How to nourish hair from the inside? Part 1: hair structure (cortex, medulla, cuticle) and keratin. Permeability, vulnerability, lipid barrier

Beautiful, healthy hair begins on the inside. Knowing your hair’s structure and what makes it strong or weak will help you nourish it to the fullest.

The cortex, medulla, and cuticle are the three main layers that make up hair. Each is essential to preserving the health and beauty of your hair. The protein keratin, which gives hair its strength, is found in the thickest layer, called the cortex. The cuticle serves as an outer layer of protection, and the medulla, which is found in thicker hair types, is the innermost layer.

The way these layers interact determines how permeability and vulnerability your hair has, particularly if the lipid barrier that shields it is damaged. You can choose the best ways to nourish your hair from the inside out by understanding its natural defenses and structure.

Keratin

Trying the post this way and that, I came to the conclusion that it makes the most sense to actually come from a distance. The problem is that we typically attempt to make up for hair damage with hair care products, but it is challenging to discuss ways to stop or make up for damage without first identifying the precise types of hair damage that can occur. So let’s go back to the beginning.

Model of two different keratin types’ alpha helices twisted into a dimer. Image sourced from proteopedia.org.

Keratin makes up 80–90% of hair. Lipids for protection and water for elasticity make up the majority of the remainder. The hair shaft is essentially made up of various forms of keratin that have been gathered and joined together with keratin cement. Although the cuticle and cortex have different structural characteristics, the entire hair is impacted by anything that affects keratin.

Composition of keratin

So what is the structure of a protein? Individual atoms (primarily carbon, hydrogen, oxygen, nitrogen, and sulfur) are assembled to form amino acid molecules. These are comparatively small molecules—around 100–150 daltons, on the smaller side—for organics. Water weighs eighteen, while a molecule of coconut oil weighs between 600 and 800.

Amino acid composition of keratins with elevated sulfur levels. They have shortened names in one letter, and each letter represents an amino acid. Image sourced from ijtrichology.com.

Natural proteins are assembled from 20 different amino acids. They can be imagined as beads. There sits such a craftswoman-Nature, she has 20 boxes with sorted beads, and she strings them as she pleases. She wants three identical ones, and she wants ten out of place. If she likes a certain sequence, she puts it aside. The resulting amino acid beads are protein. The shortest ones consist of 15-20 amino acid beads, and the longest ones number thousands. There are infinitely many variations.
Nature does not rest on this. She takes identical beads and connects them together (yes, I mean polymerization), getting very, very long beads with a repeating pattern. Of course, such long ones are inconvenient to hold, so the beads are laid out, for example, in a spiral. To prevent the spiral from unwinding, it is fixed with… uh, disulfide bonds. Here my bead analogy is stuck, so let"s go back to atoms and molecules.

Translation of the image from here: ncbi.nlm.nih.gov. An attentive reader may notice that here are intermediate filaments, and in the text further – protofilaments. This is because both fibrils and filaments are names of structures, and structures can be repeated during assembly.
Before me, Zavitushki described everything in detail and wonderfully in her post about keratin, so I will only briefly summarize everything. Keratin is a complex protein. Moreover, it is not a protein, but a group of proteins with similar properties, and even in human hair there are 15 types of them (in humans, 56 are encoded). Of the 20 possible amino acids, 18 are involved in keratin, and the weight of the molecule is about 40,000 – 70,000 daltons. Long beads! And this is only the length of the most basic sequence, this is how chains are formed from many such molecules, which are then compactly twisted.

Image sourced from saylordotorg.github.io.

Basically proteins come in two states – alpha, that is, in the form of a spiral, and beta, that is, as if simply folded in a relatively flat form. There is also a "disorderly ball" option, but our keratin is not like that! In hair, the spiral version is mainly found. Keratin is coiled into a spiral, the spirals of two different types of keratin are coiled into a double helix, a dimer. 2 dimers are coiled into a protofilament, 4 protofilaments into a protofibril. Protofibrils are coiled into a microfibril, and those massively form macrofibrils. Macrofibrils are already quite large respectable structures that are held together by keratin cement and form, in fact, the hair structures. From this, we can conclude that everything in the hair is quite twisted. And long. How does it all hold together? No, of course, with such a twist, the molecules can simply get tangled and not unravel, but it does not sound very reliable.

Keratin structure

Protein structures are classified according to their organizational level. The amino acid sequence is known as the primary structure; the protein molecule’s helix, or form of twisting and folding; the helices’ entanglement with one another to form globules or fibrils; and the arrangement of these globules/fibrils is known as the quaternary structure. Not every protein has an upper structure.

Image sourced from saylordotorg.github.io.

1. Our DNA contains a clear primary structure that the body will not deviate from (generally, a different sequence results in a different protein with a different set of properties). Peptide bonds bind the constituents of the primary structure, or amino acids. This is the strongest link, the fishing line for our beads.

2. The sequence of amino acids determines secondary structure. Certain ones have the ability to form new chemical or physical bonds. If nearby interacting amino acids of the same protein are present during the spiraling process, they form a bond and prevent the spiral from unraveling because they are further bound inside. These bonds may be disulfide, ionic, or hydrogen-based.

