To understand the mechanisms behind MPHL it is important to understand the hair growth cycle.
Hair is lost and replaced cyclically. Follicles undergo corresponding cyclic phases of growth, involution, resting and regeneration.
The hair growth cycle consists of three distinct stages – anagen, catagen and telogen phases.
The growth phase, or anagen phase, lasts an average of 3-5 years, so a full-length hair averages 18 to 30 inches. The anagen phase is generally longer in Asians, and can last as much as 7 years with hair being able to grow to 1 metre.
Hair growth remains fairly constant at around 1cm per month.
At the end of the anagen phase, your hair enters the catagen phase. A short transitional phase that lasts approximately 10 days.
Lastly, your hair enters the telogen phase, a resting phase when your hair is released and falls out. The follicle then remains inactive for 3 months and the whole process is repeated. Each hair follicle is independent and goes through the growth cycle at different times, otherwise all your hair would fall out at once. Instead, you only shed a certain number of hairs a day – up to 80 hairs on a healthy head of hair.
In MPHL, the duration of the anagen (growth) phase decreases with each cycle, whilst the length of the telogen (resting) phase remains constant or is prolonged.
Balding men often describe periods of excessive hair shedding most noticeable whilst combing or washing, this is due to the relative increase in numbers of follicles in the telogen phase being released. As the hair growth rate remains relatively constant for the duration of anagen phase, the length of this phase will determine the hair length. Thus, with each successively foreshortened hair cycle, the length of each hair shaft is reduced.
Ultimately, the anagen phase becomes so short that the growing hair fails to achieve sufficient length to reach the surface of the skin, leaving an empty follicular pore.