The Cortex: Providing Substance for the Human Hair Shaft

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If you’ve ever wondered why your hair is fine or coarse, brown or black, you’ve actually been questioning the contents of your hair cortex. Although the human hair shaft is comprised of the cuticle, cortex and, depending on the size of the strand, the medulla, it’s the cortex that makes up the bulk of the hair strand. It is also responsible for the display of your hair’s genetic code, which includes color, hair type and texture.

It’s this part of the human hair shaft that is the target for chemical alteration in regards to natural color and curl pattern and the most in need of protection by the cuticle.

What’s in your hair’s genes?

The cortex is a complex system of cells. There is a mass of cells called the dermal papilla which acts as the boss to the entire hair growth system. It is instrumental in the development of the original hair follicle and dictates the size of the hair fibre that will eventually be produced. (For more information about the hair follicle and how it reproduces hair, see Human hair follicle: Your hair growth factory.)

The larger or more broad the dermal papilla surface, the thicker the hair strand that will form within its respective hair follicle. Not every follicle on your head is exactly the same, however. This is why it’s possible to have varying textures or hair types on different parts of your head.

The hair matrix takes its direction from the dermal papilla. The cell division and replication that occurs within the hair matrix create the hair shaft and produce the necessary cellular material required to form the hair fibre.

Keratin and melanin cells both originate from this cellular process. These cells undergo a process of keratinization, which is fancy for die off and harden. The keratinized cells get pushed up into the follicle duct and advance the hair strand upward. In other words, they add to the base of your hair strand creating the growth you see above your scalp line.

Protein packs a punch.

Human hair is made up of a type of fibrous protein called keratin, which consists of a combination of 18 amino acids essential to hair health. It is an insoluble protein, which contains large amounts of cysteine. This particular amino acid is responsible for the rich sulfur content and plays an important role in the structure and cohesion of hair.

Within the cortex, the keratin is organised into protofibrils, composed of 4 chains of keratin. The protofibrils can be compared to a rope-like structure, where its strength is dependent on the bonds or bridges between the atoms of individual chains. These bonds are of variable strength.

Disulphide bonds are the strongest bonds, formed out of the attraction between amino acid cysteine and other sulfur containing molecules. The placement of these bonds form the shape and structure of your hair strand. Cysteine amino acids are capable of bonding with other cysteines further down the hair shaft and is the contributing factor to any curling of hair.

Curly hair has more of these bonds than straight hair. The follicle’s shape and resulting angle that it travels towards the surface of the skin allows for different parts of the hair shaft to come close enough together to allow a bond to form.

Acting on the sulphurated parts of the keratin amino acid chain, disulfide bridges can only be broken with the use of chemicals. Weaker bonds, such as hydrogen bonds, are ones that can be altered with the addition of water alone.

Found amongst these keratinized cells are pigment cells, or carriers of melanin.

Are you high on melanin?

The color of your hair depends on the shade and amount of pigment located in the cortex, with some influence coming from tiny air spaces found within the hair itself. Melanin represents only 1% of the total composition of your hair, so its influence is impressive despite its volume. There are two melanin types found to represent the spectrum of available hair colors; eumelanin and pheomelanin.

Eumelanin provides black and brown pigment and is particularly abundant among black populations. The absence of eumelanin dictates whether hair is blonde or not.

Pheomelanin provides pink and red colors and is the main pigment found among red-haired individuals. Aside from providing color, melanin’s functional purpose is for protection against UV-radiation. Pheomelanin is nowhere near as protective as eumelanin.

Are you starting to find white hairs? That means pigment is absent from the cortex. The contained air becomes reflective of light and is responsible for the whiteness. The difference between white and gray hair is generally associated with the overall mixture of white and colored hairs you have.

What shape do you subscribe to?

Mechanically speaking, the cortex is largely responsible for the elasticity and tensile strength of the hair fibre. It is made up of cortical cells, which when counted as one unit, comprise 75 to 80% of the strand’s overall volume.


Until closer examination, the general thought was that every hair strand had a round shaped circumference. These cross-sections, representing various hair types, highlight the fact that hair strands are as unique as the head they come from. The composition of the cortex and the shape of the cortical cells included in it, define the outer shape of each hair strand.

It’s possible for an individual cortical cell to be one of two different classifications, where the combination of these differing cells dictate your hair type and resulting curl pattern.

Cortical cells define ethnic differences.

Not all cortical cells are shaped the same. Dr. Ali N. Syed does a great job explaining the complexities of the cortex. There are two main types of cells, ortho-cortical and paracortical. Para-cortical cells are shaped like spindles and are fairly consistent in shape, although overall size can vary from one to another.

If you have straight hair of Asian or European descent, your cortex will be primarily made up of these. Bone straight hair only has para-cortical cells, as they organize themselves in a uniformly stacked fashion. When hair starts to become wavy, or expresses some curl, you can expect to see the introduction of ortho-cortical cells in its structure.

Ortho-cortical cells do not have a uniform shape to them and one might even call them random in their makeup. Much like its para-cortical counterpart, they are also generated in various sizes.

