Hair Follicle Anatomy
  1. Structure
  2. Hair growth cycle
  3. How a hair follicle grows
  4. Problems
  5. Stimulating hair growth

The average person has around 100,000 hair follicles on their scalp, according to research by the American Academy of Dermatology (AAD). You’re born with all the follicles you’ll ever have, amounting to around 5 million across the whole body (on average), and each produces hair via a natural four-stage growth cycle.

However, injuries, medical issues, and genetic conditions can all disrupt this process. How? And what can you do about it?

In this post, we’ll explore the human hair follicle in detail, covering everything from the four stages of the growth cycle to hair restoration techniques.

A Closer Look at the Hair Follicle

The hair follicle is a long, narrow shaft in the skin’s outer layer (the epidermis). Each hair sprouts from a root at the base of the follicle, formed from protein cells. Roots receive nutrients from local blood vessels in the scalp, which are essential to keep hair healthy and strong. As new cells are created, hair rises further along the follicle until it gets to the surface of the skin.

The scalp’s sebaceous glands produce sebum, a natural oil, to moisturise the skin and hair. By mixing with fat molecules (known as lipids), sebum also forms a coating that protects the scalp against germs.

How Does the Hair Follicle Growth Cycle Work?

Follicles produce hair in 4 stages:

1. Anagen (The Growth Stage)

The anagen stage typically lasts for 3 to 7 years. Hairs continue to grow until they’re trimmed or fall out. Around 85% of hairs on a healthy scalp are in the anagen stage.

2. Catagen (The Transitional Stage)

In this stage, hair growth slows as the follicle shrinks and the strand separates from the base of the follicle.

3. Telogen (The Resting Stage)

This usually takes around 3 months to end, and about 15% of hairs on the scalp are in this stage. New hairs begin to grow in follicles during this stage.

Researchers found that hair follicles do more than just “rest” in the telogen stage: a wealth of cellular activity occurs, facilitating tissue regeneration and new hair growth. That’s why the telogen stage is vital to form healthy, strong hair.

4. Exogen (The Shedding Stage)

This is an extension of the telogen stage: hair falls out of the follicles, encouraged by shampooing and combing. It’s natural to lose between 50 and 100 hairs per day, according to the NHS. The exogen stage can last for 2 to 5 months.

Follicles pass through these 4 stages at different times: one follicle could be in the anagen stage while others are in the telogen stage.

How a Hair Follicle Grows Hair and Affects Your Hair Colour

Human hair grows at a rate of around half an inch per month, though this may depend on your general health, your hair type, and your age. Follicles are responsible for hair growth and its appearance: for example, a follicle’s shape determines the hair’s appearance — curly strands grow from oval follicles, and straight hairs grow from circular follicles.

Follicles also affect your hair colour. Your hair gains pigment from melanin, two types of which exist: eumelanin and pheomelanin. Genetics determine which type of melanin you have, and its amount:

  • A large amount of eumelanin causes black hair
  • A moderate amount leads to brown hair
  • A tiny amount causes blonde hair
  • Pheomelanin produces red hair

Melanin is found in follicular cells, but a hair follicle may become unable to produce melanin as you grow older. That’s why elderly people have white or grey hair.

Hairs will usually regrow after being pulled from a follicle, but a damaged follicle will be unable to generate new hair. Alopecia and other conditions affecting hair growth can prevent follicles from producing new strands entirely.

Common Hair Follicle Problems

Issues with follicle function can cause various hair growth conditions. It’s recommended that you visit a dermatologist if you believe you may have a problem with your hair growth, or you start losing more hair than usual. Here are some of the most common conditions:

Alopecia Areata

This autoimmune disease causes the immune system to attack follicles after misidentifying them as foreign cells. As a result, hair can start to fall out in clusters. This condition can develop into alopecia universalis — complete hair loss across the entire body.

At present, no cure for alopecia areata has been discovered yet. However, you may reduce your hair loss with topical treatments or steroid injections.

alopecia areata

Androgenic Alopecia

Androgenic alopecia (or male pattern baldness) affects approximately half of men aged 50 and over in the UK. It causes the hair growth cycle to slow down and become less efficient until follicles are unable to grow new hairs.

Telogen Effluvium

Telogen effluvium increases the number of hairs in the telogen stage from around 10% to 30% (if not more). Extreme stress usually alters the hair growth cycle and causes the hair to fall out in patches across the scalp. However, it may also spread to other areas of the body, such as the eyebrows and legs.

The following events could cause high stress:

  • Switching to a new medication
  • Undergoing surgery
  • Becoming ill
  • Experiencing physical trauma
  • Giving birth

This condition tends to be temporary, and it’s unlikely that you would need treatment. Still, you should consult a dermatologist if you believe you may have telogen effluvium so they can eliminate other reasons for your hair loss.

Folliculitis

Folliculitis can affect follicles on any area of the body, including the scalp, armpits, and legs. This inflammation creates a rash of tiny bumps, which can be white, yellow, or red. They may also be filled with pus. Soreness and itching in affected areas is common.

Staph infections typically trigger folliculitis. While this condition can fade without treatment, your doctor may be able to prescribe medication (such as oral or topical solutions).

Can You Stimulate a Hair Follicle to Grow New Hair?

You may wonder if you can stimulate a hair follicle to grow new hair if you have male pattern baldness or another condition causing hair loss.

Unfortunately, a damaged follicle is unable to grow new hair, and there’s no known way to change that (at present). However, researchers have found that stem cells may be able to revive damaged or destroyed follicles, though testing is required before this treatment can be administered to patients. Read Are Stem Cell Hair Transplants the Future of Hair Restoration? to learn more about stem cell treatments.

The most effective option for people experiencing hair loss today is hair transplantation: this allows specialists to transfer healthy follicles to areas of the scalp affected by shedding or baldness. Two types of transplant are available: Follicular Unit Transplantation (FUT) and Follicular Unit Extraction (FUE). The latter is the most popular, as it allows for natural regrowth without visible scarring (unlike FUT).