How hair grows — and why it's dead by the time you see it

Hair seems simple because we only see the visible strand, but its growth is surprisingly complex. Hidden beneath the skin, a tiny living system follows a steady schedule, guided by chemical signals, to build each strand. It draws on the body for nutrients, colour and strength, anchors the hair in place, responds to water, and constantly renews itself.

Why does pulling a hair hurt, but cutting it doesn't?

At the surface, hair has two parts: the shaft, the part you see above the scalp, and the root, the part below it. The shaft is made of hardened cells that were once alive but gradually transformed into a tough, dead fibre. It has no blood, no nerves and no living cells. The root, by contrast, sits among living tissue and is anchored to it.

Cutting hair affects only the dead shaft. Pulling a hair is different: the force is transmitted to the structures holding it in place, the nearby nerves and the surrounding tissue. That is why pulling can be uncomfortable or painful even though cutting is not.

Everything that decides what a shaft will be — thick or fine, straight or curved, coloured or grey — is settled lower down, at the root, by the follicle.

The follicle, in three zones

A follicle is a small tube of skin that folds down into the scalp and wraps around the hair root — a little like a pen cap gripping a pen. It anchors the root in place, receives signals from the body, and helps decide when that hair grows, rests and sheds. It's easiest to picture it top to bottom, in three zones, each with one main job.

Upper — the oil supply. A small sebaceous gland opens into the follicle and releases sebum — the scalp's natural oil — which coats the strand near the scalp, smooths it and gives it a little natural water resistance. Without it, hair tends to feel drier and more brittle^[13].

Sebum doesn't travel the whole strand, though — it spreads down from the scalp slowly, and only so far. A L'Oréal study of 123 Chinese men measured how far the oil had crept after shampooing^[10]: up to about 3.3 inches from the roots within 24 hours, and up to about 5.7 inches by 48 hours. How far it gets depends mainly on whether the scalp is oily or dry, with hair thickness, density and grooming likely shaping the rest. On long hair, that means the ends rarely receive the scalp's own oil — they're the oldest, most exposed part of the strand, and tend to be the driest.

Middle — the warehouse. The bulge, a quieter region that stores the follicle's stem cells — reserve cells that help build a new hair strand each cycle. We'll come back to why this matters.

Lower — the factory. The bulb, where a new hair strand is actually built, and just above it is the area where that soft strand is hardened and anchored. This is where growth happens — so it's where we go next.

Inside the bulb: where a hair is built

At the base of the bulb, two structures work as a pair. The first is a cluster of living cells that divide quickly, each new cell shoving the older one upward. These are called the matrix cells, and they're among the fastest-dividing in the whole body^[1]. As they divide, the cells stack up into a column and reach the suprabulbar region just above the bulb (supra simply means above), where they fill with keratin proteins and harden — a process called keratinization — and die. So a finished strand is really a column of cells frozen at the moment they set. "Growth" is mostly this: the matrix pushing finished, dead cells out of the scalp, at roughly a centimetre a month for most people.

The second is the dermal papilla — a small cluster of cells at the base, often called the follicle's control centre. Those fast-dividing matrix cells need feeding and direction, and that's its job. It does two things. It carries a loop of tiny blood vessels up into the bulb, so the matrix gets oxygen, sugar, the amino acids it needs to build keratin, and minerals such as iron and zinc — straight from the bloodstream^[1]. And it issues the instructions: chemical signals telling the matrix when to divide and when to stop. Because it reads the blood, the papilla is also where the rest of the body reaches the follicle — hormones, and the after-effects of illness, stress or poor nutrition, all arrive here.

One way to hold this together is to picture the base of the bulb as an automated production line, run from a single control booth. The matrix cells are the robots on the line; the papilla is the engineer in the booth — it programs them (run, slow, rest) and feeds them power and raw materials through the blood. The robots don't just build the strand, they lock into it and become it. When a whole new line has to be built from scratch, extra hands are called in from the warehouse — the bulge. At the far end, the finished strand gets its packaging — the sebum — to smooth and protect it on the way out.

What grips a hair — and where does its strength come from?

In the suprabulbar region, a close-fitting layer of tissue surrounds the new hair. It hardens before the still-soft strand inside it, helping to hold the strand steady as it is shaped and pushed upward. Think of it as a garden clip holding a young stem against a stake while it firms up.^[1]

Farther up, in the middle zone of the follicle, this supporting layer has finished its job. It breaks down and releases the shaft, allowing the hair to continue moving towards the surface. The strand is therefore held most securely deep in the follicle, while that temporary support gradually disappears higher up.

