grow several months later, once the follicle enters the anagen stage.
ANAGEN PHASE (growth phase)
The anagen phase is the growing phase of the cycle. Anywhere from 85 – 90% of all hairs are in this growing phase at any one time. The Anagen growth phase can vary from two to six years. Hairs in this stage will generally grow approximately 10cm per year.
CATAGEN PHASE (transitional phase)
At the end of the anagen phase, the hairs enter into a Catagen phase, which is considered the transitional or breakdown phase. During the catagen phase, which lasts a few weeks, the hair follicle shrinks to about 1/6 of the normal length. The root shrinks and the dermal papilla breaks away to rest below. The hair detaches from the root and moves closer to the surface of the scalp.
TELOGEN PHASE (resting phase)
This is the dormant or resting phase. The follicle continues to shrink during this phase. This phase could last several weeks. Approximately 10-15% of all hairs are in this phase at any one time. During this phase, the hair will be shed or be pushed out by new hairs in the anagen phase. In an individual not experiencing hair loss, the number of hairs on the scalp remains relatively consistent due to the balanced nature of this growth cycle.
Genetics, Hormones and Growth Cycle
In patients with Androgenetic Alopecia, when genetically susceptible hair follicles are consistently exposed to the DHT hormone in the scalp, the length of the anagen phase is reduced, while the length of the telogen phase increases.
With each new growth cycle the hair that does grow is thinner, shorter, weaker and more brittle, and doesn’t grow more than a few centimetres during the anagen phase. This process is known as miniaturization.
With each new cycle the telogen phase becomes longer and the hair spends more time in a resting state where it is prone to shedding. With each passing phase of growth, the hair shaft becomes smaller until eventually there is no growth, seen as baldness.
Hair loss does not occur at a consistent rate or all at once, but rather in cycles. People who are losing their hair experience alternating periods of slow and rapid hair loss and sometimes experience periods of stability. Many of the factors that cause the rate of loss to accelerate or decelerate are unknown.