The Hair Follicle: How It Works & What Makes It Fail

A hair follicle is a tube-like structure in your skin that surrounds the root and strand of a hair and grows it in cycles. A follicle hurt by an injury can repair itself, but scarring alopecia causes permanent hair loss.

This guide explains what a follicle is made of, how its growth cycle works, why follicles shrink in pattern hair loss, and which kinds of damage can reverse. It also covers what a follicle does in a hair transplant and answers the questions people ask most.

Hair does not grow from the visible strand. It grows from the follicle underneath the skin, and what happens there decides how thick, how long and how permanent your hair is.

What a Hair Follicle Is

Cleveland Clinic describes a hair follicle as a tube-like structure (pore) that surrounds the root and strand of a hair. It sits in the top two layers of skin, and you cannot see it without a microscope. Its own comparison is a sock: the follicle is the sock, and the hair is the foot inside it.

A follicle does more than grow hair. According to Cleveland Clinic, it also:

• grows each hair on your head and body
• helps repair the skin after a wound
• helps form new blood vessels and neurons

It is also one of a few structures in the body that can stop working and then start again, which is the reason hair can fall out and later return.

Hair Follicle Anatomy, Layer by Layer

A medical review of hair anatomy in Medscape divides the follicle into three regions, from the bottom of the follicle to the skin surface.

Three parts deserve a closer look:

• Dermal papilla. A cluster of cells at the base that Medscape calls required for hair follicle growth in each cycle.
• Matrix. Rapidly dividing cells around the papilla that give rise to the hair shaft and its pigment.
• Bulge. The area marked by the muscle attachment. It holds stem cells that are essential for regenerating the follicle and healing wounds.

The hair strand itself is wrapped in sheath layers. The outer root sheath is continuous with the skin surface, and the inner root sheath coats and supports the shaft up to the isthmus.

The bulge matters because it is where the follicle renews itself. Damage that reaches the stem cells there is what separates a follicle that can recover from one that cannot. That is the logic behind the reversible and permanent sections below.

The Hair Growth Cycle in Brief

Every follicle cycles through growth, a short transition and rest. Cleveland Clinic puts the growing phase (anagen) at two to seven years, with hair lengthening about 1 cm a month. The transition phase (catagen) lasts about two weeks, and the resting phase (telogen) up to four months.

Medscape gives scalp figures that are slightly different: anagen about 3 to 4 years, catagen about 2 to 3 weeks and telogen about 3 months. It also says that about 84% of scalp hairs are in anagen, 1% to 2% in catagen and 10% to 15% in telogen at any moment. Because follicles cycle out of step with each other, you lose a few hairs every day without going bald. The NHS says it is normal to lose between 50 and 100 hairs a day.

For the full cycle, the timings and what disturbs it, see our guide to the hair growth cycle.

How Many Hair Follicles Do You Have?

Cleveland Clinic says you are born with over 5 million hair follicles in your body and over one million on your head. That is a count of follicles, and each one grows hair through the cycle described above. For a closer look at how many strands a typical head carries, see how many hairs are on a head.

Why Hair Follicles Shrink

The most common follicle problem is not death but shrinking. Medscape explains that in people who are predisposed to hair loss, terminal hairs on the adult scalp can undergo involutional miniaturization and become vellus, the fine, short, pale hair.

MedlinePlus links pattern hair loss in men to the hormone DHT. It states that too much stimulation of hair follicles by androgens may lead to a shorter growth period, resulting in shorter and thinner strands of hair. The growth of new hair to replace shed strands is also delayed. In women, androgens play a role too, but MedlinePlus says other unknown factors are involved.

Miniaturisation is the hallmark of male pattern hair loss, female pattern hair loss and alopecia areata, according to a 2012 study in the International Journal of Trichology. The difference is the outcome. Hair in alopecia areata can regrow fully, while pattern hair loss is either irreversible or shows only partial regrowth. The authors suggest the cause may lie in the muscle attachment, and they present it as a model, not a settled fact.

For the hormone side, read DHT and hair loss. For the typical stages and patterns, see male pattern baldness.

Reversible vs Permanent Hair Follicle Damage

Whether a follicle can recover depends on what hurt it. The table below sums up what the sources say.

What Damages Hair Follicles

Cleveland Clinic lists the conditions that most often affect follicles: alopecia areata, baldness, folliculitis, hidradenitis suppurativa, stress, telogen effluvium, and whiteheads, blackheads or a dilated pore of Winer.

Pulling and Tension

You cannot pull out a follicle, but you can pull out the hair, and a bulb at the end of the strand is the root. Repeated tension is another matter. A 2018 review in Clinical, Cosmetic and Investigational Dermatology says traction alopecia affects one third of women of African descent who wear traumatic hairstyles for a long time. Risk rises with the amount of pulling, how long it lasts and chemical relaxers. Tight buns, ponytails, weaves, extensions and tight braids carry the highest risk. Our guide to traction alopecia covers early signs and what to change.

Infection and Inflammation

Folliculitis is inflammation of the follicle. Cleveland Clinic lists pimples, fluid-filled bumps, swelling, an itchy rash, and a wound that does not heal, has a yellow crust or leaks white to yellow fluid as signs of infection.

A doctor examines the skin and can run a blood test, a biopsy or a pull-and-tug test. Cleveland Clinic lists antibiotics among the treatments for infections, and the choice and the course are for a doctor to decide.

Stress and Shedding

Stress and telogen effluvium are both on Cleveland Clinic's list of follicle conditions. In telogen effluvium, more hairs than usual move into the resting phase and shed together. Read more in our guide to telogen effluvium.

Scarring

Frequent injury to skin and follicles can produce scars that make growing hair difficult, and cicatricial alopecias, the group that destroys follicles, always result in permanent hair loss and progress rapidly, according to a 2021 review in the Indian Journal of Plastic Surgery.

Can Hair Follicles Grow Back?

It depends on whether the follicle is alive and has not scarred. Cleveland Clinic says that when you damage hair follicles in an injury, they can repair themselves and your hair will grow back, with new growth taking up to four years depending on the severity.

A follicle that has shrunk but is still alive is a different case, and so is a follicle in a scar. For the signs and the options, read about dead hair follicles.

How to Keep Hair Follicles Healthy

Cleveland Clinic recommends these steps to keep follicles healthy and strong:

• avoid pulling your hair out
• eat a healthy, well-balanced diet
• reduce stress
• wear a helmet when your head is at risk of injury
• wear sunscreen
• clean wounds so they heal without infection

These steps protect the follicles you have. They are not a treatment for pattern hair loss, which MedlinePlus links to androgens acting on the follicle.

Hair Follicles and Hair Transplants

When pattern hair loss has progressed, the question becomes whether enough healthy follicles remain. Cleveland Clinic lists hair transplant among the treatments for follicle conditions. In a FUE hair transplant, follicles are moved from one area of the scalp to another, so the quality and number of your own follicles shape the plan.

The cause of the loss comes first, so the medical team looks at your photos before discussing any treatment. See what causes hair loss if you are not yet sure what is behind yours.

Frequently Asked Questions

Worried About Your Hair Follicles?

If your hair is thinning and you are not sure whether the follicles are shrinking, damaged or scarred, clear photos can show the pattern of loss. Send them for a free hair analysis, and the medical team will tell you what they see.

Medical disclaimer: This article is for informational purposes only and is not medical advice. Individual results and recovery vary from person to person. Always follow the written aftercare plan from your own medical team.