The shaft of a hair is made up of three concentric layers: an outer protective cuticle, a middle cortex that provides strength and color, and in thicker hairs, a central core called the medulla. All three layers are built primarily from a tough protein called keratin, the same material found in your fingernails and the outer layer of your skin. Each layer has a distinct structure and serves a different purpose.
The Cuticle: Your Hair’s Outer Armor
The outermost layer of the hair shaft is the cuticle, a thin but remarkably tough shell made of flat, overlapping cells. These cells are arranged like shingles on a roof, with each one slightly overlapping the next from root to tip. A single strand of hair can have five to twelve layers of these shingle-like cells stacked on top of one another.
The cuticle’s primary job is protection. It shields the inner layers from physical damage, heat, chemical exposure, and water loss. When your hair looks shiny and smooth, that’s largely because the cuticle cells are lying flat and reflecting light evenly. When hair feels rough or frizzy, the cuticle scales have been lifted or chipped away by damage from heat styling, chemical treatments, UV exposure, or simple mechanical wear from brushing.
Each cuticle cell is itself layered, containing several thin membranes rich in proteins and lipids. The outermost surface is coated with a very thin layer of fatty acids that makes healthy hair naturally water-repellent. This is why undamaged hair initially resists getting wet and why it takes a moment for water to fully saturate a healthy strand.
The Cortex: Where Strength and Color Live
Beneath the cuticle sits the cortex, which makes up the bulk of the hair shaft, roughly 80% of its total mass. This is the layer that determines your hair’s mechanical properties: its strength, elasticity, thickness, and texture. It’s also where your natural hair color comes from.
The cortex is composed of long, spindle-shaped cells packed tightly together and running parallel to the length of the hair. Inside each of these cells are smaller structures called macrofibrils, which are bundles of even smaller microfibrils, which are themselves twisted ropes of keratin protein chains. Think of it like a cable: individual protein strands twist together into tiny fibers, those fibers bundle into larger fibers, and those bundles fill each cortex cell. This hierarchical rope-within-a-rope structure is what gives hair its remarkable tensile strength. A single healthy strand can support about 100 grams of weight before breaking, and the combined strength of a full head of hair could theoretically hold several tons.
The keratin chains in the cortex are held together by several types of chemical bonds. The strongest are disulfide bonds, which are permanent cross-links between sulfur atoms in neighboring protein chains. These bonds are what give hair its natural shape. When you get a perm or a chemical straightening treatment, those processes work by breaking and reforming disulfide bonds to reshape the cortex’s internal structure. Weaker hydrogen bonds also connect the protein chains, and these are temporarily broken by water and heat, which is why wet hair can be reshaped with a blow dryer or curling iron but eventually reverts to its natural form.
Scattered throughout the cortex are granules of melanin, the pigment responsible for hair color. Two types of melanin determine the full spectrum of natural hair colors. Eumelanin produces brown and black shades, while pheomelanin produces red and yellow tones. The specific ratio, amount, and distribution of these two pigments creates everything from jet black to strawberry blonde. When melanin production slows with age, the cortex fills with tiny air pockets instead, and the hair appears gray or white.
The Medulla: A Hollow Core
At the very center of some hair shafts sits the medulla, a loosely structured column of cells with air spaces between them. Unlike the cuticle and cortex, the medulla isn’t always present. Fine or light-colored hair often lacks a medulla entirely, while thick or coarse hair typically has a continuous one. Medium-thickness hair may have a fragmented medulla that appears and disappears along the length of the strand.
The medulla’s cells are only loosely packed and partially broken down, creating a spongy, air-filled channel running through the core of the hair. Its exact function in humans isn’t entirely clear. In many animals, the medulla plays an important role in insulation by trapping air to regulate body temperature. In human hair, it may contribute slightly to the stiffness and width of thicker strands, but it doesn’t appear to be essential for hair’s mechanical performance. People with fine, medulla-free hair don’t have meaningfully weaker strands because the cortex does most of the structural work.
What Holds It All Together
Between the major layers, thin membranes called cell membrane complexes act as a kind of biological glue. These lipid-rich layers cement adjacent cells together within each layer and bind the cuticle to the cortex. When hair is severely damaged, it’s often because these intercellular membranes have broken down, allowing the cuticle to peel away and exposing the cortex to further damage. This is the mechanism behind split ends: the binding material at the tip of the hair fails, and the layers separate and fray.
The overall composition of a hair shaft by weight is roughly 65 to 95% protein (mostly keratin), with the remainder being water, lipids, pigment, and trace minerals. The exact proportions vary depending on ethnicity, hair thickness, and how much damage the strand has accumulated. Hair also absorbs small amounts of elements from the environment and bloodstream, which is why hair analysis is sometimes used in forensic and toxicology testing.
How Hair Structure Varies by Type
The basic three-layer architecture is the same in all human hair, but the proportions and geometry differ. Straight hair tends to have a round cross-section, while curly hair is more oval or asymmetrical. This shape difference is set deep in the follicle before the hair even emerges from the scalp, and it affects how the cortex cells are distributed around the strand. In curly hair, the cortex cells on one side of the oval are often denser or differently oriented than those on the other side, creating an internal asymmetry that causes the strand to bend.
Thicker hairs generally have more cuticle layers and a more prominent medulla. Asian hair, for example, tends to have a larger overall diameter and a more developed medulla than European or African hair types. African hair typically has a flatter cross-section and fewer cuticle layers on the outer curve of each curl, making those points more vulnerable to breakage. These structural differences explain why different hair types respond differently to the same styling products, heat levels, and chemical treatments.