Every strand you can see, brush or cut is dead tissue, which is why a haircut never hurts.1Theradome. Is Hair Dead or Alive? The Science behind Your Strands But hair is not dead all the way down, because each strand is made by living tissue under the skin.2Visible Body Blog. Let’s Untangle This: All About Hair A better way to picture a hair is as the dead output of a small living organ, closer to a fingernail growing from a living nail bed than to a lifeless thread.
How a Living Cell Turns Into Dead Hair
The cells that become hair begin as active cells that move, divide and make molecules like any other living cell.3British Journal of Dermatology. The walking dead: sequential nuclear and organelle destruction during hair development To become hair, they fill up with keratin, a tough fiber-like protein that makes hair strong.4British Journal of Dermatology. The walking dead: sequential nuclear and organelle destruction during hair development To make room, each cell throws out its contents, starting with its nucleus, the cell’s control center.5British Journal of Dermatology. The walking dead: sequential nuclear and organelle destruction during hair development For a short time the cells keep their energy-making parts and keep working on one job: making keratin.6British Journal of Dermatology. The walking dead: sequential nuclear and organelle destruction during hair development They throw out those energy parts at the last moment, once no space is left.7British Journal of Dermatology. The walking dead: sequential nuclear and organelle destruction during hair development All of this happens within the first millimeter of the follicle, deep in the skin.8British Journal of Dermatology. The walking dead: sequential nuclear and organelle destruction during hair development
Keratin molecules are long chains that bundle into fibers, and in the finished strand these fibers run lengthwise, which is why hair resists stretching so well. Bonds between neighboring chains lock the fibers together, and these links help hold a hair’s shape, including whether it is straight or curly.
As the cells move up toward the surface, they are cut off from nourishment and die.9Visible Body Blog. Let’s Untangle This: All About Hair So the hair shaft, the part of a hair above the skin, is not pure keratin but dead cells filled with and surrounded by it.10Visible Body Blog. Let’s Untangle This: All About Hair By the time it clears the skin, it has no living cells at all.11Theradome. Is Hair Dead or Alive? The Science behind Your Strands Nothing in the shaft can sense, heal or replace itself, so any change you see along a strand reflects what happened to it after it left the scalp. The tip of long hair may be years old and has been dead the whole time.
The same process also explains how hair gets longer. The cells at the base of the follicle are the only ones still dividing, so new material is added only at the bottom, and the shaft is pushed up as a column. Cutting the tip does not change how fast the root makes new hair, so trimming doesn’t make hair grow faster or thicker.
The Living Part of Hair Lives Under Your Skin
A hair has two parts: the shaft above the skin and the root below it.12Visible Body Blog. Let’s Untangle This: All About Hair The living root sits in the middle layer of skin, wrapped in a pocket called the follicle, usually linked to an oil gland.13Visible Body Blog. Let’s Untangle This: All About Hair Actively growing follicles reach down into the fatty layer below the skin.14PubMed. The hair follicle as a dynamic miniorgan
There is no visible line between the shaft and the root, because cells harden and lose their life gradually as they travel up out of the follicle. That is why washing, coloring or conditioning the visible strand never reaches the living part under the skin.
At the base, the root swells into a bulb around a small cluster of tissue that carries tiny blood vessels.15TeachMeAnatomy. Hair Follicle – Anatomy, Structure and Growth Cycle That blood supply brings the oxygen and nutrients the follicle needs while hair is actively growing.16TeachMeAnatomy. Hair Follicle – Anatomy, Structure and Growth Cycle So whatever reaches your hair through your body has to come through this base, not through the strand itself.
What you eat can affect the hair being made, not the hair already grown. A shampoo or mask that claims to nourish the strand can’t feed it, because the dead shaft has no way to take in and use nutrients.
The oil gland empties into the follicle and releases sebum, an oily substance that travels up along the shaft. Sebum coats the strand and the skin, which is why the scalp’s oil level affects how hair looks and feels between washes. Because the oil is made in living skin and spreads outward, hair near the scalp often looks oilier than the ends, which are farthest from the gland.