3. The location of the globules in relation to one another and the manner in which the protein folds into globules are determined by tertiary and quaternary structure. Since keratin starts combining with other molecules after the secondary (helix), it lacks these structures.

Bonds in keratin and ways to break them

1. A peptide is the link that exists between the carboxyl and amino groups, which is a feature of amino acids. We can discuss the breakdown of the protein if the peptide bond breaks, and this type of reaction is thought to be irreversible. Ammonium hydroxide is one of the alkalis and acids that can break this bond. Additionally, I came across information about how heating keratin to temperatures above 140 degrees causes appreciable changes in its characteristics. Additionally, proteins are destroyed by direct UV exposure. To put it briefly, you can learn how to ruin keratin irreversibly. It can only be patched up; restoration is not feasible.

2. Disulfide bonds guarantee the protein’s strength at the macro level if peptide bonds are the fundamental component of proteins. Two sulfur atoms can bond together to form disulfide bonds. Though not as strong as a peptide bond, this is a fully formed chemical bond that is extremely powerful. It guarantees the protein’s shape stability.

Image sourced from saylordotorg.github.io. This picture displays every conceivable bond as well as an illustration of a secondary structure. a) Ionic bonds between amino acid charges; b) Hydrogen bonds; c) Disulfide bonds (sulfur is represented by the letter S, not by the number 5); d) Repulsive forces, which also help to maintain the chain’s shape.

The number of sulfide bonds depends on the amount of sulfur in the protein. In human hair keratins, it is 5% on average. But the sulfur content is uneven: there is more of it in the upper layers of the cuticle, ensuring its strength, and less in the hair fibrils themselves. It is important to understand that due to disulfide bonds, not only a spiral or flat lay is formed within one protein molecule. Most of these bonds are intermolecular. Two spirals of different keratins are held together by disulfide bonds. There is also a lot of sulfur in the matrix, the cementing substance between the fibrils, since it needs to bind the fibrils.
In short, the strength of the hair from the base spiral of the protein to the bundle of macrofibrils and the upper layer of the cuticle is provided by disulfide bonds. Having seriously damaged them, you cannot expect that the hair will be in order.

The same conditions that break peptide bonds also break disulfide bonds: light, exposure to acids and alkalis, and temperatures higher than 80 degrees. It is simpler to break them. Additionally, there are specific procedures like keratin straightening and perm that are meant to break disulfide bonds. Special reagents cause disulfide bonds to break, causing the hair to soften and change shape before additional reagents are added to repair the bonds. However, not all bonds are strengthened, which eventually weakens the hair structure.

3. Ionic bonds: connections between amino acids and charged particles. Certain amino acid components can become charged in water (and hair does too), and the attraction between these parts keeps the hair in its original form.

These are more disordered, readily broken and reformed physical bonds rather than chemical ones. They also help hair keep its structure. For instance, the majority of the amino acids in the matrix have a second amino group (positive charge), and the surface of the fibrils has a second acid group (negative charge), which adds strength and preserves mobility (the bonds are non-directional and can break and re-form).

The reasoning behind hydrogen bonds is the same as that of ionic bonds: they are physical bonds that are far weaker than chemical ones. Since hydrogen bonds are what make water a liquid rather than a gas, they are extremely common. Water molecules are drawn to one another and form chains because they have different charges at different ends. Hydrogen bond formation is another ability of protein OH groups. Image sourced from saylordotorg.github.io.

Although hydrogen bonds are frequently broken and then reformed, they help keep hair in its natural shape. They are somewhat controllable because hydrogen bonds are formed when the hair dries, and if we style it using a brush or curl it, the hydrogen bonds will hold it in place for a while. The hair will absorb moisture from the rain or simply from the humid air, allowing the hydrogen bonds to reorganize and return to their original positions. Unfortunately, the way your hair is designed to lie causes frizz or straightening.

Morality

A complex arrangement of amino acids called spirals, double spirals, bundles of spirals, bundles of bundles, and so on make up the keratin found in hair. This keratin is filled with a cementing type, and it is covered in more than six layers of denser keratin on the outside. With wet styling and external fixation, we can virtually painlessly alter the shape of the hair; however, any internal color change (herbal coloring is a different story) or long-term structural alteration results in irreversible or nearly irreversible damage. As you were aware of it already.

Knowing the structure and functions of hair is crucial to nourishing it from the inside out. The cortex, medulla, and cuticle are the three layers that make up hair. The medulla is the innermost portion, the cortex gives structure and strength, and the cuticle serves as a layer of defense. Hair’s vital protein, keratin, is essential to its health, and the lipid barrier shields hair from harm. Selecting the proper nutrients and treatments to maintain strong, healthy hair requires an understanding of the permeability and vulnerability of these layers.

Knowing the structure of hair is crucial to properly nourishing it from the inside out. Your hair’s strength and condition are influenced by its cortex, medulla, and cuticle, with keratin playing a crucial role in preserving its resilience.

Hair is vulnerable to damage due to its permeability, especially if the lipid barrier is compromised. Maintaining moisture and shielding the hair from outside stressors are made possible by this natural barrier.

Understanding these foundational elements of hair will help you make better decisions about nutrition and care, which will ultimately support the growth of healthier, stronger hair from the inside out.

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