Once the new hair strand has hardened into a finished fibre, its material structure is the next part of the story. Inside a strand explains what gives the finished fibre its strength, flexibility and vulnerability to wear. The strand can still continue to lengthen as the follicle adds new cells.

The rhythm: growing, resting, shedding

A follicle builds a strand like that — and then, in time, lets it go. No follicle grows forever; each one moves through a repeating cycle. For everyday purposes, three phases are worth knowing — the table is the quick reference; the detail follows.

Anagen is the long build phase, when the matrix is dividing and laying down fibre. An easy way to remember it is: A = active, Gen = generation. It's the only phase in which a hair actually gets longer, so how long anagen lasts is what sets a hair's maximum length: the years-long anagen of the scalp is why head hair can reach the waist, while the weeks-long anagen of an eyelash keeps it short^[3].

Telogen is the rest phase. The matrix has switched off, and the finished strand is simply held in the follicle, no longer growing — anchored but idle. It's a genuine pause, not a slow decline: the hair waits, fully formed, until the next phase lets it go^[3].

Exogen is the release. The resting hair finally lets go — usually nudged out as a new anagen hair starts growing up beneath it. This is why shedding and regrowth run together, and why losing 50–100 hairs a day is normal rather than alarming: the hairs in the brush are exogen hairs, the system working, not failing.

The cycle is steady but not fixed — it answers to the body. A physiological shock such as a serious illness, childbirth, crash dieting or sustained stress can tip many follicles into the resting phase at once, so they shed together a few months later (telogen effluvium^[3]). That, and follicles making a slightly finer hair each cycle, is the backdrop to ordinary hair loss — which the companion piece, Hair Loss Is Not One Thing, takes up in detail. After exogen comes anagen again: the cycle turns over, and the follicle has to build a whole new strand from scratch. Restarting that line takes a fresh supply of cells, and the follicle keeps one in reserve, dormant, close by.

The dormant warehouse — the bulge

The follicle's stem cells are stored in the bulge. Most of the time they do very little: the pool is largely quiescent (held in a low, inactive state), dividing only rarely — on average just two or three times across a whole hair cycle, and only about a hundred times in a lifetime^[11].

When a new growth cycle (early anagen) begins, an activating signal rouses some of them. The main burst of division comes then — they divide only rarely otherwise — and the new cells they produce take up two roles: some stay behind as stem cells, restocking the warehouse, while others travel down to help rebuild the matrix and bulb, and a fresh hair starts^[11]. So the reserve is tapped at the start of each cycle, not continuously — and it replenishes itself as it goes.

What keeps it healthy is, oddly, rest — and that rest is actively defended, not passive. Resting cells hold a molecular brake on their own division, kept in place by quieting signals from the cells around them; remove the brake and they wake too early, cycle too often, and the reserve is spent faster^[19]. So staying mostly quiet is what preserves the cells' long-term ability to regenerate hair, and regrowth suffers with age when follicles are pushed too hard^[12]. The balance between resting and waking is held by these opposing signals — one set keeping the cells quiet, another rousing them — and getting it right is much of what keeps a follicle cycling normally. The warehouse is not bottomless, though: as it gradually runs down over a lifetime, the follicle cycles less vigorously, and the hair it makes can grow finer^[12].

Where does hair get its colour — and why does it run out?

Colour is added right at the start, in the bulb, by a second workforce tucked among the matrix cells: melanocytes — cells that make pigment^[1]. They hand packets of pigment to the cells that will form the shaft. The pigment, melanin, comes in two broad types — eumelanin, which is brown-black, and pheomelanin, which is red-yellow — and the mix and amount, set by your genes, is essentially your hair colour^[4].

There's a neat twist that answers an obvious question: where do these pigment cells keep coming from, cycle after cycle? Colour has its own reserve — a store of melanocyte stem cells held higher up, near the bulge. The key point is that those reserve cells are deliberately kept immature: they hold the full instructions for making pigment but keep them switched off, so they make no colour while they wait. Only when a new cycle begins do the new cells they produce travel down into the bulb and switch those instructions on — becoming active, pigment-producing cells^[4].

So, like the hair, colour keeps a reserve near the bulge — two stores side by side, running on different clocks. The colour reserve tends to empty first. When it does, new strands grow in with little or no pigment — and that, as far as we currently understand it, is much of what greying is: not the hair failing, but its colour supply running out ahead of the hair's^[4]. The follicle keeps building. It simply builds without colour.

