The Biology of Hair Keratin: Structure, Strength, and Degradation

Quick answer: The Biology of Hair Keratin explains how specialised, insoluble structural proteins form roughly 85 to 90 percent of the hair shaft. Synthesised in the hair follicle bulb by matrix cells, keratin filaments cross-link via strong disulphide bonds to provide high tensile strength, elasticity, and thermal resilience. However, once hair keratin fully cornifies and emerges past the scalp surface, it is biologically dead tissue; topical salon treatments cannot biologically regenerate lost keratin, meaning preventing structural damage remains the only permanent defence.

Key takeaways

• Keratin makes up 85 to 90 percent of human hair by dry weight, accompanied by water, lipids, trace elements, and melanin.
• The fibre relies on two main structural components: intermediate filament proteins (low-sulphur alpha-keratins) and a surrounding matrix of keratin-associated proteins (KAPs, rich in cysteine).
• Chemical bonds dictate hair behaviour: covalent disulphide bonds provide permanent mechanical strength, while hydrogen and salt bonds provide temporary styling flexibility.
• Once keratinised in the follicle zone of elongation, the hair shaft has no living cells, blood supply, or repair mechanisms.
• Topical "keratin treatments" do not rebuild biological keratin; they temporarily coat or cross-link the fibre using external cosmetic proteins and conditioning agents.

The Biology of Hair Keratin: what the evidence shows

Understanding the biology of hair keratin starts deep within the dermal papilla and hair follicle bulb. Trichocytes (follicular epithelial cells) rapidly divide and produce fibrous proteins as they migrate upward. As these cells move through the keratogenous zone, they undergo a programmed cell death known as cornification. During this transition, cellular organelles, nuclei, and cell membranes are systematically degraded and compacted. What remains is a densely packed composite structure built almost entirely of keratin intermediate filaments embedded within an amorphous protein matrix.

At a molecular level, hair keratins are fibrous proteins arranged in an alpha-helical conformation. Human hair contains specific "hard" alpha-keratins (classified as epithelial K31 through K38 for type I acidic keratins, and K81 through K86 for type II basic-to-neutral keratins). These pair together as heterodimers, assemble into tetramers, and pack into protofilaments and protofibrils, eventually creating the 7 to 8 nanometre intermediate filaments that run parallel along the hair shaft.

Surrounding these organised filaments are keratin-associated proteins (KAPs). These matrix proteins are exceptionally rich in the amino acid cysteine. When cysteine residues align between adjacent peptide chains, their thiol groups oxidise to form covalent disulphide bonds (cystine). This dense disulphide cross-linking is precisely what gives hair its remarkable resistance to enzymatic digestion, mechanical stretching, water dissolution, and environmental stress.

Individual Keratin Alpha-Helices
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 Heterodimers (Type I + Type II)
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Tetramers & Protofilaments
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Intermediate Filaments (7 to 8 nm)
 │ + Keratin-Associated Proteins (KAPs Matrix)
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Cortical Macrofibrils ──> Hair Cortex (85-90% of Fibre)

The hair shaft itself divides into three primary anatomical regions, each with a distinct keratin profile:

1. The Cuticle: The outermost protective shield, made of 6 to 10 overlapping scales. Each scale contains an outer epicuticle, an exocuticle (extremely rich in cysteine and heavily cross-linked), and an endocuticle (lower in sulphur, higher in swelling capacity).
2. The Cortex: The structural core, representing roughly 85 to 90 percent of the hair volume. It houses the tightly bundled macrofibrils of alpha-keratin and the bulk of the hair's melanin pigment.
3. The Medulla: The central, discontinuous core found primarily in thick or coarse terminal hairs, composed of loosely connected, specialised structural proteins and air pockets.

Chemical bonds that hold keratin together

The mechanical durability of the hair fibre depends on three types of chemical bonds within and between keratin chains. Each behaves differently when exposed to water, heat, and salon chemicals.

Bond Type	Relative Abundance	Bond Strength	How It Breaks	How It Reforms
Hydrogen Bonds	Very high (roughly one-third of fibre strength)	Weak individually, powerful in aggregate	Water, moisture, high humidity	Neutralised and reset upon thorough drying
Salt (Ionic) Bonds	High	Moderate	Extreme shifts away from hair's isoelectric point (pH 3.67 to 5.5)	Return of fibre to acidic pH range
Disulphide Bonds	Moderate (governed by cysteine content)	Very strong covalent bond	Chemical reduction (perms, relaxers, bleaching agents)	Chemical oxidation (neutralisers), though only 80 to 90% re-link

Hydrogen bonds occur between the oxygen of one peptide group and the hydrogen of an adjacent amine group. When you wet your hair, water molecules insert themselves between these bonds, sliding the alpha-helical spirals apart. Blow-drying or heat-styling locks them into a new temporary position until humidity re-enters the fibre.

