Inside a strand: cuticle, cortex and medulla — and what “nourish” can mean on a dead fibre

Walk into any beauty store in Singapore, or scroll Shopee and TikTok for ten minutes, and a single strand of hair is described as if it were alive — wounded, starving, in need of rescue. Strip the brand names away, and the words fall into three kinds. They run from honest to impossible.

The honest kind is damage language: hair that is "rough," "broken," "brittle," "dull," split at the ends. This mostly describes something real — a fibre slowly wearing out.

The middle kind is repair language: products that "repair," "restore," "strengthen," "bond" or "reverse" the damage. These can be true — but only if they say what they actually fix, and how you would measure it. Left vague, they promise more than they deliver.

The impossible kind is nourishment language: hair "fed," "nourished," "hydrated from within," its "nutrients" returned. That fits a living root, which blood feeds. It cannot fit the strand: there is nothing in it that is alive to feed.

Most of the shelf speaks the last two kinds. Of fifty popular hair masks in Singapore, more than nine in ten promise to repair, restore, nourish or strengthen. And it is not the cheap jars overclaiming: the same promises run from a S$5 mask to a S$90 one, and the dearest salon jars claim just as boldly. All of it uses the same science-sounding words — keratin, bonds, ceramides, pH — which make a claim sound proven when it is only a mood.

And yet the strand is already dead — a finished fibre with no blood, no living cells and no repair crew, as How hair grows explains. So how do you repair, feed or reverse a dead thing? Hold that question. To answer it, you first need to see how a strand is built. Not a solid rod, but three layers, one inside the next, each with one job.

The strand, in three layers

Cut a hair across and you would see three layers, one wrapped around the next — like the core, the body and the outer sheath of a cable ^[1]. The outer layer is the cuticle: a thin, clear sheath of overlapping scales, and the only part your fingers touch. Under it is the cortex: the thick core that is most of the strand, and that holds its strength and its colour. At the centre, in some hairs but not all, runs the medulla: a narrow, often hollow channel we have only begun to understand.

If hair is dead, can it still be damaged?

Yes. Dead means the visible fibre cannot do what living tissue does: it cannot grow new cells, mend a split, or replace what it loses. But dead is not the same as unchanging. A strand still soaks up water and swells, bends and springs back, loses the thin oils that coat it, roughens, and — under enough stress — weakens and finally breaks. So damage here is not a wound the body heals; nothing comes to fix it. It is weathering — the slow wear that wind, rain and sun work on a stone statue, except the material is keratin and the weather is heat, friction, sunlight, water and chemistry. The three layers are simply where that wear lands.

The cuticle — the sacrificial barrier you actually touch

Everything you do to your hair, you do to the cuticle first. It is the strand's outer skin: a thin, colourless layer of dead, flat cells that overlap like fish scales, all pointing from root to tip ^[2].

That shape has an everyday effect. As the strand slowly moves outward, its scales — which point towards the tip — nudge dirt and loose flakes towards the end and off the hair. They act as a passive escalator, helping to stop debris from catching on the surface.^[11]

The deeper point is simple: this layer exists to be used up. It takes the friction of brushing, the heat of a dryer, the sun and the pull of chemicals. By wearing out first, it protects the cortex underneath. It is the strand’s bumper.

It is also where most of the shine comes from. A smooth, flat cuticle reflects light evenly, so the hair looks glossy; lifted or chipped scales scatter the light, so it looks dull ^[3]. A product can improve this surface — sometimes the moment you rinse — by smoothing or coating the scales. So "shinier" is often real. What shine cannot tell you is the state of the cortex underneath. There is no potion that adds gloss by mending the inside.

The cuticle has one older job too. As the strand forms below the scalp, its scales lock into a matching layer lining the follicle — like the teeth of two combs meshing — and that grip helps hold the growing hair in place ^[2]. By the time the hair reaches the air that job is done, and all the cuticle has left to do is protect.

The cortex — where strength and colour live

Lift the cuticle away and you reach the cortex: the thick core that makes up most of the strand ^[2]. Nearly everything we care about in a hair — that it holds together, stretches, and carries a colour — happens here. By weight it is mostly protein — somewhere between two-thirds and nine-tenths, depending on humidity — with up to about a third as water and only a little oil and pigment; its behaviour is ruled by a tough protein called α-keratin ^[10].

