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Hair Cuticle: Hair’s First Line of Defense

April 5, 2023


Verna Meachum

Image of a hair cuticle - healthy and damaged.

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Curl care

With extensive experience in the beauty industry, I specialize in writing for curly hair care brands, websites, and magazines.

Not only do I have curly hair, but my children, friends, family members, and even friends who are professional curly hairstylists, each with their unique curly textures.

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I also partner with a friend who holds a Ph.D. in chemistry and works as an R&D Chemist, ensuring our content is scientifically accurate and help us navigate through the misinformation around curly hair care. 

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Last Updated on April 24, 2023 by Verna Meachum

Hair is an amazing thing. It’s our crowning glory and the first line of defense against environmental damage.

And at the heart of it, all is the hair cuticle – the outermost protective layer made up of many overlapping cells that guard hair strands.

Not only does this provide protection, but it also helps keep moisture in. Hair cuticles are like tiny shields for your hair which is why it’s important to care for them properly.

The outermost layer of our hair, the cuticles, are often displayed to the world. Consequently, taking care of them by using suitable products and grooming techniques is a necessary step in achieving healthy hair that looks smooth, glossy, and well-moisturized.

By exploring the science of our hair cuticles, their importance in hair care science, and their functional benefits in hair outlook. 

Hair Anatomy

Illustration of a hair structure.

Human hair is a remarkable composite material composed of protein fibers made up of keratin, with characteristic structural units.

Hair has a cylindrical shape comprising three main layers.

  • The uppermost layer is the “Cuticle.
  • The “Cortex” lies underneath the cuticles.
  • The innermost region in the middle of the fiber is the “Medulla”.1
Illustration of medula, cortex, and hair cuticle.

The cuticles and cortex are the key components of the hair structure.

  • Cortex makes up most of the mass of hair and is responsible for the mechanical strength of hair.
  • Cuticles are the outermost layer and the site of the first interaction with cosmetic products. The cuticles play a crucial role in protecting the inner core of the hair and therefore have a vital role in hair structure and its cosmetic properties.

What are Cuticles?

Cuticles appear as small scales over the hair shaft like tiles covering the roof and their external edges point towards the hair tips (ends).

Every single cuticle is approximately 0.5 – 1.0 um thick and 40 – 50 um long while one single spot, might have 5 – 10 cuticles superimposed on each other making a layered sheet over the hair shaft.2-3 

Despite being tiny in thickness, they play a vital role in defining the surface properties of the hair fiber.

Additionally, they act as a barrier against external aggressions like penetration of water, active ingredients, and humidity changes.

The cuticles are the first point of contact for any treatment or chemical applied to hair. All ingredients, actives, and chemicals (such as coloring dyes) must pass through the cuticles.

Cuticles, just like the rest of the hair, are made of keratin. The amino acid analysis of isolated cuticles demonstrates the higher cystine content compared to the cortex.

Cystine is a sulfur-rich amino acid, which indicates that cuticles are highly cross-linked, the structure is compact, and the protein molecules are tightly packed.

The cuticle is further divided into sub-layers.

  • Epicuticle – the outer layer of the cuticle layer.
  • Exocuticle – the middle segment of the cuticle layer.
  • Endocuticle – the innermost portion of the cuticle layer.

The epicuticle has a hydrophobic lipid layer on its outer surface. This layer is known as the “F-Layer”, which is a water-repellent layer containing a unique fatty acid called, methyl-eicosanoic acid (18-MEA).

This fatty layer is responsible for the sheen, gloss, and water-repellency of the hair fiber. This can also be called a natural lubricant and conditioner for the hair surface.4 

How the Hair Cuticle Protects Your Hair

The cuticle layer covers the entire surface of the hair shaft, protecting the inner cortex from the outer physical and chemical insults (harm).

Additionally, this layer is responsible for defining the surface properties of a hair fiber such as its bending (flexibility) – an essential element when styling.

Our hair is exposed to physical and chemical wear-and-tear daily, such as brushing, combing, and applying chemicals.

Chemical treatments initially act on the cuticles. Likewise, thermal styling applies heat to the cuticles of hair strands.

With all this stress levied upon them, it’s no wonder why most of us suffer from split ends or dullness at some point in life.

For the hair to endure physical wear and tear as well as harsh chemical treatments, it needs a strong and healthy cuticle layer. This will serve as armor against such aggressions while safeguarding the inner cortex beneath.

The compact nature of cuticle protein resists any passage of actives under normal conditions, while its F-layer – which is resistant to water – protects against dramatic fluctuations in humidity and moisture content in the surroundings. 

Factors that Can Damage the Hair Cuticle

Image showing levels of damage to the hair cuticle.

The cuticle can be damaged by any of the following factors.

Physical abrasion – repeated and aggressive combing, brushing

Scientific studies have found aggressive combing and brushing can remove cuticles from the shaft.

This occurs more at the tips (ends) of hair because of the aging factor, meaning the cuticles on the tips are the oldest part of our hair and are more fragile and prone to breakage.

The tips are always more damaged compared to the hair at the root area. Moreover, wet hair is especially sensitive to this type of physical damage.

Therefore, it’s best to be especially gentle when brushing or combing wet hair. This warning rings even truer for those with chemically-treated strands – the risk of causing trauma and harm increases exponentially.5 

Aggressive Chemical Treatments

Alkaline bleaching and permanent coloring contain hydrogen peroxide. The pH of these products is generally 12 – 13 using an ammonia solution. Ammonia dilates the cuticle layer and opens up the passage channel making it easier for dye molecules to penetrate the hair.

