Hair structure, from follicle to fiber

Key takeaways

• A hair fiber has up to three layers: an outer cuticle (overlapping dead cells, about 10% of the weight), a central cortex (86-90% of the weight, where the strength is) and sometimes a medulla, a hollow or loosely packed core that is more common in thicker fibers Plowman 2018.
• The cuticle is the face your hair shows the world. Its scales control shine, friction, tangling and how water gets in. It wears away first. In hair longer than 2.4 m, the cuticle wore away progressively over roughly the first metre before the cortex was affected Thibaut 2010.
• One fatty acid, 18-methyleicosanoic acid (18-MEA), is bonded to the outside of cuticle cells and makes the fiber water-repelling. Removing it makes the fiber water-loving, and alkaline treatments are reported to strip it Jones and Rivett 1997.
• The cortex is a composite of stiff keratin filaments in a softer protein matrix. Cross-linking in the matrix keeps the filaments stiff even when wet Greenberg and Fudge 2013.
• The join between cuticle and cortex is a weak point. In stress tests of curly hair from one donor, cracks started there and merged until the fiber snapped, at unpredictable places Camacho-Bragado 2015.
• Breakage is usually cumulative: grooming and environmental stress add up as internal cracks. Gentle handling every day matters more than one rescue product.
• Inner fine structure is nearly identical between people. In 12 people, including twins, X-ray diffraction found the same keratin and lipid signals; only some extra lipid signals differed, and those ran in families Yang 2014.

The follicle makes the fiber

Hair is produced in the follicle bulb and hardens as it rises; by the time it emerges, the cells are dead. The shape of the follicle sets the curve of the fiber (see curl pattern), and the growth cycle is covered in the hair growth cycle. Nothing you apply changes what the follicle already made. Products only change what happens to that fiber afterward, so "repair" claims really mean coating, filling or cross-linking a fixed structure.

Layer by layer

Bonds that hold keratin together

• Hydrogen bonds are weak and numerous. Water disrupts the protein-protein ones, which is why hair set wet and dried holds its shape until it gets wet again Harland 2022.
• Disulfide bonds are strong and permanent. Perms, relaxers and bleach act on them. Under tension some break first near the protein ends, and this differs between wet and dry fibers (a sheep wool study, not human hair) Harland 2022.

Water and the fiber

Hair absorbs water and swells. In a tensile study of bleached and unbleached hair given heat treatments, wet-state stiffness (modulus) was judged the best single measure of damage from chemical and physical treatments Wortmann 2022. Calcium and magnesium from hard water build up in the cuticle, more so in chemically damaged hair, and stiffen the fiber. Style retention improved in virgin hair but dropped slightly in bleached hair Evans 2011 (industry-affiliated authors).

How hair ages from root to tip

In very long virgin hair, the cuticle wore away over about the first metre while the cortex stayed intact. Further along, scale damage was significant and lined up with falling ceramide and 18-MEA levels. Most mechanical and optical properties then decayed Thibaut 2010 (industry-funded). This is why ends look and feel worse than roots even without styling damage, and why trimming is the only true fix for old ends.

Lipids

A 2023 systematic review of 59 papers found that hair lipid composition varies by ancestry-linked hair type and by layer, and that lipid loss is accelerated by bleach, dye, perms, straightening, surfactants, sun and aging, leading to dehydrated, breakable, dull hair Csuka 2023. Lab studies on African hair found more, and more disordered, cuticle lipids and faster water diffusion Marti 2016, Oliver 2020, Barba 2022. Between Caucasian, Asian and African samples no difference in inner fiber structure was found, while geometry, mechanics and water swelling did differ Franbourg 2003. More in hair by ancestry.

What it means for your hair

• Curl pattern: bends concentrate stress. See curl pattern and brushing and detangling.
• Strand thickness: coarse fibers are more likely to have a medulla. See strand thickness.
• Porosity: in practice a marker of cuticle and lipid-layer damage. See hair porosity.
• Length: longer hair has had more exposure. Assume ends older than a couple of years are cuticle-worn.

Myths and facts

• "Hair can be repaired." Largely false, because the fiber is dead. Products can smooth, coat or add temporary cross-links; see bond builders.
• "Curly hair is a different kind of keratin." Not supported at the nanoscale Franbourg 2003, Yang 2014. Geometry and surface lipids differ.
• "Breakage is one bad event." More often it is cumulative internal cracking Camacho-Bragado 2015.

Practical points

1. Treat the ends as the oldest, most worn part of the fiber. Expect them to need more conditioning and eventual trimming.
2. Keep wet-hair mechanical stress low: detangle gently with conditioner slip.
3. Avoid stacking chemical and heat damage. They add up.
4. Be skeptical of any "rebuilds the cortex from the outside" claim that shows no penetration and strength data.
5. If hair breaks near the scalp or comes out with a white bulb, think shedding or scalp rather than fiber damage.

Bottom line

Hair is dead keratin: protect it, don't expect to repair it. The cuticle wears first, breakage accumulates, and the ends are always the oldest part. Inner structure is much the same across people; what differs is shape, surface lipids and history of treatment. Several of the key studies are small or industry-funded, so treat the details as a good working model rather than settled fact.