Hair Has Three Layers: Cuticle, Cortex and Medulla

A single hair is roughly 0.05–0.1 mm thick. Looking at its cross-section, it is made of three layers, from the outside in.

Cuticle (outer layer)

Thin, scale-like cells overlap in about 4–10 layers, pointing from the root toward the ends. This layer protects the inside of the hair and determines shine and feel. It opens slightly when wet and closes when dry. When it peels or chips from friction, heat or alkali (color and perm products), components inside the hair escape more easily.

Cortex (middle layer)

This layer makes up 85–90% of the hair. It consists of a fibrous protein called keratin, the lipids that fill the gaps between the fibers (CMC), and melanin pigment. The hair's strength, elasticity, color and moisture are largely determined here. Color changing with hair color treatments, and shape changing with perms or straightening, both happen in this layer.

Medulla (center)

The core at the center of the hair. Thick hair has one, but fine hair and vellus hair may not. Its role is still being studied, and hair care products rarely act on it directly.

Points to remember for hair care

• Shine and feel = mainly the condition of the cuticle
• Strength and manageability = mainly the condition of the cortex (keratin, lipids and moisture)
• Shampoo works mainly on the surface of the cuticle; treatments and repairing ingredients act on the cortex through the gaps in the cuticle

Related: Hair damage / Treatments