Updated: Sep 16, 2020
The three main layers of the hair shaft are the hair cuticle, cortex, and medulla.
The hair cuticle (HAYR_KYOO-ti-kul)
It's the outermost layer of the hair. It consists of a single overlapping layer of transparent, scale-like cells that look like shingles on the rooftop. The cuticle layer provides a shield that protects the inner structure of the hair as it lies tightly against the cortex. It is responsible for creating the shine and the smooth, silky feel of healthy hair. And even though all of the cuticles overlap, each individual one is attached to the cortex. Each overlapping scale makes up the cuticle layer. Putting solutions like hair color swells and raises the cuticle layer and opens space between the scales, which allow liquid to get inside the cortex.
To feel your cuticle, grab a strand of hair near the root (scalp) then glide your fingers from the scalp up to the ends. The strand should feel sleek and smooth. Now take the same strand of hair and start from the ends and slide down toward the scalp in the opposite direction and explore the cuticles rising. That’s because you’re going against the natural flow. A healthy compact cuticle layer is the hair’s primary defense against damage.
The cortex (KOR-teks)
is the middle layer of the hair right beneath the cuticle. It’s a fibrous (fiber) protein core formed by elongated cells containing melanin pigment. About 90% of the total weight of hair comes from the cortex. The elasticity of the hair and its natural color are the results of the unique protein structures located within the cortex. Anything that has to do with coloring, wet sets, heat, perms, and relaxing takes place within the cortex.
The medulla (muh-DUH-uh)
The medulla is the inner layer of the hair and is composed of round cells called melanin within the elongated cells. Very fine and naturally blond hair to entirely lack a medulla. Generally, only thick, coarse hair and all male beards contain a medulla.