Hair Growth Cycle

 

Understanding the basics of  the hair growth cycle can be useful to anyone experiencing hair loss issues, as it will help explain why a particular treatment, whether it be a shampoo for hair loss or a more intensive treatment, works the way it does.

There are 3 main phases of hair growth in addition to follicle formation (follicular morphogenesis) and shedding (exogen). The hair growth cycle takes approximately 4 years to complete on the scalp; from new hair formation by the follicle cells to the hair falling out. The cycle length differs depending on body location, and genetic predisposition, follicle shape and hair color can affect phase lengths.

Hair Growth Cycle

Hair Growth Cycle

With regards to the scalp, the hair growth cycle has the following characteristics:

  • Anagen Phase is the growth phase and accounts for approximately 90% of hair follicles.

    The growth period is genetically determined and lasts approximately 2-3+ years. The hair grows at a rate of about 1/2″ (1cm) a month or 6″ (15cm) a year.

  • Catagen Phase is the regressing phase and accounts for approximately 1-3% of hair follicles.

    This phases lasts around 2-4 weeks and involves follicle shrinkage and detachment from the blood supply as hair converts to club hair.

  • Telogen Phase is the resting phase and accounts for approximately 10-14% of hair follicles.

    The resting phase lasts approximately 2-4 months and normally 50 – 100 club hairs a day are shed from the scalp.

DHT and Hair Loss

Although follicles do sometimes rest and temporarily fail to produce a new hair, this can become a permanent feature, such that no further hair will be produced in that location. Male and female pattern hair loss is associated with androgens (Testosterone, DHT) and characterized by shorter periods of growth, and smaller, finer and lighter hair. However, scalp hair growth is not androgen dependent.

The enzyme 5-alpha-reductase converts Testosterone into the hormone DHT (Dihydrotestosterone) in various parts of the body including the adrenal glands, prostate, testes and of particular interest regarding hair loss, the hair follicles. In someone who is a genetically susceptible individual to Androgenic alopecia, DHT will have the following effects on genetically predisposed hair follicles:

  • Shrinkage (Miniaturization)

  • Inhibited growth, and ultimately

  • Follicle death.

Androgenic alopecia is the most common cause of hair loss and can affect both men (male pattern baldness or MPB) and women (female pattern baldness or FPB) but rarely causes total baldness. Although DHT is the main factor of male hair loss (male pattern baldness) it is just one of several possible causes related to female hair loss.

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