The most important part of the hair follicle is the dermal papilla, which is responsible for the growth cycle of hair and the formation of new hair follicles.
It receives nourishment from the surrounding blood supply. An interesting characteristic of hair is that, contrary to the commonly held belief that it grows as individual strands, it actually emerges from the scalp in groups of 1-4 hairs. The reason for this is that hair follicles are not solitary structures, but are arranged in the skin in naturally occurring groups called “follicular units”.
Each individual hair that is visible on the human head is called a hair shaft. Millions of dollars are spent styling, cutting and coloring these hairs despite the fact that they are considered “dead.” They don’t have a blood supply, nerves or muscles, so when it’s cut it doesn’t hurt or bleed. Each hair shaft grows from the hair follicle that lies beneath the scalp, and is considered the only living portion of the hair.
On average, about 110,000 to 150,000 hair follicles exist on a normal scalp.
50-100 of those hairs are lost on any given day, while new hairs form in the same place as the old ones. That is why you may notice hairs on your pillow, back of your sweater, brush and even in the shower drain. The way our hair grows is in an asynchronous pattern consisting of 3 distinct stages. If our hair grew in a synchronous cycle, all our hairs would be shed at the same time, leading us to be completely bald, while waiting for the hair to return. As a person ages, the timing of these growth cycles change, where the growth (anagen) stage becomes shorter and the resting (telogen) stage lasts longer.
The first phase of hair growth is the anagen phase. This is the phase where the hair is actively growing. Around 85-90% of hair is in this phase at any point in time and can last anywhere from 2 to 6 years. Typically, hair growth for most individuals is close to half an inch each month. People who have difficulty growing their hair long have naturally shorter growth periods.
At the end of the anagen phase, hair enters what is known as the catagen phase. This is a transitional phase that can last for about 2 weeks. It occurs when the bulb detaches from the normal blood supply, causing the hair shaft to be pushed up.
The telogen phase is the final phase in the hair growth cycle lasting 3 to 4 months, and is commonly known as the “resting” phase where there is no additional growth. At the end of this phase, the hair follicle re-enters the anagen phase and a new hair begins to form. If the old hair has not already been shed, the new hair pushes out the old one. Approximately 10-15% of hair is in this phase at once.