Follicles rebuild themselves again and again, thanks to a store of stem cells.17PubMed. Hair follicle bulge: a fascinating reservoir of epithelial stem cells These sit in a spot called the bulge, partway up the follicle, which marks the bottom of the permanent part of the follicle.18PubMed. Hair follicle bulge: a fascinating reservoir of epithelial stem cells They can regenerate not only follicles but also oil glands and skin.19PubMed. Hair follicle bulge: a fascinating reservoir of epithelial stem cells
Why Pulling Hair Hurts When Cutting Doesn’t
The shaft has no nerves, so cutting it causes no feeling. The follicle, though, is a sensory organ, and the skin around it responds strongly to touch and temperature.20PubMed. The male beard hair and facial skin – challenges for shaving That skin is rich in blood vessels, nerves and immune cells.21PubMed. The male beard hair and facial skin – challenges for shaving A tug on a strand moves the follicle, and the nerves around it register the pull or pain.
The soreness often shows up when the hair is let down. A tight ponytail or braid keeps tugging on the follicles for hours, and once the skin is no longer held in tension, it becomes more aware of the irritation. Repeated, long-term tension on the same follicles can damage them, which is why very tight styles are best worn gently or varied from day to day.
Plucking hurts more than you’d expect for something so thin. Pulling a hair out by the root drags the base of the follicle itself, so the nerves around it are stretched all at once. The pain is sharp but brief, because it comes from that single pull on living tissue and ends when the hair is out.
Each follicle also has a tiny muscle attached that can pull the hair upright.22PubMed Central. Beyond Goosebumps: Does the Arrector Pili Muscle Have a Role in Hair Loss? Nerves from the body’s automatic stress-and-cold system work with that muscle and the follicle to raise goosebumps.23PubMed. A Hairy End to a Chilling Event The nerves are densest where the muscle attaches to the follicle.24PubMed Central. Beyond Goosebumps: Does the Arrector Pili Muscle Have a Role in Hair Loss?
Can Damaged Hair Be Repaired
The shaft is sensitive to mechanical, physical and chemical wear.25PubMed. UV damage of the hair Too much sun is the most common cause of damage to its structure, because UVB light breaks down hair proteins, while UVA light mostly fades color.26PubMed. UV damage of the hair Heat from styling and long sun exposure break the chemical links that hold keratin together.27PubMed. Key locations of oxidative damage in human hair keratins after heat and ultraviolet light exposure Repeated styling and sun can leave hair weaker over time.28PubMed. Key locations of oxidative damage in human hair keratins after heat and ultraviolet light exposure
Because the shaft has no living tissue, it cannot repair itself.29Ethique. Hair Myths: Top 6 Busted by Ethique Hair treatments mainly deposit material on the surface to add shine, smoothness and strength.30PubMed. Wanted, dead and alive: Why a multidisciplinary approach is needed to unlock hair treatment potential Silicones, for example, smooth hair’s outer layer and make it look healthier without truly repairing it.31Ethique. Hair Myths: Top 6 Busted by Ethique The effect is temporary because these coatings come out in the wash.32Ethique. Hair Myths: Top 6 Busted by Ethique Natural keratin is also so tightly bonded that most ingredients bind to it only weakly.33PubMed. Reactive keratin derivatives: A promising strategy for covalent binding to hair
So read “repairing” on a label as making hair look and feel smoother for now, and expect to reapply after washing. Real improvement comes from new hair growing in undamaged.
Damage shows up at the ends first. They are the oldest part of the hair, so they have had the most time to collect wear from brushing, washing, sun and styling. That is why split ends and dryness are usually worst at the tips, while the newer hair near the scalp often looks healthy.
A split end happens when the outer protective layer wears away at the tip and the inner fibers separate. Once a strand has begun to split, it can keep splitting further up, so leaving it often lets the damage travel along the hair. Only cutting above the split removes it. Sealing products can hold the pieces together for a short time but do not rejoin them. Protection comes down to a few habits: lower heat settings and fewer heat passes reduce bond damage, a hat helps in strong sun, and trimming removes damaged ends that can’t be fixed.
