Anatomy of Hair: Hair Care Begins with Understanding Its Structure

｜What does hair look like?

The skin is composed of a sophisticated structure consisting of the stratum corneum, dermis, and subcutaneous fat. This structure forms the skin barrier, firmly supports the skin, and maintains the balance of oil and moisture. Hair is not so different. Although it appears to be nothing more than a thin strand, inside lies a complex world where each layer performs a different role, much like the skin. From here, we will take a closer look at the basic structure of hair and the role of each part, step by step. By the time you reach the end of this content, you will have a clear understanding of the solutions your hair truly needs.

｜1. The Hair’s Protective Shield: The Cuticle

Take a few strands of your hair and run your fingers from top to bottom. Your fingers will glide down smoothly. However, if you swipe upward from the bottom, you'll feel a stiff sensation as if something is catching somewhere. This subtle difference is the moment when you are feeling the cuticle, an invisible layer that constantly surrounds the hair, through your fingertips.

The cuticle is a protective barrier on the outermost surface of the hair that shields it from external stress, but it also serves another important function. At the same time, it plays a key role in determining the hair’s shine and smoothness. The cuticle is composed of multiple thin cell layers stacked on top of one another, like fish scales. Its surface is thinly coated with a natural lipid layer known as 18-MEA (the F-layer), which gives it a smooth, soft feel.

The scale structure of the cuticle affects how much light is reflected from the hair surface, depending on how flat and densely aligned it is. Well-organized cuticles create shine, whereas damaged and cracked cuticles result in a dull and rough appearance. The cuticle can also slightly lift or swell in response to environmental factors such as heat, humidity, and pH, changing how the hair feels and looks. In this way, the cuticle goes beyond simply protecting the inner structure; it also plays a central role in the hair’s natural shine.

｜2. The Core That Gives Hair Its Vitality: The Cortex

When you pull a single strand of hair, you may notice that it stretches more than expected. This remarkable elasticity comes from the hair cortex, which lies beneath the cuticle and makes up the majority of the hair shaft. The hair cortex is the key component that determines the hair’s characteristics, and it has a highly delicate and complex structure. It is composed of fibrous structures assembled from keratin proteins, along with matrix proteins that fill the spaces between them.

Within the hair cortex, numerous keratin fiber structures known as intermediate filaments come together to form a strong framework. These fibers are created as thread-like keratin chains twist together, and those threads then assemble into progressively thicker bundles. Like multiple strands of thread twisting into a single, sturdy rope, the keratin fibers formed in this way are further organized into larger bundle structures, completing the skeletal framework of the hair. As a result, the hair becomes strong and resistant to breakage.

However, strength alone does not explain the hair’s natural movement and bounce. Between the rigid keratin fibers lies a flexible protein layer called the matrix, which is one of the key elements that gives hair its movement. The matrix is a soft protein layer without a fixed shape, and it retains moisture and surrounds the keratin fibers.

Through the harmony between rigid keratin fibers and flexible matrix proteins, hair is able to bend without breaking and move with a natural, flowing bounce. In other words, the hair’s strength, elasticity, and flexibility all originate in the hair cortex. If your hair breaks easily or feels stiff, it may be a sign that damage has progressed into the hair cortex, which makes up the largest portion of the hair structure.

｜3. The Center of the Hair: The Medulla

The medulla is a layer located at the very center of the hair, like the core of a pencil. It is largely an air-filled space, and its functional role is still not well understood. In fine hair, the medulla may be absent or appear only intermittently. When the medulla is present, chemical reactions during treatments such as perming or dyeing may occur more slowly, as chemical agents must penetrate through the hair’s internal structure to take effect.

｜4. The Adhesive of Hair Structure: CMC

How are the cuticle, hair cortex, and medulla described above connected to one another?

The Cell Membrane Complex (CMC) acts as an interface that invisibly links the hair structure into a single unit. It allows each structural component to remain securely in its proper position. The CMC is composed of lipids such as ceramides, fatty acids, and cholesterol. It not only fills the spaces between each tissue, but also serves as a pathway for moisture movement. Although it is an extremely thin layer that cannot be seen with the naked eye, the health of the CMC is essential for maintaining the integrity of the entire hair structure.

｜Why Understanding Hair Structure Matters

Most hair concerns, such as damage, split ends, and dryness, begin when the hair structure breaks down. For example, when the cuticle is damaged, hair loses its shine and becomes more vulnerable to external stress. When the hair cortex is damaged, the hair’s strength and elasticity decrease, and protein loss causes it to break more easily. Furthermore, when the CMC that connects hair strands weakens, damage can progress more rapidly.

By understanding the structure, it becomes possible to identify the root cause of the concerns more accurately and determine the right approach to hair care. Healthy hair doesn’t begin with shine alone. True hair care starts with an understanding of the hair's structure from the inside out.