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Human hair may look simple, but at the microscopic level it reveals a layered and sophisticated architecture. Each strand of hair on your head is composed of three primary layers, each with its own function. The innermost layer is called the medulla. Not all hair types have a medulla; it is present in thicker or coarser hair, while fine hair may lack it altogether. Surrounding the medulla is the cortex, the thickest and most important structural layer.
The cortex gives hair its elasticity, strength, color, and much of its texture. It's here that the protein keratin is densely packed and where natural pigments-melanins-determine your hair's color. Around the cortex lies the cuticle, a thin, protective outer layer made up of overlapping cells. The cuticle acts much like shingles on a roof, shielding the more delicate inner structures from environmental stress, moisture loss, and chemical exposure.As you go forward, keep in mind that hair's structure determines how it responds to various care routines and treatments. Some ingredients will coat or seal the cuticle, others are designed to penetrate and strengthen the cortex, and a few play roles at the scalp or follicle level.
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