Why Hair Grows, Stops and Falls Out
Each follicle cycles through three phases. In the growth phase, called anagen, cells in the bulb multiply rapidly and hair grows fast.34PubMed Central. Morphogenesis, Growth Cycle and Molecular Regulation of Hair Follicles In the shutdown phase, catagen, growth stops, cells die off on schedule and the follicle shrinks.35PubMed Central. Morphogenesis, Growth Cycle and Molecular Regulation of Hair Follicles In the resting phase, telogen, the follicle is least active and the old hair falls out.36PubMed Central. Morphogenesis, Growth Cycle and Molecular Regulation of Hair Follicles Even while resting, the follicle is ramping up signals to start the next growth phase.37PubMed Central. Morphogenesis, Growth Cycle and Molecular Regulation of Hair Follicles
The length your hair can reach depends largely on how long its follicles stay in the growth phase before shutting down. Scalp hair has a long growth phase, so it can grow much longer than hair on the arms, which has a much shorter one. That is why body hair stops at a short length while scalp hair keeps going for years.
Shedding is part of this cycle. Hairs in the brush are often resting-phase hairs that finished their run. Follicles cycle out of step, so you don’t lose all your hair at once. During rest, the old hair is held in the follicle while preparations for a new one begin underneath, and the old one is usually pushed out when the new one starts growing. A shed hair usually means a new one is on the way, and normal daily shedding is a sign the cycle is running as it should.
Timing can mislead, though. A major illness, surgery or stressful period can push many growing follicles into the shutdown and resting phases earlier than usual. Because hair is shed at the end of the resting phase, the extra fall-out shows up weeks to months after the event, so the cause may seem unrelated. In most cases the follicles themselves are not harmed, so hair usually grows back once the body recovers.
Growing follicles are partly shielded from the immune system, from the bulge down to the bulb.38PubMed. Hair follicle immune privilege and its collapse in alopecia areata They hide markers immune cells look for and send “no danger” signals.39PubMed. Hair follicle immune privilege and its collapse in alopecia areata The protection is only partial, and when it fails the immune system can attack follicles, as happens in alopecia areata, a patchy hair loss.40PubMed. Hair follicle immune privilege and its collapse in alopecia areata
Why Hair Turns Gray
During growth, pigment cells in the bulb pass packets of pigment to the cells that become hair.41Origenere. What Causes Grey Hair? The Science of Hair Greying The color is locked into each strand before it leaves the scalp. So gray hair grows out gray, and an existing colored strand does not turn gray along its length.
In people, graying starts with pigment cells in the growing bulb making and handing over less pigment.42PubMed. The biology of human hair greying It is driven by pigment stem cells failing to maintain themselves.43PubMed. Mechanisms of hair graying: incomplete melanocyte stem cell maintenance in the niche With age, more of those stem cells get stuck where they can’t renew pigment.44Origenere. What Causes Grey Hair? The Science of Hair Greying Once the stem cell pool is used up, graying becomes largely permanent.45PubMed. The biology of human hair greying
Individual follicles go gray at different times, which gives the “salt and pepper” look. Early graying may be partly reversible before the stem cells are gone, while late graying is largely fixed.
Each follicle makes its own color, so pulling out one gray strand has no effect on its neighbors. The new hair from that same follicle will usually come in the same color as before, since its pigment cells haven’t changed. The idea that plucking gray hairs makes more appear is a myth. More grays show up over time because more follicles are running out of pigment.
Gray or white hair has little or no pigment in the shaft, so light scatters differently and it often looks dull or wiry. Many people find it feels coarser or harder to style, though that is not true for everyone.
The same logic applies to color you add. Dye coats or alters the dead shaft, not the follicle, and sun fades color in the shaft over time. A real color change has to start in the living bulb.