How Does Armpit Hair Know When to Stop Growing?

Armpit hair stops growing because the hair follicles have a genetically predetermined growth cycle, which is regulated by chemical signals.

Here's a more detailed explanation:

• Hair Growth Cycle: Hair growth occurs in cycles with three main phases:

  • Anagen (Growth Phase): This is the active growth phase where hair cells rapidly divide, and the hair shaft grows longer. The length of the anagen phase varies depending on the body location. Armpit hair has a relatively short anagen phase (weeks to months), compared to scalp hair (years).
  • Catagen (Transition Phase): A short transitional phase where hair growth slows down, and the hair follicle shrinks.
  • Telogen (Resting Phase): The hair follicle is dormant, and the hair eventually sheds.

• Genetic Predetermination: The length of the anagen phase, and therefore the ultimate length of the hair, is genetically determined for each hair follicle. This means your genes dictate how long armpit hair will actively grow.

• Chemical Signaling: Hormones and other chemical signals influence the hair growth cycle. These signals can affect the duration of the anagen phase. The brain plays a role in this signaling.

• Follicle Signal Termination: The reference mentions the brain stopping signals to hair follicles, ultimately halting hair growth. More specifically, signals within the hair follicle itself, and from surrounding cells, instruct the follicle when to transition to the catagen phase. This could involve growth factors, proteins, and other molecules that trigger the switch. When these internal signals trigger the catagen phase, the hair stops growing.

• Shedding: After the telogen phase, the hair sheds (exogen phase), and a new hair growth cycle begins.

In summary, armpit hair doesn't "know" when to stop growing in a conscious sense. Instead, its growth is controlled by a genetically programmed cycle, influenced by hormonal and chemical signals within the hair follicle, which ultimately determines the length of the anagen phase. When this phase ends, the hair stops growing and eventually sheds.