While everyone has different ratios of layering in their hair, we generally have three layers to our hair shaft: the cuticle, the cortex, and the medulla.
Outer Layer: Cuticle
The cuticle is the outer layer. The cuticle is a hard and resistant protein layer compared to the soft, elastic layer of the cortex. In very dark hair, the cuticle may contain melanin (hair pigment). Regardless if the cuticle has pigment or not, it’s always affected by bleach.
Microscopically, a normal cuticle layer has the appearance of shingles on a roof, like multiple scales stacked flat on top of each other. During a lightening service, the cuticle is altered to open these scales, which allows the product to penetrate the cortex. If the lightening process is rushed or if the lightener is left on too long, it could permanently break off the cuticle scales, leaving the cortex vulnerable.
Middle Layer: Cortex
The cortex contains the majority of melanin (hair pigment). Since color change primarily occurs in the cortex layer, if you have a very thick cuticle layer, it may take longer for color change to happen because it’s harder for the color to penetrate.
The cortex or middle layer has chains of protein formed by amino acids, which give the hair its strength and elasticity. These acids are linked together by peptide bonds. Decolorizers (bleach) break some of these bonds in the cortex of the hair. In the decolorizing process, as much as 15 – 20% of the bonds are permanently broken. The longer a bleach mixture remains in contact with the hair, and the higher the pH, the higher the percentage of broken bonds. The destruction of the bonds leads to the production of a new molecule called cysteic acid, which produces hair that is weak, easily broken, and “gummy” or stretchy when wet.
Inner Layer: Medulla
The medulla, which isn’t always present in hair, is the innermost layer running down the middle of the hair shaft. In humans, it has no known function.