Salt bonds form between the acidic side chains of aspartic and glutamic acids and the basic side chains of lysine and arginine. They remain stable in healthy, slightly acidic environments. Alkaline solutions (such as bleach or permanent hair dyes above pH 9.0) strip the electrical charges from these amino acids, breaking the ionic bridges and causing the cortex to swell.

Disulphide bonds are covalent links formed between two sulphur atoms of neighbouring cysteine amino acids. Because they are covalent, neither water nor routine heat can dismantle them. Only aggressive chemical reducing agents (such as ammonium thioglycolate) or strong oxidisers (such as hydrogen peroxide during bleaching) cleave disulphide bonds.

What the evidence does not show

There is a widespread misconception across salon marketing that topical cosmetic products can biologically "rebuild", "feed", or "resurrect" broken keratin. The biological reality of hair keratin contradicts this claim:

• Keratin cannot be biologically regenerated: The hair shaft outside the follicle contains no living cellular machinery, ribosomes, or blood vessels. Once a disulphide bond or polypeptide backbone is severed by bleaching or mechanical trauma, the fibre cannot synthesise replacement protein.
• Topical keratin is not structural keratin: Shampoos, masks, and salon treatments containing "hydrolysed keratin" deposit broken-down protein fragments (peptides with molecular weights of 500 to 3,000 Daltons) onto the cuticle surface or into superficial cortical voids. They act as film-forming humectants, smoothing roughness and reducing friction. They do not re-establish the complex alpha-helical crystalline matrix of natural hair.
• Protein overload is a cosmetic, not biological, phenomenon: Hair does not become "allergic" to protein. However, excessive application of heavy hydrolysed proteins and conditioning polymers without balanced emollients can leave a rigid, unyielding film on the cuticle. This makes the fibre feel stiff, brittle, and prone to snapping under tension.

How keratin degrades: signs of mechanical and chemical damage

Keratin degradation occurs gradually along the hair length, making older ends substantially more fragile than fresh growth near the scalp.

1. Cuticular weathering and lift

Friction from vigorous brushing, towel-drying, and rough handling chips away at the low-sulphur endocuticle and shears off the protective 18-methyleicosanoic acid (18-MEA) lipid layer. Once this hydrophobic barrier disappears, the underlying keratin scales lift and crack.

2. Disulphide bond loss (cysteic acid formation)

When hair is bleached or exposed to ultraviolet radiation (UVB and UVA), the disulphide bridges between cysteine molecules oxidise irreversibly into cysteic acid groups. This permanently reduces the wet and dry tensile strength of the cortex. High concentrations of cysteic acid increase the porosity of the hair, causing it to absorb excess water, swell excessively, and stretch into a fragile, rubbery state when wet.

3. Thermal denaturing

Applying heated tools above 185 to 200 degrees Celsius begins to denature alpha-keratin, causing the structured alpha-helical coils to transition into disordered beta-sheet configurations. Above 230 degrees Celsius, the protein undergoes irreversible thermal decomposition, melting internal structural components and causing bubble hair deformity.

Practical steps to protect hair keratin integrity

Because the body cannot biologically repair hair keratin once it emerges, long-term hair quality depends on preventative preservation.

• Minimise chemical oxidation: Space out lightening and permanent colour sessions by at least 8 to 10 weeks. Use the lowest effective volume of developer to prevent excessive disulphide cleavage.
• Shield against thermal oxidation: Always use heat protectants containing silicones, polymers, or polyols before heat-styling. These ingredients conduct heat evenly and slow down the rate of cuticular moisture loss. Keep heated styling tools below 185 degrees Celsius.
• Maintain an acidic pH balance: Wash and condition hair with formulations maintained between pH 4.5 and 5.5. An acidic environment preserves the hair's isoelectric point, keeping the cuticular scales tightly flattened against the cortex and reinforcing ionic salt bonds.
• Reduce wet manipulation: Hair keratin loses approximately one-third of its structural strength when fully saturated with water due to disrupted hydrogen bonds. Detangle hair gently using a wide-tooth comb or flexible-bristle brush, starting at the ends and working upward, ideally while saturated with a slip-inducing conditioner.