Curlier hair types have a cortex filled with a majority share of these ortho-cortical cells, with a small amount of para-cortical cells, typically lining one side of the hair shaft. Considered an example of mixed race hair, you can expect larger, fuller curl patterns as a result of this composition.

The curliest hair types have equal shares of both otho- and para-cortical cells. It may be the struggle for cortex domination occurring between the two types that causes the exaggerated curl definition found in African descent hair.

An interesting hair type cross-section comparison between straight and curly hair shows the uniformity of straight hair clearly. The curly haired strand presents itself in a whorl pattern, much like you would see in a fingerprint. If you think about the vast number of fingerprints in the world, this could be some indication of the vast number of hair combinations that are actually possible. Left, curly hair – Right, represents straight hair.

Dry, broken and lacking elasticity.

The extent of the dryness of your hair can be attributed to something called cell membrane complex. There are two forms in the human hair strand. One type is found at the cuticle layer and another within the cortex used to bind the keratinized cells together.


Cell membrane complex is a lipid material formed out of essential fatty acids, ceramides and cholesterol. It performs an important role in structure, tensile strength and elasticity. Let’s try to apply an analogy here to improve your understanding.

Think of the internal cell membrane like window putty. In older windows, when seating a pane of glass, they often used a putty because it was flexible and would allow for expansion and contraction of the glass during changes of temperature, without compromising its function. If that putty were to dry out, the glass would become loose and rattle if windy or possibly even fall out if moved.

Cell membrane complex operates in much the same way. It binds the cortex together, allows for flexibility in the hair shaft and provides a built-in pocket of moisture to keep the hair strand pliable and responsive to manipulation.

The cuticle has its own version of cell membrane complex with a slightly different composition. Responsible for protection and moisture retention, the cuticle is the first line of defense when washing your hair. A study was conducted measuring the effects shampoo had on the lipid layers of hair.

One shampoo application can be responsible for extracting approximately 50% of cuticle layer fats, with repeated shampooing increasing the reduction to between 70 and 90%. When the cuticle is doing its job, it is believed that the internal fats found in the cortex are not affected to the same extent.

When an extreme depletion occurs, the internal fats migrate from inside the cortex of the hair shaft to the outside cuticle layer in an attempt to remedy the depletion. You can imagine that if this cycle happens enough, the cortex will become dry and brittle and you will experience breakage.

Because the hair shaft is dead, the lipids don’t have the ability to restore themselves. Much like the window analogy, when the cell membrane complex is dried out it loses its functionality.

The sebum produced from the sebaceous gland would be ineffective in trying to coat the curly hair shaft since it would not have the benefit of gravity to help it slide along the shaft.

In most cases of extreme dryness, new growth is the best replacement.

There are reports that coconut oil has a natural penetrating ability when applied to human hair because of the chemistry of its fatty chains. Applying coconut oil may provide some temporary relief from dried out hair but the jury is still out on whether or not it would revitalize a cortex depleted of all its built-in moisture.

Water: Friend or foe?

We tend to have a love/hate relationship with moisture. On the one hand, we crave it to keep our hair looking healthy and free of frizz. On the other hand, when moisture asserts itself into your hair situation when you least expect it—think high humidity—you are less than impressed with what it does to your style.

So what’s a girl to do? Knowledge is the first line of defence. The proteins that make up the human hair have a natural affinity for water. When water is introduced, either directly or indirectly, as in the atmosphere, your hair strand seeks to soak it up.

Dr. Andrew Avarbock, who writes for the New York Times, claims that the better hydrated your hair is the less it will react to environmental moisture conditions. He explains that the swelling of the hair strand, evidenced by the appearance of frizz, is a direct result of dry, or porous hair soaking up water from the atmosphere. The introduction of water alters any existing bonds between your keratin proteins.

Many a battle has been lost with humidity, but if optimal cortical health plays a part in conquering this one, improved care is so worth it.

Give your cortex a head start

Knowing that your hair is made of protein, water and essential fatty acids, direct attention to your diet can provide positive results. Most people are not deficient in protein so we’ll focus on the increase of essential fatty acids and water.

One of the key essential fatty acids is Omega-3. You can find this abundantly in foods like salmon, mackerel, tuna, white fish, sardines, egg yolks, walnuts and hemp seeds. If these food sources don’t appeal to you, you can always try taking an omega-3 or fish oil supplement.

Increase your water intake. The more water you drink, the better your skin looks. Since hair is a direct descendant of your skin, keratinized protein, it makes sense that it will improve how your hair looks and functions as well.

Not sure you can see a difference? Watch your fingernails for clues. Also made of keratinized protein, your fingernails will display hydration deficiencies that are easy to recognize. If your fingernails begin to break more, look dry or lose their natural luster, you can be certain your hair will do the same, if it isn’t already.

As with most things, what you put into it is what you get out of it. Our bodies are no different. Try increasing your essential fatty acids and water intake and watch the natural shine and elasticity of your hair come back to life.

Have you made any observations about your hair’s cortex and how it responds to treatment? If you’ve got some hidden gems, please share with us below. It’s our goal to make everyone’s hair life better than ever.

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