Hair Follicles: Function and Anatomy

Normally, hair follicles are responsible for hair growth, which occurs in a cycle of three different phases. These follicles also determine your hair type, such as straight or curly. Once damaged, hair follicles can stop producing hair and cause the hair growth cycle to slow down.

Anatomy of a Hair Follicle

A hair follicle is a sac-like structure located in the epidermis (outermost layer) of the skin. According to the American Academy of Dermatology, humans have an average of about 100,000 hair follicles on their scalp. Hair begins to grow at the bottom of the hair follicle. The root of the hair is made up of protein cells and is nourished by nearby blood vessels.

As more cells are produced, the hair protrudes from the skin and extends beyond the surface of the scalp. Meanwhile, sebaceous glands near the hair follicles produce oil, which helps keep hair and skin healthy and moisturized for growth.

The Hair Growth Cycle

Hair typically grows out of the follicle and develops in a specific cycle with three distinct phases:

• Anagen (Growth Phase): Hair begins to grow from the root. This phase typically lasts from three to seven years.
• Catagen (Transition Phase): Hair growth begins to slow down, and the hair follicle shrinks. Catagen typically lasts from two to four months.
• Telogen (Resting Phase): Old hairs fall out and are replaced by new hairs that begin to grow from the same follicle. This usually lasts from three to four months.

Recent studies have shown that hair follicles aren't simply "resting" during the telogen phase. A lot of cellular activity occurs in this phase to help regenerate tissues and facilitate more hair growth. In other words, the telogen phase plays a crucial role in forming healthy hair.

Each hair follicle goes through different phases of the growth cycle. Some follicles may be in the growth phase, while others are in the resting phase. Some hairs may be growing, while others are falling out.

Researchers say that the average person loses about 100 hairs each day. Around 90% of your hair follicles are likely in the anagen phase at any given time.

The Role of Hair Follicles

On average, your hair grows about half an inch per month. Your hair growth rate can be affected by age, hair type, and overall health.

Hair follicles are not only responsible for the rate of hair growth, but they also significantly influence the appearance of hair. The shape of the hair follicle determines how curly your hair is. Round follicles produce straight hair, while oval follicles produce curlier hair.

Hair follicles also play a part in determining a person's hair color. Like skin, our hair gets its pigment from melanin. Two main types of melanin determine human hair color: eumelanin and pheomelanin.

Genes determine whether you have eumelanin or pheomelanin, as well as the amount of pigment you have. Abundant eumelanin results in black hair, a moderate amount results in brown hair, and little eumelanin results in blonde hair. Pheomelanin produces red hair.

Both types of melanin are stored in hair follicle cells, which then determine hair color. At some point, hair follicles can lose their ability to produce melanin pigments (especially in older adults), leading to the growth of gray or white hair.

If hair is pulled out of the follicle, it can grow back. However, if the follicle is damaged, it can stop producing hair. In some conditions, such as alopecia, hair follicles can stop producing hair altogether.

Hair Follicle Problems

Several hair conditions can stem from problems with hair follicles. If you experience any of the following, or have unexplained symptoms (like hair loss), consult a dermatologist.

1. Androgenetic Alopecia

Also known as male pattern baldness, this condition affects the growth cycle of hair follicles on the scalp. The hair growth cycle slows down, weakens, and eventually stops, preventing follicles from producing new hair. According to the U.S. National Library of Medicine, about 50 million men and 30 million women are affected by androgenetic alopecia.

2. Alopecia Areata

This is an autoimmune disease where the immune system mistakenly attacks hair follicles, causing hair to fall out in patches. It can even lead to alopecia universalis, where all body hair is lost. There’s no cure for alopecia areata, but steroid injections or topical treatments can help slow hair loss.

3. Folliculitis 

This is an inflammation of the hair follicles. It can occur anywhere on the body where hair grows. Folliculitis often looks like a rash with small pimples or bumps. It can be itchy and painful. The primary cause is often a staphylococcal infection. While it can clear up without treatment, a doctor can diagnose and prescribe medication to help manage the condition.

4. Telogen Effluvium (TE)

This is a common type of temporary hair loss. A stressful event can cause hair follicles to enter the telogen phase prematurely, leading to thinning hair and shedding. Hair typically falls out in patches on the scalp, but in severe cases, it can fall out elsewhere on the body. TE is temporary and doesn't require treatment, but it’s important to speak with a dermatologist to rule out other causes. 

Can Hair Regrow After Falling Out?

Can we stimulate hair follicles to regrow hair in conditions like alopecia or baldness? If a hair follicle is damaged, you cannot reactivate it. Researchers are exploring new methods to reactivate dead or damaged hair follicles, but these treatments haven't been tested on humans and aren't FDA approved.

The first step to regaining your hair is finding the right approach. Start with our personalized quiz for MEN and personalized quiz for WOMEN.