When hair thinning is biological rather than structural

It is essential to separate fibre damage from follicular hair disorders. Hair breakage occurs when the keratinised shaft fractures under mechanical or chemical tension. The root remains firmly seated inside the scalp.

Conversely, conditions like androgenetic alopecia, telogen effluvium, or alopecia areata involve the biological mini-organs within the scalp dermis. If you are finding entire hairs shed from the root (characterised by a small white bulb at the end) or observing widening parts and diffuse shedding across the crown, the underlying cause is not keratin shaft degradation. These conditions require evaluation of genetics, hormonal balance, ferritin levels, and thyroid function.

Supporting the follicle: the Watermans approach

Preserving your existing lengths through sound handling protects the outer keratin, but healthy fibre production depends entirely on the microenvironment of the living follicle. While dead hair cannot be "fed", matrix cells in the bulb require steady microcirculation, balanced sebum levels, and freedom from chronic oxidative stress.

Formulations such as Watermans GrowMe Shampoo are created with this scalp-first principle in mind. By combining biotin, rosemary extract, caffeine, niacinamide, and hydrolysed lupin protein, it focuses on clarifying the scalp surface, supporting local microcirculation, and optimising the conditions where new keratin filaments are initially assembled.

Frequently asked questions about The Biology of Hair Keratin

What is the primary role of keratin in hair?

The biology of hair keratin dictates that it acts as the primary structural scaffold of the hair fibre. Making up to 90 percent of the shaft, keratin proteins fold into microscopic alpha-helices and cross-link via disulphide bonds. This provides tensile strength, water resistance, flexibility, and protection against daily mechanical stress.

Can salon treatments permanently restore damaged hair keratin?

No. While many salon services market themselves as keratin-restoring, the hair shaft is composed of non-living cornified tissue. Topical treatments use hydrolysed proteins, silicones, or chemical cross-linkers to fill surface voids and temporarily smooth the cuticle. They do not regenerate the native, living keratin architecture of the natural cortex.

How does water affect the biology of hair keratin?

Water readily disrupts the weaker hydrogen bonds that stabilise the keratin alpha-helix. When wet, the hair fibre swells radially, increases in elasticity, and loses up to one-third of its tensile strength. This makes wet hair significantly more vulnerable to stretching, deformation, and cuticle tearing during brushing.

Why does bleaching permanently weaken hair keratin?

Bleaching uses alkaline agents and hydrogen peroxide to penetrate the cortex and dissolve melanin granules. In the process, the peroxide aggressively oxidises the disulphide bonds connecting cysteine residues, converting them into cysteic acid. Once these covalent bonds are destroyed, the cortical protein matrix loses structural rigidity and cannot naturally reform.

Is keratin in hair identical to keratin in skin and nails?

Hair, skin, and nails all belong to the keratin family, but they feature different subtypes and cross-linking levels. Stratum corneum skin cells contain "soft" keratins that desquamate continuously. Hair and nails are composed of "hard" alpha-keratins, which contain substantially higher cysteine concentrations and extensive disulphide bonding, making them rigid, insoluble, and mechanically resilient.

Preserving your hair's structural integrity

The Biology of Hair Keratin demonstrates that human hair is an extraordinarily engineered biological material, capable of withstanding immense daily mechanical and environmental stress. Yet because the emerged hair shaft lacks living cells, this complex protein structure is finite and non-renewable. By understanding the chemical bonds that keep hair intact, maintaining an optimal acidic pH, and protecting the fibre from aggressive oxidative and thermal trauma, you can preserve the natural strength and elasticity of your hair for years to come.

Sources and references

• Robbins, C. R. (2012). Chemical and Physical Behavior of Human Hair. 5th Edition, Springer-Verlag New York.
• Langbein, L., et al. (2001). "The Catalog of Human Hair Keratins: I. Expression of the nine type I and eight type II keratins in the hair follicle." Journal of Biological Chemistry, 276(37), 35123-35132.
• Schweizer, J., et al. (2006). "New consensus nomenclature for mammalian keratins." Journal of Cell Biology, 174(2), 169-174.
• Dawber, R. (1996). "Hair: its structure and response to cosmetic preparations." Clinics in Dermatology, 14(1), 105-112.
• Cruz, C. F., et al. (2016). "Human hair characteristics: A review on keratin, lipids and non-keratin proteins." International Journal of Molecular Sciences, 18(4), 706.
• Rogers, M. A., et al. (2006). "Hair keratin associated proteins: characterization of a diverse multigene family." International Review of Cytology, 251, 209-263.

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Find these ingredients in Watermans products

• Niacinamide: you will find this ingredient in our Hair Mask.