The outer layer of hair is called the cuticle.
The hair shaft, the part extending beyond the skin’s surface, is composed of three distinct layers. The outermost layer, the cuticle, acts as the hair’s primary protective shield against environmental stressors and physical damage. This non-living layer is fundamental to the overall health and appearance of the hair strand.
The Hair Cuticle: Structure and Function
The cuticle is formed from multiple layers of flat, dead cells that overlap in a specific pattern. These cells are arranged similarly to shingles on a roof, with their free edges pointing toward the tip of the hair shaft. This scale-like structure is tightly bound and gives the hair strand a smooth, uniform surface when healthy.
The cuticle cells are primarily made of keratin proteins, which are highly cross-linked, contributing to their toughness and stability. When the cuticle is smooth and intact, it efficiently reflects light, which is the source of hair’s natural shine. The surface is also coated with a thin layer of lipids, including 18-methyleicosanoic acid, providing a necessary hydrophobic quality. This lipid layer helps the hair repel water and controls the movement of moisture into and out of the internal layers.
The Inner Layers: Cortex and Medulla
Directly beneath the cuticle lies the cortex, which constitutes the majority of the hair shaft’s mass. The cortex is a dense, fibrous layer composed of twisted keratin protein chains responsible for the hair’s mechanical strength and elasticity. The cells within the cortex also contain melanin, the pigment that determines the hair’s natural color.
The innermost core of the hair shaft is the medulla, a soft, often discontinuous region of loosely packed cells and air spaces. This layer is not present in all hair types, frequently being absent in fine hair. While its purpose is not fully understood, it is believed to play a role in maintaining the hair’s structure and thermal regulation.
How Cuticle Damage Affects Hair Health
Damage to the cuticle compromises the hair’s structural integrity and is the primary cause of many common hair problems. When exposed to harsh treatments like chemical processing, excessive heat styling, or vigorous friction, the cuticle scales can lift, crack, or chip away. Alkaline chemical treatments, such as those used in coloring or perming, cause the cuticle to swell dramatically, allowing chemicals to penetrate the cortex.
Once the cuticle is damaged, the hair’s internal structure is exposed, leading to rapid moisture loss. This loss results in a rough, dry texture and a noticeable reduction in shine, as the uneven surface scatters light instead of reflecting it. Raised cuticles also cause hair strands to snag against each other, increasing friction, which manifests as tangling and frizz. If the protective barrier is completely lost, the inner cortex becomes vulnerable to fraying, ultimately resulting in split ends.