Its strength is built before the strand reaches the surface. In the bulb, the dermal papilla provides the growing follicle with blood and nutrients. Those conditions help determine how well the new cortex is formed before it hardens into a non-living fibre.^[1]

Inside the cortical cell, keratin proteins are spun into fine threads. The threads bundle together and sit in softer protein-rich material, rather like stiff rods held in flexible glue.^[10] More than one kind of bond helps hold them in place: strong sulfur-to-sulfur disulfide bonds, like rungs tying two ropes into a ladder, and weaker bonds that water can loosen and reset.^[4] Around the cortical cells, a thin film of fat and protein — the cell membrane complex — helps hold neighbouring cells together.^[2]

Strength comes from this whole arrangement, not from one bond alone. Water softens the surrounding material, which is why wet hair is weaker.^[10] The cuticle still matters as protection, but it is not the main load-bearing layer: when researchers damaged the cuticle alone with chemicals, the fibre's strength barely changed.^[10]

That same structure lets hair stretch and spring back: pull gently and the coiled keratin gives, then recoils. That is why healthy hair can bend all day without breaking — and why wet hair, softened by water, stretches further and is more vulnerable to snapping.^[10]

Because each section of the shaft is made only once, it can carry a record of interruptions while it was being built. Severe nutritional deficiency can disrupt hair-shaft formation, leaving a weaker, narrowed section that breaks more easily.^[14] Sudden weight loss or reduced protein intake can also trigger telogen effluvium - a form of shedding often confused with female pattern hair loss though the two are biologically distinct, as Hair loss is not one thing explains.^[15] The cortex records more than the quality of its construction: it is also where the strand receives its colour.

Melanin — the pigment made at the root — is packed into the cortex as the strand forms ^[1]. The point here is simple: colour lives in the core, not on the surface.

So this is the layer the bigger claims aim at. "Strengthen," "rebuild" and "bond repair" all point here. "Nourish" has nowhere to go — there is no blood and no living cells in the core to take a meal. And because strength and colour both live here, it is the cortex breaking down, not the cuticle wearing thin, that we feel as weakness, breakage and hair that will not behave.

This is also how the harshest products reach it. Colouring, perming and relaxing all need an alkaline, high-pH mix to work: the high pH swells the cuticle and lifts its scales, opening a path down to the cortex — and leaving the surface rougher to the touch ^[12].

The medulla — the layer we understand least

At the very centre of some strands runs the medulla: the innermost layer, and the least understood. It is a narrow channel, often full of air, and frequently patchy or missing — especially in fine and pale hair ^[1]. Plenty of healthy hair has little or none. Its absence is normal, not a flaw.

For a long time it seemed to do nothing. Then optical studies of hair gave it a small role: its inner structure changes how light moves inside the strand. When the channel is full of tiny pores, light scatters from the core and the hair looks a little whiter and less rich in colour; when the core is solid, it does not ^[5]. The same study, on Japanese women's hair, found these pores can form during heat-drying — one way hot drying can dull a strand ^[5].

How much the medulla matters depends on the hair. When researchers measured how much each layer dims a beam of light, the medulla mattered far more than the cortex in blond and grey hair — but barely at all in the dark-haired Asian samples used in that experiment, where the colour-packed cortex soaks up most of the light itself ^[6]. (These are lab readings of light through fibres, not a judgement on everyday shine.)

So what does "damage" really mean?

Now the word is easy to define. Damage to a strand is weathering — physical and chemical change to a finished fibre. It usually begins where the world touches first, at the surface, and works inward. Friction, sun and everyday handling wear the cuticle: scales lift and chip, the oils that kept it sleek rub off, and the hair turns rough, dull, dry and lifeless ^[3]. Once the cuticle is chipped, the cortex is exposed and starts to weaken — strength and stretch go, the strand snaps more easily, and a split end is just the cortex fraying after the cuticle that held it has worn away ^[2].

But that order is a tendency, not a rule. Some damage skips the queue. Bleach and strong chemicals, high heat, sunlight, and the daily swelling of wet-then-dry all reach past the surface and hit the cortex's bonds and oils directly ^[4].

Whichever route the damage takes, the practical result is the same: a cuticle that lets more in, and holds less back. That trait has a name of its own: porosity. A tightly closed cuticle resists water and product; a lifted, chipped one takes both up quickly and loses them just as fast. Hair porosity explained covers how to read that from your hair's own history, rather than treating it as a fixed "type."

When does smoothness count as repair?

In a June 2026 AYUVAY audit of the fifty popular hair masks in Singapore, more than nine in ten promised to repair, restore, nourish or strengthen. Yet a sentiment analysis of the 209 usable buyer reviews for those masks, read product by product, finds about seven in ten centre on a surface result — softness, smoothness, less frizz — and only about one in ten on anything structural like repair or strength (full breakdown in the appendix). The audit reads what a product claims and what buyers say back, not what was measured on the hair.

