Hair is formed deep in the skin, inside a tiny organ called a hair follicle. At the base of each follicle, rapidly dividing cells push upward, harden into a tough protein called keratin, and emerge as the visible strand you see on your head or body. The entire process is driven by a small cluster of cells that acts as a command center, telling the follicle when to grow, when to rest, and when to start over again.
Inside the Hair Follicle
A hair follicle is a tube-shaped structure that extends from the surface of your skin down into its deeper layers. The bottom of the follicle widens into a rounded area called the bulb, and nestled inside that bulb is the dermal papilla, a tiny mound of tissue packed with blood vessels. The dermal papilla is the control hub of the entire operation. It delivers nutrients from your bloodstream and sends chemical signals to the surrounding cells, telling them to multiply and start building hair.
Wrapped around the dermal papilla are rapidly dividing cells known as matrix cells. These are the actual construction workers. As they multiply, they get pushed upward and away from the blood supply. Without that nourishment, they begin to die, hardening into keratin as they go. By the time the cells reach the skin’s surface, they’re completely dead, compacted into the tough, flexible fiber you recognize as a strand of hair. Protein makes up roughly 65 to 95% of the hair’s total weight.
Scattered among the matrix cells are pigment-producing cells called melanocytes. As new hair cells form, melanocytes inject tiny packets of pigment into them. The type and amount of pigment transferred determines whether your hair appears black, brown, blonde, or red. When melanocytes slow down or stop producing pigment (typically with age), the incoming hair grows in gray or white.
Layers of a Hair Strand
A single strand of hair isn’t just one solid material. It has three distinct layers. At the center is the medulla, a soft inner core that’s sometimes absent in finer hairs. Surrounding it is the cortex, which makes up the bulk of the strand and gives hair its strength, elasticity, and color. On the outside is the cuticle, a single layer of overlapping cells that lies flat like shingles on a roof. The cuticle protects the cortex from damage. When those shingle-like cells get roughed up (from heat styling or chemical treatments, for example), hair looks frizzy and feels brittle.
While the hair strand is being shaped inside the follicle, it’s encased in an inner root sheath that acts like a mold, guiding the strand’s shape as it grows upward. This sheath eventually disintegrates about halfway up the follicle, releasing the fully formed hair to continue its journey to the surface.
The Four Phases of Hair Growth
Hair doesn’t grow continuously. Each follicle cycles through four phases independently of its neighbors, which is why you don’t shed all your hair at once.
- Anagen (growth): The active phase when matrix cells are dividing and producing new hair. On your scalp, this lasts two to eight years. Eyebrow follicles stay in anagen for only two to three months, which is why eyebrows stay short.
- Catagen (regression): A brief transition lasting about two weeks. The follicle shrinks and detaches from the dermal papilla, cutting off the hair’s nutrient supply. Cell division stops.
- Telogen (rest): The follicle sits dormant for two to three months. About 9% of your scalp hair is in this resting phase at any given time, compared to 40 to 50% of the hair on your torso.
- Exogen (shedding): The old hair loosens and falls out as a new hair begins growing beneath it. Losing 50 to 100 hairs a day during this phase is normal.
On average, scalp hair grows between 0.5 and 1.7 centimeters per month, or roughly 6 to 20 centimeters per year. The wide range reflects differences in genetics, age, and overall health.
How Stem Cells Restart the Cycle
Each new growth cycle depends on stem cells stored in a region of the follicle called the bulge, located about midway down the follicle where a tiny muscle (the arrector pili, the one that gives you goosebumps) attaches. These stem cells are the follicle’s long-term reserve. Without them, a follicle could never regenerate.
The restart process happens in two steps. First, a small group of cells just below the bulge, called the hair germ, wakes up. Signals from the dermal papilla, particularly growth factors like FGF7, stimulate these hair germ cells to begin dividing. Several days later, the bulge stem cells activate and take over, supplying the sustained cell production needed to build a full new hair. Think of the hair germ as the ignition and the bulge as the engine.
Vellus Hair vs. Terminal Hair
Not all hair on your body forms the same way. Vellus hair is the fine, nearly invisible fuzz covering most of your skin. It comes from small, shallow follicles that produce thin, short strands with little or no pigment. Terminal hair is the thicker, longer, pigmented hair on your scalp, eyebrows, and (after puberty) your underarms and face.
The follicles themselves can convert from one type to the other. During puberty, rising hormone levels transform many vellus follicles into terminal ones, which is why teenagers develop body hair and facial hair that wasn’t there before. The reverse also happens: in pattern baldness, hormones gradually shrink terminal follicles back into vellus-like ones, producing thinner and shorter strands with each cycle until the hair is barely visible. One key structural difference is that miniaturized follicles that were once terminal still retain an attached arrector pili muscle, while follicles that have always been vellus typically lack one.
What Influences Hair Growth
Because the dermal papilla controls the follicle through chemical signals, anything that alters those signals can change how hair forms. Hormones are the most powerful influence. Dihydrotestosterone (DHT), a byproduct of testosterone, binds to receptors in dermal papilla cells and shortens the growth phase while lengthening the resting phase. Over time, this produces thinner, weaker hairs. Importantly, DHT doesn’t act on the hair-building cells directly. It works indirectly by changing the signals the dermal papilla sends to the surrounding matrix cells and melanocytes.
Thyroid hormones also play a role. Both underactive and overactive thyroid function can cause diffuse hair thinning by prematurely pushing follicles from the growth phase into the resting phase. This type of shedding is usually reversible once thyroid levels stabilize.
Chronic stress raises cortisol levels, which disrupts the hair cycle’s timing and can trigger a condition called telogen effluvium, where a large number of follicles enter the resting phase simultaneously. The result is noticeable shedding that typically appears two to three months after the stressful event. Nutritional deficiencies, poor sleep, and chronic inflammation can produce similar effects. Even your body’s internal clock plays a part: circadian rhythm genes help regulate when follicles transition between resting and growth phases.
The dermal papilla needs a steady supply of blood to fuel the matrix cells that build hair. Conditions that reduce blood flow or nutrient delivery to the scalp can slow growth or weaken the hair that forms. This is one reason why a balanced diet with adequate protein and iron supports healthy hair, since the follicle is one of the fastest-dividing structures in the human body and demands a constant supply of raw materials.