Hair, a protein filament that grows from follicles embedded in the skin. While the hair root, located beneath the skin, is the living part of the hair, the visible portion extending above the skin surface is known as the hair shaft. This shaft, composed of dead, keratin-filled cells, forms a complex structure. Understanding its fundamental layers is insightful for comprehending hair’s appearance and overall health.
The Hair Shaft’s Primary Layers
The hair shaft has three layers: the outermost cuticle, the middle cortex, and the innermost medulla. Each layer contributes to the hair’s properties and integrity.
The cuticle is the outermost layer, consisting of several layers of flat, thin cells that overlap like shingles on a roof. Made of keratin proteins, these cells serve as a protective barrier for the inner hair layers. A healthy, smooth cuticle reflects light, contributing to the hair’s shine, and helps to maintain its hydration balance. Damage to this layer can lead to increased porosity, frizz, and a dull appearance.
Beneath the cuticle lies the cortex, the thickest layer and majority of the hair shaft’s volume. The cortex is composed of tightly packed, keratin-filled cells arranged in long, spiral chains. This layer is responsible for the hair’s strength, elasticity, and texture, with its structure determining whether hair is straight or curly. The cortex also contains melanin, the pigment that gives hair its natural color.
The medulla is the innermost, central core. This layer is composed of loosely packed, translucent cells and is often fragmented or discontinuous. It is more commonly found in thicker, coarser hairs and may be absent in fine or vellus hair. While its exact function is not fully understood, it contributes to the hair’s overall structure and appearance.
Understanding Hair Health Through Its Structure
Understanding the hair shaft’s layers provides insights into maintaining hair health. The cuticle, while protective, is vulnerable to damage from external factors. Heat styling, for instance, can cause cuticle cells to lift and separate, leading to rough, porous hair and increased moisture loss. Chemical treatments like bleaching and perming can also disrupt the cuticle, leaving the inner cortex more exposed.
When the cuticle is compromised, the underlying cortex becomes more susceptible to damage. Prolonged heat can break down the keratin chains within the cortex, diminishing hair’s strength and elasticity, and leading to breakage. Chemical processes can similarly alter the cortex’s internal structure, affecting the hair’s texture and overall integrity. Such damage can manifest as dryness, frizz, dullness, or a change in curl pattern.
Hair care products often target specific layers to improve hair’s condition. Conditioners, for example, work to smooth the lifted scales of the cuticle, reducing frizz and enhancing shine. Treatments designed to strengthen hair may focus on repairing the protein bonds within the cortex. Understanding how external stressors affect these layers allows for more informed hair care choices.