Quick definition: A hair follicle is a small, tube-shaped structure in the skin that produces and anchors a single hair. At its base, a bulb of rapidly dividing cells wraps around the dermal papilla, which supplies blood and nutrients. Each follicle cycles through growth, transition, and rest throughout your life.
What is a hair follicle?
The word "follicle" means a small sac or cavity, and in skin it almost always refers to the hair follicle. Wikipedia's hair follicle entry describes it as an organ found in mammalian skin, which is a useful way to think about it: each follicle is a miniature, self-renewing hair factory with its own blood supply, oil gland, muscle, and stem cells.
"a tube-like structure (pore) that surrounds the root and strand of a hair" — Cleveland Clinic
Hair follicles cover most of your skin. According to Cleveland Clinic, you're born with more than 5 million hair follicles on your body and over one million on your head, so the ones you have are worth looking after.
Anatomy of a hair follicle
A follicle starts at the surface of the epidermis and extends down into the dermis. Its key parts:
Dermal papilla. A cluster of specialized cells at the very base. The NCBI StatPearls chapter on hair physiology describes it as containing nerve fibers and a loop of capillary, and notes that it sends the signals that set the size and color of the hair shaft.
Hair bulb and matrix. The bulb surrounds the papilla. Matrix cells here divide rapidly and become the hair shaft as they're pushed upward.
Root sheaths. Inner and outer layers that line the follicle and guide the hair as it grows.
Bulge. A region in the outer root sheath near where the arrector pili muscle attaches. This is where the follicle's stem cells live.
Sebaceous gland. Attached to the follicle, it produces sebum, the oil that conditions hair and skin.
Arrector pili muscle. A tiny muscle that contracts to make hair stand up ("goosebumps").
"stem cells are thought to reside in a distinct bulge area" — StatPearls: Physiology, Hair
That stem cell reservoir is what allows a follicle to rebuild its lower portion again and again across many hair cycles.
How hair follicles cycle
Hair follicles don't grow hair continuously. Each one repeats a cycle on its own schedule:
Anagen (growth): the follicle is fully formed and building hair. On the scalp this lasts two to seven years.
Catagen (transition): growth stops and the lower follicle shrinks over about two to three weeks.
Telogen (rest): the follicle rests for about three months before the old hair sheds and a new anagen phase begins.
Wikipedia notes that up to 85% of scalp follicles are normally in anagen, 10–14% in telogen, and 1–2% in catagen. Because the follicles are out of sync, you shed a little every day rather than all at once.
Dormant hair follicles: what the term means
"Dormant hair follicles" is a popular phrase, but it isn't a single medical diagnosis. It usually refers to one of three situations:
Resting follicles. Follicles in telogen are temporarily inactive. This is normal and they return to growth.
Miniaturized follicles. In pattern hair loss, follicles gradually shrink and produce finer, shorter hairs over successive cycles.
Damaged follicles. Some conditions cause inflammation or scarring that can permanently destroy follicles.
Cleveland Clinic explains that the hair follicle is one of the few structures in the body that can stop functioning and begin functioning again, and that follicles can often repair themselves after injury. Whether a particular follicle can recover depends heavily on the cause, which is why a dermatologist's diagnosis matters. If you notice an area where hair no longer grows, Cleveland Clinic recommends talking to your healthcare provider.
Hair follicle stimulation: what the research says
"Hair follicle stimulation" is a term used across haircare marketing, so it helps to separate research from claims. Medical treatments for hair loss are prescribed and monitored by doctors. Light-based approaches have also been studied: a 2016 evidence-based review in Lasers in Medical Science by Zarei and colleagues examined 21 studies and concluded that FDA-cleared low-level laser therapy devices were safe and effective for people with pattern hair loss who hadn't responded to or tolerated standard treatments. Those findings apply to the devices and populations studied. Learn more in our photobiomodulation entry.
Hair follicles in beauty and personal care
Much of haircare is really follicle care. Practical ways the beauty industry works with follicles:
Scalp cleansing. Removing excess sebum, product buildup, and flakes keeps follicle openings clear and the scalp comfortable.
Gentle styling. Constant tension from tight ponytails, braids, or extensions can strain follicles over time. Varying styles and loosening tension helps.
Heat and chemical care. Harsh chemical processing mainly damages the hair shaft, but irritation at the scalp can affect the skin around follicles.
Nutrition. Matrix cells divide quickly and rely on adequate protein, iron, and micronutrients such as zinc.
At-home light devices. Combs, caps, and helmets use red and other wavelengths as part of scalp-care routines.
Professional care. Dermatologists use trichoscopy and other tools to assess follicle health and diagnose conditions like folliculitis, alopecia areata, and telogen effluvium.
How Reseen fits in
Reseen's scalp tools are designed around making follicle-friendly scalp care easy to do consistently at home. Weave is a handheld light comb with 39 × 660nm red VIPE™ precision light emitters, made for a healthier-looking scalp and fuller-looking hair.
Halo, coming soon, is a wireless magnetic cap liner with 256 precision light points (128 red at 660nm and 128 blue at 460nm) and 10- to 15-minute modes. Both are cosmetic devices that support scalp health; they are not intended to treat hair loss.
Frequently asked questions
What does a hair follicle do?
A hair follicle produces and anchors a single hair. Cells in the bulb divide to build the hair shaft, the dermal papilla supplies blood and signals, and the attached sebaceous gland keeps hair and skin conditioned with sebum.
How many hair follicles are on the scalp?
According to Cleveland Clinic, people are born with over one million hair follicles on the head and more than 5 million across the body.
Can dormant hair follicles be reactivated?
It depends on why they're inactive. Resting (telogen) follicles restart naturally, while scarred follicles generally can't. Because causes vary, a board-certified dermatologist should evaluate areas where hair has stopped growing.
What is the difference between a follicle and a pore?
"Pore" is an everyday word for the opening of a follicle at the skin's surface. The follicle is the whole structure below that opening, reaching down into the dermis.
Browse every term in the Reseen glossary.
Sources
Hair Follicle: Function, Structure & Associated Conditions — Cleveland Clinic, accessed 2026
Physiology, Hair — Hoover E, Alhajj M, Flores JL; StatPearls, NCBI Bookshelf, 2023
Hair follicle — Wikipedia, accessed 2026
Low level laser therapy and hair regrowth: an evidence-based review — Zarei M, et al.; Lasers in Medical Science, 2016
This article is for general education and is not medical advice. Reseen devices are cosmetic devices and are not intended to diagnose, treat, cure, or prevent any disease. If you have a skin or scalp condition, talk to a board-certified dermatologist.