Yet, the strong and aggressive nature of ammonia and hydrogen peroxide also oxidizes the cuticle layer by depleting its thickness.

With frequent and repeated use, alkaline hydrogen peroxide can erode the cuticles exposing the cortex.6

Alkaline hair relaxers containing caustic soda, guanidine hydroxide, and perming or texturizing cream containing thioglycolic acid also significantly damage the cuticle layer.7

Thermal Treatments 

Blowdrying and flat ironing your hair can be damaging, as the generated heat energy is strong enough to denature proteins in hair.

Consequently, this could leave cuticles ruptured and chipped off, forming bubbles on the surface – a sign of extensive damage that has been done.8 

Solar Radiation

UV radiation possesses the capability to oxidize hair proteins by emitting high-energy that initially reacts with the cuticle layers of hair, as they are exposed to the surroundings.

When hair is exposed to the environment repeatedly, oxidation occurs on the sulfur-rich cuticles which makes them more soluble in water (cystine is converted to cystic acid).

As a result, these oxidized protein components are dissolved in water and then rinsed off when shampooing.9

What a Healthy and Damaged Cuticle Looks Like

Image showing damaged hair versus healthy hair.

When examined under a microscope, healthy cuticles appear smooth and sleek while the damaged cuticles look ruptured and blistered.

The surface appears to have bumps with eroded patches all along the shaft line.  The damaged cuticle layer also lacks shine under the microscope due to the complete removal of the F-layer

When hair is damaged you can feel it by running your fingers through their hair. The strands lack the smoothness and shine of healthy hair, feeling rough, dry, and brittle instead.

Can you Open/Close a Hair Cuticle?

Yes! Certain chemicals can dilate the cuticle openings. In general, hair swells under alkaline pH conditions (high pH above 9.0) and cuticle pores open up.

Hair dyeing and bleaching are used to increase the penetration of dye molecules, taking advantage of this phenomenon.

Cuticles can be closed back again by lowering the pH using a dilute solution of citric acid or phosphoric acid.

After using hair bleaching, dyeing, or straightening products, a neutralizing shampoo can be utilized to lower the pH level of your hair fibers and ensure healthy results.

How to Protect the Hair Cuticle

Taking just a few simple steps will ensure that the integrity of the cuticle layer remains intact and defended against harsh treatments.

Basic steps

How to Repair or “Fix” Damaged Cuticles

Damaged cuticles require a customized approach to reconstruction, recovery, and restoration. Through the use of custom-crafted products and an optimized hair care regimen, healthy hair can be achieved once more.

Key points:

How to “Seal” the Hair Cuticle

Cuticle sealants are hydrophobic materials such as oils and waxes. Silicone oils are top-rated for their sealant properties. They offer superior lubrication without any greasy impact. However, silicones are synthetic and are not sustainable ingredients. 

Natural oils such as coconut oil, sunflower oil, and olive oil are excellent sealants for cuticles. A delicate layer is formed over the hair shaft, protecting cuticles against UV radiation and physical abrasion during brushing or combing.


The cuticle is the outermost layer of our hair fiber, a vital protector for its inner core, and an essential contributor to strength, appearance, and surface properties.

Acting as armor against environmental harm, it shields our hair from external insults.

Beware of aggressive chemical treatments and physical abrasion, which can alter the physical and morphological properties of the fiber, resulting in damaged hair that looks dull and lacks luster.

To keep your hair looking its best, it is essential to preserve and protect your hair’s delicate cuticles.


1. Zavik, C.; Milliquent, J., Hair Structure, Function, and Physicochemical Properties In The Science of Hair Care, 2nd ed.; Bouillon, C.; Wilkison, J., Eds. Taylor & Francis Group, LLC: London, 2005; pp 29-35.

2. Swift, J., Human hair cuticle: biologically conspired to the owner’s advantage. J. Cosmet. Sci. 1999, 50, 23-47.

3. Wolfram, L. J.; Lindemann, M. K., Some observations on the hair cuticle. J Soc Cosmet Chem 1971, 22, 839-850.

4. Tanamachi, H.; Tokunaga, S.; Tanji, N.; Oguri, M.; Inoue, S., 18-MEA and hair appearance. J. Cosmet. Sci. 2010, 61 (2), 147-160.

5. Robbins, C.; Kamath, Y., Hair breakage during combing. IV. Brushing and combing hair. J. Cosmet. Sci. 2007, 58 (6), 629-636.

6. Jeong, M.-S.; Lee, C.-M.; Jeong, W.-J.; Kim, S.-J.; Lee, K.-Y., Significant damage of the skin and hair following hair bleaching. The Journal of Dermatology 2010, 37 (10), 882-887.

7. Ruetsch, S. B.; Yang, B.; Kamath, Y. K., Cuticular damage to African; American hair during relaxer treatments; A microfluorometric and SEM Study. Inter. J. of Cosmet. Sci 2009, 31 (3), 244-245.

8. Wortmann, F. J.; Wortmann, G.; Marsh, J.; Meinert, K., Thermal denaturation and structural changes of α-helical proteins in keratins. Journal of Structural Biology 2012, 177 (2), 553-560.

9. Nogueira, A. C. S.; Dicelio, L. E.; Joekes, I., About photo-damage of human hair. Photochem. Photobiol. Sci. 2006, 5 (2), 165-169.


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