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Hair Strand Anatomy: Cuticle, Cortex and Medulla Explained

Writer: Maggie. R
Maggie. R
2 days ago
4 min read

Every strand of hair has a protective outer layer called the cuticle, a keratin-rich middle called the cortex that supplies most of its strength, and, in some hair, a loosely packed centre called the medulla. Most of what happens on wash day, from shampooing to brushing to blow-drying, acts on that outer cuticle first. Here is what published research says about each layer, and where the science is still unclear.



The Three Layers of a Hair Strand


A hair has two parts. The follicle sits beneath the skin, and the shaft is the strand you see above it. According to a StatPearls anatomy review from the National Library of Medicine, the shaft has an inner core called the medulla, surrounded by the cortex, which makes up the bulk of the hair, with the cuticle on the outside.

Inside the follicle, cells divide to form the shaft, and growth runs in cycles. The same review reports that scalp hair spends roughly 2 to 6 years in its active phase, that these hairs lengthen by approximately 1 cm per month, and that people lose about 100 hairs a day on average, a number that can double with shampooing. Some daily shedding is part of the normal cycle.

The follicle also has a sebaceous gland, which releases sebum. The review describes sebum as lipid-rich and says it protects the hair and lubricates and waterproofs the hair and skin.



The Cuticle: The Outer Shield


The cuticle is made of flat, overlapping cells, often compared to roof shingles. A 2015 review in the International Journal of Trichology describes it as a chemically resistant region, and a 2023 review in the journal Polymers says it acts as a barrier that protects the cortex and is responsible for the hair's texture and smooth appearance. The two reviews give different figures for how many overlapping cells make up the cuticle (the 2023 paper says about 6 to 10), so treat any single number as approximate.

On the surface sits a thin fatty layer. The 2023 review identifies it as 18-methyleicosanoic acid, or 18-MEA, bonded to keratin proteins and described as the hair's first hydrophobic, or water-repelling, defensive barrier. It also reduces friction between fibres. Both reviews report that alkaline chemical treatments can remove it, which leaves the surface more water-attracting and increases friction.



The Cortex and Medulla: Strength and a Quiet Core


Beneath the cuticle, the cortex accounts for about 90 percent of the hair's weight, according to the 2023 review, and it is primarily responsible for the hair's mechanical properties. It is built from keratin, a fibrous protein, held in shape by bonds including hydrogen bonds and disulfide bonds. The 2015 review notes that relaxers and perms work by breaking and reforming some of these bonds, while the 2023 review says bleaching disrupts disulfide bonds and makes the cuticle more porous.

The medulla is the least understood layer. The 2015 review reports that it is present in coarser hair, such as grey, thick and beard hair, and absent in fine children's hair. The 2023 review describes it as loosely packed and disordered, and calls its contribution to the hair's mechanical properties negligible. So if you have heard that the medulla decides how strong your hair is, these reviews do not support that.



What Everyday Habits Do to the Outer Layer


The 2023 review states that mechanical damage is the most frequent source of damage to hair, and that repeated brushing movements in the same area can cause cuticle edges to peel off. It also reports that anionic surfactants, a common class of cleansing ingredient, can dissolve structural lipids and proteins, and that blow-drying can make the scale edges concave, with damage increasing as temperature rises. The 2015 review lists rough washing, unprotected drying, friction, sunlight and alkaline chemical treatments as factors that reduce the surface lipid.

Putting that together, a few practical takeaways follow from the research:

  • Friction adds up. Detangle gently and avoid repeatedly working over the same section.

  • Heat matters. Lower temperatures cause less cuticle damage than higher ones, according to the 2023 review.

  • Chemical services change the bonds in the cortex. Ask a licensed professional about the risks before bleaching, relaxing or perming.

If a gentler detangling step is on your list, the Crown Care Detangling Hair Brush is one option to look at.

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What Conditioning Products Can and Cannot Do


The 2015 review explains that conditioners act partly by neutralizing the negative electrical charge of hair fibres and by lubricating the cuticle, which improves combability and reduces frizz. The 2023 review adds that conditioners made for dry or damaged hair contain molecules that build up on damaged cuticle scales and fill the fractures, and that intensive conditioners can temporarily replace the 18-MEA layer.

The key word there is temporary. In the passages we reviewed, the effects are described as coating, lubricating and smoothing the surface. This article does not claim that any conditioner permanently rebuilds a damaged strand, and these reviews do not describe it that way. Smoothing and lubricating the surface is still a worthwhile job, because it reduces the friction that the research links to everyday damage.

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Hair varies from person to person in diameter, texture and treatment history, and the science continues to evolve. If you are worried about shedding, itching or changes in your scalp, a board-certified dermatologist can examine your scalp and explain what is going on.



Sources


This article is for educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any condition. Talk with a qualified healthcare provider about skin or scalp concerns.

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