How the follicle hides from your immune system

A growing follicle has an awkward problem. As it builds a hair, it also makes proteins the immune system has never familiarised itself with — including some tied to active growth and to colour-making. Unfamiliar proteins like these would normally be flagged and attacked.

The follicle gets around it by turning its lower half — from the bulge down to the bulb — into an immune-privileged zone: a small, protected patch the immune system's patrols mostly leave alone. It earns that status in two ways. It switches off the "ID badges" (called MHC class I) that cells usually display for inspection, so its unusual proteins are never shown to the patrolling immune cells; and it releases local "stand down" signals that keep those patrols quiet nearby^[7].

The closest everyday parallel is diplomatic immunity — the lower follicle behaves a bit like an embassy, where the usual authorities don't barge in or demand papers. When the privilege holds, the follicle grows in peace. When it collapses and the badges switch back on, the immune system can turn on the growing bulb — which is what happens in alopecia areata, a form of patchy hair loss. Tellingly, the reserve higher up (in the bulge) is usually spared even then, which is why hair can return once the attack settles^[7].

Why does the body make dead hair?

Because dead hair is useful. A dead shaft of keratin needs no blood or nerves to do its jobs: it insulates, and it shields the scalp from the sun's UV — denser, darker hair blocks more^[14]. It registers movement and touch through the living follicle it's rooted in, and it signals things like age, identity and belonging. A living fibre would be fragile and painful to maintain. A dead keratin fibre can be cut, brushed, exposed to sunlight, washed repeatedly and withstand daily wear without injury. Building the fibre dead isn't a flaw in the design; it's what lets a single strand stay useful for years without the body having to keep it alive.

Is all hair built the same?

The process is the same in every follicle — but the settings differ from person to person, and on average from one population to another. East Asian hair — most hair in Singapore — tends to be the roundest in cross-section and the thickest, around 85 micrometres across or ~16% thicker than European and African hair (~73 micrometres). Further, rounder strands tend to grow straight; flatter, more oval ones tend to curl^[16].

The contrasts run beyond thickness. Next to American (Caucasian) hair, Chinese hair grows roughly 10% faster, yet it's the least densely packed — American hair carries around 30% more strands per patch of scalp, with Indian hair in between. These figures come from a L'Oréal study of about 2,250 young adults (aged 18–35) across 24 ethnic groups on five continents, measured with non-invasive methods^[16].

They are broad averages from a young, healthy cohort — not a guide to any individual; and even between groups, the traits blur into a continuum rather than sharp lines^[16]. All of it, though, is variation in the settings, not the system — the machine underneath is the same in everyone.

The whole system, in one view

Set side by side, the parts sort themselves by job — though several carry a built-in tension. The bulb builds the hair by dividing into new cells while pushing the older, now-dead ones out. The papilla feeds and directs the work through the blood it carries — though that same line also delivers the body's stresses to the follicle. The bulge holds the reserve, sending cells down to rebuild each cycle — but the reserve is limited, and pushing the whole process too fast leaves the hair weaker. The zone just above the bulb hardens and grips the strand, anchoring it. The pigment cells colour it on the way up. The oil gland adds a little protection near the scalp. And the immune-privileged zone keeps the whole operation shielded while it runs.

For something we treat as surface, hair turns out to be almost entirely a story of depth. The part we wash, cut and worry over is the finished, dead output of a living organ that is — at every moment — dividing, feeding, signalling, hardening and deciding when to rest. Seen that way, a lot of its everyday behaviour stops being mysterious. It grows slowly because building is slow. It sheds because shedding is part of the cycle. It changes in the rain because it's built from a material that answers to water. It greys because one of its two reserves empties before the other. None of this says what to do about any of it — but it makes every later question, about shedding, dryness, breakage or thinning, easier to think through, because it starts from what is actually happening beneath the surface.

Common questions

Is hair alive or dead?

Both — and that's the whole point. The visible strand above the scalp is dead (which is why cutting it doesn't hurt); the follicle below the scalp is living tissue. Hair is alive while it is being made, but dead by the time it reaches the surface^[1].

Does shaving a baby's first hair make it grow back thicker and darker?

