A Simple Guide to Hair Structure: The Cuticle, Cortex & Medulla

You’ve probably heard the words “cuticle” or “cortex” thrown around when it comes to hair—but do you actually know what they are or why they matter? Understanding the structure of your hair can explain so much about why your hair behaves the way it does (or doesn’t).

Hair structure plays a huge role in everything from how your hair holds a style to how it reacts to color, heat, and humidity. So in this post, I’m breaking down the anatomy of a single hair strand—what each part does, how it impacts your hair’s overall health, and how structure ties into things like texture, density, and wave pattern (AKA your actual hair type).

Once you understand what’s really going on inside your hair, it becomes a lot easier to care for it, style it, and pick products that actually work for you.

Hair Structure 101

Before digging into the hair type categories, let’s first begin by understanding the anatomy and structure of a single strand of hair.

The 3 layers of a single hair strand

Every strand of hair is made up of two to three layers. Naturally blonde and fine hair contain two of these layers, while coarse hair contains all three, but we’ll get to that in just a moment. The three layers of a hair strand are the cuticle, cortex, and medulla.

Cuticle – Outer layer of a hair strand

The cuticle is the outermost layer of a hair strand. It resembles fish scales that open and close. This is similar to the pores on your skin when heated or cooled, as well as with water or steam.

Hair that is damaged has a roughened and very open cuticle, while hair that is healthy has smooth scales that close and seal nicely. This is why damaged hair appears frizzy, and also why it does not hold on to hair color as well.

If the outer scales don’t close properly, the color molecules can slip out much more easily when shampooed, or even onto your pillowcase. This layer is the protective covering for the next layer, the cortex.

Cortex – Middle layer of a hair strand

The cortex is the middle layer of your hair structure. This is the layer that gives hair its strength, elasticity, color, and thickness.

When hair is colored (with permanent color), the color molecules need to get through the cuticle and penetrate this layer. If the cortex is weak or damaged, then the elasticity will be poor, and the hair will not stretch and return to its original state.

If you take a strand of wet hair and stretch it, it should feel strong and sturdy. When damaged hair is stretched, it acts like bubble gum and typically just breaks off. The cortex becomes damaged when the cuticle is damaged, because there is no longer that protective layer of scales covering it to keep it in good health.

If this layer is damaged, the hair will also lose its wave pattern and go straight and limp. So if you have curly or wavy hair and the ends of your hair are straight and stringy, your hair is likely damaged, and those straight ends need to be cut off.

Medulla – Inner layer of a hair strand

The medulla is the deepest, innermost layer that makes up your hair structure. Not every hair type will contain this layer. It is mostly seen in coarse and naturally dark hair strands. Naturally blonde and fine hair does not have this layer. The medulla is very small in diameter and is the most fragile of all three layers.

The purpose of this layer is still not completely known, considering not all hair even contains it. But hair that does contain this layer is typically much stronger and can take more abuse than hair that does not have a medulla, due to the extra reinforcement.

That being said, hair with a medulla will also be more difficult to lighten and will take a higher temperature to curl or straighten.

The 3 Categories That Determine Hair Type

Now, it is really easy to get confused about what hair type you have. A lot of people believe that if you have fine hair, then you have thin hair and vice versa. But that’s not always the case!

All heads of hair are different, and you can have any combination of the 3 categories. As a hair stylist, I have seen it all.

The three main categories that will determine your hair type are texture, density, and wave pattern.

I originally had all of this information crammed into one blog post, but I realized it was just TOO much. The post was over 6,000 words! So I divided each category into its own post that you can access below to learn more about if you want more info than the summary I provide here.

On each individual post, I describe the hair type, its strengths and weaknesses, as well as style and product recommendations that will work for you based on what category you fit into.

Hair texture

The texture of your hair is the thickness of each individual hair strand. It can be fine, medium, or coarse.

To learn more about what texture your hair is, visit my post here.

Hair density

Your hair density is the amount of hair that you have per square inch. This can be thin, medium, or thick.

To discover the density of your hair, see my post here.

Wave pattern

The wave pattern of your hair is the shape your hair takes in its natural state. It can be straight, wavy, or curly. Curly hair can also come in many shapes and sizes.

To read my post on wave pattern, click here.

Hair Structure Conclusion

To conclude, understanding the structure of your hair is important for making informed decisions about hairstyles and hair care.

Every strand of hair is made up of three layers: the cuticle, cortex, and medulla, each playing a specific role in determining the health, strength, and appearance of your hair.

The cuticle is the protective outer layer (resembling fish scales), and its condition affects how well your hair holds onto color and resists damage.

The cortex (the middle layer) gives the hair its strength, elasticity, and thickness. Damage to the cortex can lead to weak, brittle hair that loses its natural wave pattern.

Lastly, the medulla (not found in thin and naturally blonde hair) offers extra reinforcement to the strand, making it stronger but also more resistant.

Recognizing your hair type is equally as important, as it falls into three main categories: texture, density, and wave pattern.

Hair texture refers to the thickness of each individual hair strand, which can be fine, medium, or coarse. Hair density relates to the amount of hair per square inch, varying from thin to thick. And the hair wave pattern is the natural shape your hair takes, like straight, wavy, or curly.

By learning about your hair type, you can choose hairstyles and hair care routines that will work for you instead of against you. Hairstyles that work well for one hair type may not be as manageable for another.

Next time you bring a picture to your stylist, make sure to ask if it will work well with your hair type instead of landing on something that may not be the best option for you.

If you have any further questions for me on this topic, leave them for me in the comments. And as always, Have A Good Hair Day!