The Three Layers of a Strand: Cuticle, Cortex, Medulla

The cuticle — the protective layer

The cuticle is the outer layer of the strand. It is made of tiny transparent scales that overlap like scales on a fish or shingles on a roof. When those scales lie flat, hair is shiny, flexible, and holds water well. When they lift or chip away, hair becomes dry, tangles easily, and loses water.

Almost everything we do to hair — how we wash, how we comb, heat, products — affects the cuticle before anything else. Keeping the cuticle flat is keeping hair healthy.

The cortex — where strength and color live

Beneath the cuticle is the cortex. It is the largest part of the strand, and it is where hair's strength lives. The cortex holds keratin fibers that give hair its strength and elasticity — its ability to stretch slightly and return without breaking. The cortex is also where the pigment (melanin) that gives hair its color sits.

When chemical products like color or relaxers work, they must open the cuticle to reach the cortex. That is why those treatments can weaken hair if not done with care.

The medulla — the center

The medulla is the channel in the very center of the strand. Very fine hair may not have a medulla at all, and it plays little role in daily care. What matters most to know: it is the cuticle (protection) and the cortex (strength) that decide whether your hair stays healthy or breaks.