Across many Asian cultures, a child's first haircut is a ritual as much as a grooming choice. In parts of India, for example, the first tonsure is associated with purification, renewal, family blessing and the symbolic removal of birth hair. Among Chinese families, the first haircut traditionally comes at the "full month" celebration — the "shaving of the fetal hair," marking a baby's safe arrival at one month and standing for health and a fresh start; the shorn hair is sometimes saved, or even made into a small calligraphy brush kept as a blessing for the child's learning and future^[18]. Other communities have their own versions, often tied to health, luck, identity or transition. Running through several of these traditions is the same belief — that shaving helps the hair grow back thicker or darker.

Scientifically, cutting or shaving a child's hair does not change the follicle beneath the skin. It cannot increase the number of follicles, change the genetic diameter of the hair, or make the bulb divide faster. Newborn scalp hair is often temporary in appearance: during the first months after birth, some of it naturally sheds as follicles settle into their early growth cycles, with more permanent hair often appearing between six and twelve months^[9]. A shaved hair also grows back with a blunt edge rather than a naturally tapered tip, which can make the regrowth feel coarser or appear thicker even though the follicle has not changed^[8]. The ritual may be meaningful; the thicker-looking regrowth is usually a combination of timing and appearance rather than a biological change.

Does cutting hair make it grow back thicker or faster?

No. A freshly cut end is blunt rather than finely tapered, so it can feel coarser for a while — which is where the impression comes from^[8].

If you pull out one grey hair, do many more grow back in its place?

No. Each follicle runs on its own colour schedule, so pulling one can't turn its neighbours grey; the follicle you plucked just regrows a single hair, the same colour as before. More greys over time is simply ageing continuing, not spreading^[17]. Worth knowing: pulling repeatedly from one spot can irritate and, over time, damage the follicle.

Why does the same hair misbehave on humid days?

Water slips into the surface scales and lifts them open, and the fibre swells and softens — so it loses its shape and stretches out of line more easily. Humid air does this continuously^[5].

Can a product "feed" the hair you can see?

Not in the living sense — the shaft is dead. Nourishment only reaches hair at the root, through the blood supply at the papilla^[1]. A product can coat, smooth or protect the strand, but the growing happens from the inside.

Source notes

1. [1] Martel JL, et al. Anatomy, Hair Follicle — StatPearls
2. [2] Histology, Hair and Follicle — StatPearls
3. [3] Hughes EC, Saleh D. Telogen Effluvium — StatPearls
4. [4] Nishimura EK, et al. Melanocyte stem cell maintenance and hair graying — Cell
5. [5] Breakspear S, et al. Chemical bonds and hair behaviour: a review — International Journal of Cosmetic Science (2024)
6. [6] Why Humidity Makes Your Hair Curl — Smithsonian Magazine
7. [7] Bertolini M, et al. Hair follicle immune privilege and its collapse in alopecia areata — Experimental Dermatology (2020)
8. [8] Hair removal: Does shaved hair grow back thicker? — Mayo Clinic
9. [9] Hair Loss (newborn shedding and regrowth) — Seattle Children's
10. [10] Gao J, et al. Revisiting, in vivo, the hair regreasing process by the Sebuprint method — Skin Research and Technology (2019)
11. [11] Yi R. Mechanisms of Quiescent Hair Follicle Stem Cell Regulation — Stem Cells (2017)
12. [12] Lay K, Kume T, Fuchs E. FOXC1 governs stem cell quiescence to preserve long-term tissue-regenerating potential — PNAS (2016)
13. [13] Sebaceous Glands — Cleveland Clinic
14. [14] de Gálvez MV, et al. Human Hair as a Natural Sun Protection Agent: A Quantitative Study — Photochemistry and Photobiology (2015)
15. [15] Russell E, Koren G, Rieder M, Van Uum S. Hair cortisol as a biological marker of chronic stress — Psychoneuroendocrinology (2012)
16. [16] Loussouarn G, et al. Diversity in human hair growth, diameter, colour and shape (24 ethnic groups) — European Journal of Dermatology (2016)
17. [17] Does Pulling One Gray Hair Cause More to Grow in its Place? — UAMS Health
18. [18] China's 'hair-shaving' ritual for newborns — South China Morning Post; Calligraphy brushes made with baby hair (胎毛笔) — Goldthread
19. [19] H2AK119ub dynamics control hair follicle stem cell quiescence — Nature Communications (2025)
20. [20] Draelos ZD. Hair Care: An Illustrated Dermatologic Handbook. Taylor & Francis; 2005. See Table 1.1, “Scalp hair density variation with age.”