What Controls the Hair Cycle?
Posted by Fran Dubsky

Unlike any other organ in the body, hair follicles go through three distinct stages, where the follicle cycles repeatedly to maintain a continuum of cell regression and regrowth throughout ones’ life. While there are over 100,000 hairs on a normal scalp, each hair is individually controlled by your genes to enter each phase at different times so that the overall appearance of your hair does not change regardless of the cycle your hair is in. The important controller of hair growth in a normal cycle is the signaling mechanisms in your genes that provide communication between the dermal papilla and the overlying epidermal basement membrane as to which cycle the hair enters into.

 Below is a pictorial representation of these cycles.

Hair Cycle

The growth phase of this cycle is the Anagen Phase where the hair regenerates and grows new hairs. This phase of hair growth is controlled by your genes as far as when hair enters this cycle and the health of the hair that grows in. Typically, in a normal cycle of hair growth, the anagen phase lasts 2 to 8 years unlike other hairs on your body that have a shorter phase length.

 However, in androgenetic alopecia (or pattern baldness), the cycle is interrupted and the anagen hairs do not mature into terminal hairs, but instead become miniaturized, do not grow, and eventually enter and remain in the telogen phase and become vellus hairs.

The next normal cycle is the Categen Phase which is when the hair stops growing and the follicle begins to regress. What controls this action are your genes which program the regression of the follicle which destroys most of the epithelial cells in the follicle, but leaves the dermal papilla intact as shown below. When hair leaves the Catagen Phase, the lower part of the hair follicle starts to deteriorate and the dermal papilla (which stays intact) starts to move upward. When hair enters the Anagen Phase, it is hyposthesized that the dermal papilla sends signals to the epidermal basement membrane to grow and that starts the regrowth of the hair and entry into the this phase.

Biological view of hair follicle

After the catagen phase, the hair enters the Telogen Phase, which is when the hair follicle rests and the hair falls out. This fall out of each individual hair is controlled by the genes and since hairs enter the Telogen Phase at different times, hair loss is usually not noticable. This phase usually lasts about 3 months and the whole cycle repeats itself when androgenetic alopecia is not present. If you have androgenetic alopecia, then hairs tend to stay in the telogen phase and hair takes on a wispy unpigmented appearance refered to as vellus hairs. Hair at this stage in androgenetic alopecia does not regenerate and can lead to follicle death which is irreversible.

The key to treatment of genetic diseases of the hair is to restore the normal cycle of growth in the hair follicle. Some drugs proven effective for hair loss act on signally pathways to stimulate the hairs to enter the Anagen phase. On the other hand, it is hypothesized that the HairMax LaserComb acts as an Anagen inductor through the process of photo-biostimulation. This means that the laser energy of the HairMax causes cell proliferation in the epithelia of the hair follicle marked by thicker hair growth and an increase in shedding of Catagen hairs at the onset of treatment and stimulation of entry into the Anagen or growth phase of the cycle. Since the dermal papilla consists of certain fibroblasts and specialized matrix cells at the base of the hair follicle, the laser light of the HairMax apparently interacts with these cells causing cell proliferation. 

In conclusion, the cycles of hair growth are a complex mechanism that is not completely understood as to how they are controlled. However, research continues in an attempt to understand why the signaling system of the normal hair follicle is interrupted. Other research is focused on the role that photo-biostimulation plays in devices such as the HairMax LaserComb in the stimulation of hair regrowth. In the years to come, there will be a greater understanding of the reasons for hair loss and this will lead to the introduction of many other efficacious treatments including more widespread use of laser energy to treat diseases of the hair.

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