That difference matters, but it does not make smoothness trivial. Once the cuticle has been weathered, smoothness is often the first change a treatment can make. Hair slips more easily through the fingers, combing takes less effort, and light reflects more evenly from the surface. Those are real improvements. What smoothness cannot tell you on its own is what kind of improvement happened — or how long it will remain.

At one end is temporary conditioning. A product deposits a film that makes the strand feel softer and more slippery for a wash or two. That is still useful: lower friction can make detangling easier and reduce some of the breakage caused by handling. But once that film washes away, the fibre may behave much as it did before.

At the other end is durable surface repair. Here, the treatment does more than sit briefly on the strand. It survives washing or rubbing and restores a function the cuticle has lost: smoother movement between fibres, better water resistance, or protection against further wear. This is still surface repair. A repaired bumper is no less repaired because it sits on the outside of a car; what matters is whether it has been securely restored to its job.

One study showed one version of this in the laboratory. Hair treated with highly charged cationic polymers retained more fatty acids and cholesterol after a surfactant wash challenge than untreated hair. The treatment did not rebuild the hair's original internal lipid architecture. But it did create a durable change at the surface that protected part of the fibre's barrier from further loss^[8].

Then there is structural reinforcement: a treatment that changes how the cortex behaves. Certain cross-linking chemistries, for example, have improved tensile properties in laboratory tests on damaged hair fibres. That is a different claim from surface smoothness. It needs different evidence: controlled measurements of force, stretch, breakage resistance or other mechanical behaviour — not just how hair feels after rinsing^[9].

So the useful distinction is not surface versus real. Surface repair can be real, and temporary conditioning can still be useful. The questions are more specific: what function did the treatment restore or protect, how long did that benefit survive, and was the result actually measured?

A result that disappears after the first rinse may still be good conditioning. A result that survives repeated washing is stronger evidence of durable repair. But duration alone is not a map of the mechanism: a benefit may persist because a protective surface layer remains, because material has entered the fibre, or because a new chemical link has formed within it.

What can hair-care words honestly mean?

The useful distinction is not whether a product uses words such as “repair,” “strengthen” or “nourish.” It is whether the word is attached to a clear result.

Repair can be fair when a treatment restores a function that weathering has taken away. That may be surface repair — smoother movement, lower friction or better protection against further wear — or structural reinforcement, where a treatment measurably improves how the fibre withstands force. The claim becomes vague only when it does not say what was repaired.

Strengthen is a more specific claim. It should mean an improvement in a mechanical outcome: less breakage, greater resistance to pulling, or a measurable change in tensile behaviour. Some cross-linking chemistries have improved mechanical properties in laboratory tests on damaged hair fibres, but that does not make every product labelled “bond repair” equivalent^[9].

Restore is fair when it names the condition being restored: smoothness, gloss, water resistance, easier combing or a measured reduction in breakage. It cannot mean returning an old strand to the untouched state it had when it first grew.

Nourish is different. On a living follicle, nourishment has a literal biological meaning: cells receive oxygen, nutrients and signals as they build a strand. On the visible dead fibre, the word is a marketing gloss. It borrows the language of feeding and renewal without saying what, materially, has changed.

Can you test repair at home?

Not reliably. There is no home test that can confirm cortex repair or prove that a product has strengthened a strand internally. True fibre strength is measured by comparing treated and untreated hair under controlled tension in a laboratory^[9]. At home, you can still notice useful outcomes — easier detangling, less breakage during handling, fewer new split ends or a benefit that lasts through several washes — but those observations cannot tell you exactly where in the fibre the change happened.

What a dead fibre can — and cannot — become

Set the three layers side by side and each has one honest job. The cuticle is the surface - the sacrificial shield, anchors the hair, the passive cleaner of dirt, and gives most of the shine. The cortex gives the strength, the stretch, and the colour. The medulla is the quiet core: often absent, at most a fine-tuner of the shine.

Which answers the question we started with. A dead fibre cannot be fed, or healed. What can be done is a small set of physical acts dressed in the language of biology — coat, smooth, fill, sometimes work in, sometimes re-bond. Care can materially change a dead fibre; it just cannot turn material care into biological healing. Once you know which layer a worry belongs to, the rest — frizz, breakage, colour, the time a treatment needs — gets easier to think about, each in its place.

Common questions

Can damaged hair be repaired?

Partly. A strand is dead, so it cannot heal itself or grow back what it lost — in that sense, no. But as a material, it can be repaired from the outside: a treatment may restore surface smoothness or protection, and certain chemistries can reinforce damaged fibres in laboratory tests.^[9] That is material repair, not biological healing. How long the benefit lasts depends on what the treatment actually changes.

