As hair cells emerge from the skin, they are exposed to oxygen and die while becoming packed tightly with filaments of the protein keratin. While those hair cells are dead, multiple chemical bonds continue forming and reforming, meaning every single strand of hair (dead or alive) is constantly changing, breaking, and reconnecting. 

When you look at a single strand of hair, you may see it as one slim, solid string-like extension. Maybe you’ve never given it much thought. Hair is not just a fiber, it is a complex biosubstrate with many, many layers (they’re just hard to see!) 

A single hair strand has 3 main layers: The cuticle, the cortex, and the medulla. Under a microscope, the outside layer of hair, the cuticle, looks like overlapping tiles on a roof. Healthy cuticles lay flat—they are porous and can absorb and retain moisture. Unhealthy cuticles do not lay flat—they allow moisture to enter and escape easily, causing frizz and dryness.

The complexity of hair becomes clearer as we move from the cuticle to inside the center cortex, which is occupied by cortical cells filled with filaments of the protein keratin. Hair is made up of 95% keratin, a fibrous, helicoidal protein ( a fancy way of saying it’s shaped like a helix.) A multitude of amino acids can be found in hair, and strings or chains of amino acids are arranged in a very particular order that gives the resulting keratin protein certain properties. The cortex, contains bundles of fibrous protein and imparts the majority of hair’s strength and elasticity. 

In the cortex, keratin strands will self-assemble into a coiled-coil (a helical shape that is formed by multiple protein chains). Those coiled-coils form bundles of intermediate fibers, which in turn interweave to form filaments and fibrils, that, in total, make up the hair fibers.