1. Cuticle: This is the outermost layer, forming a protective barrier for the hair. It's made of overlapping, scale-like cells that point away from the scalp. The cuticle's health determines the hair's shine, smoothness, and resistance to damage. When the cuticle is intact and lies flat, the hair reflects light and appears shiny. Damage to the cuticle (from heat, chemicals, or friction) can cause it to lift, making the hair look dull, rough, and prone to breakage.
2. Cortex: The cortex is the thickest layer, located beneath the cuticle. It makes up the bulk of the hair shaft and contains the pigment (melanin) that gives hair its color. The cortex is responsible for the hair's strength, elasticity, and texture. It is made up of keratin protein fibers arranged in a specific pattern. The pattern and amount of these fibers determine the hair's shape (straight, wavy, curly).
3. Medulla: This is the innermost core of the hair shaft. It is not always present in all hairs, especially fine or very blonde hair. When present, it is a soft, spongy layer composed of loosely packed cells and air spaces. The exact function of the medulla is not fully understood, but it is believed to contribute to the hair's insulation, flexibility, and potentially even color perception.