Can a "bond repair" product really rebuild my hair?

It depends on the specific product. The honest answer has two parts:

What is a split end, really?

The cortex fraying. Once the cuticle at the tip — the oldest, most worn part — wears away, the rope-like core underneath has nothing holding it together, and it splits ^[2]. The two halves cannot be joined back; trimming above the split is the only real fix. Smoothing products can hide it for a while, but the fibre is already divided.

Why does my hair look dull?

Usually because the cuticle surface is no longer smooth: lifted or chipped scales scatter light instead of reflecting it ^[3]. A smoothing product can bring the gloss back, sometimes at once — but that is a surface change. It says nothing about the cortex underneath.

Does the medulla matter if mine is barely there?

For many people it is thin, broken or missing, and that is completely normal — not a defect. Its main known effect is a small one on the light inside the fibre, smaller still in dark hair. Its absence is nothing to treat ^[1].

Editorial note. This piece is about the hair you can see — the dead fibre — not the scalp or the living follicle, and it is educational, not medical or product advice. Where the evidence is only mechanistic or from the lab, the text says so.

Source notes

1. [1] Martel JL, et al. Anatomy, Hair Follicle — StatPearls
2. [2] Histology, Hair and Follicle — StatPearls
3. [3] Okamoto M, et al. Influence of internal structures of hair fiber on hair appearance. III. Generation of light-scattering factors in hair cuticles and the influence on hair shine — Journal of Cosmetic Science (2003)
4. [4] Breakspear S, et al. Chemical bonds and hair behaviour: a review — International Journal of Cosmetic Science (2024)
5. [5] Nagase S, et al. Influence of internal structures of hair fiber on hair appearance. I. Light scattering from the porous structure of the medulla of human hair — Journal of Cosmetic Science (2002)
6. [6] Kharin A, et al. Optical properties of the medulla and the cortex of human scalp hair — Journal of Biomedical Optics (2009)
7. [7] Marsh JM, et al. Strengthening benefits of panthenol for hair: mechanistic evidence from advanced spectroscopic techniques — International Journal of Cosmetic Science (2026)
8. [8] Song M, et al. Prevention of lipid loss from hair by surface and internal modification — Scientific Reports (2019)
9. [9] Novel Compounds for Hair Repair: Chemical Characterization and In Vitro Analysis of Thiol Cross-Linking Agents — Pharmaceuticals (2025)
10. [10] Structure and mechanical behavior of human hair — Materials Science and Engineering: C (2017) (compiles hair composition and the finding that tensile strength comes from the cortex, not the cuticle — Velasco et al. 2009; Kreplak et al. 2001; Robbins & Crawford)
11. [11] Aswathy P, Aravind S. Keshyam: A Handbook on Ayurveda Trichology (book).
12. [12] Khosa K. Beauty Unbottled (book).
13. [13] Salon / trichology descriptions of the at-home "elasticity" or stretch test (industry sources, not peer-reviewed): HairKnowHow, Ogario London.
14. [14] Saleh, D. et al. Anagen Effluvium — StatPearls (updated 2024).
15. [15] Guo, E.L. and Katta, R. Diet and hair loss: effects of nutrient deficiency and supplement use — Dermatology Practical & Conceptual (2017).

---

Appendix — what the reviews said

How the masks were chosen. We scanned the two big online marketplaces, Shopee and Lazada, plus a reader survey. Shopee matters most: it is the channel Singaporeans most often buy hair care from — first for shoppers under 35, and second for those 35 and older — in a ranking that includes both online and physical stores (Daily Vanity SG Consumer Behaviour Report 2025). Rather than eyeballing "popular," we ranked each platform's demand signal on a log-normal distribution — a bell curve on a log scale — split masks into low, mid and high at one standard deviation either side of the mean, and kept the 50 masks in the mid and high tiers. We looked only at rinse-off hair masks priced roughly S$5 to S$90 a jar; shampoos, scalp and growth treatments, and heat protectants were left out, since repairing the fibre isn't their job.

Alongside the listing claims, we looked at what buyers of those same 50 masks actually reported — a sentiment analysis of their 209 usable reviews across Shopee, Lazada and Amazon, tagging the positive themes each mask's buyers raised (a mask can raise more than one):

Method. Themes overlap — a mask can raise both a surface and a structural result. The point is linguistic rather than clinical: listings often lead with repair language, while buyers more often name immediate sensory results such as softness, smoothness and shine. This audit does not tell us whether a product repaired hair — only which benefits brands promise and which buyers choose to name.