The hair shaft is the visible, thread-like structure extending above the skin’s surface, representing the non-living portion of the hair. Its composition and physical organization determine its strength, texture, color retention, and resistance to damage. This understanding is foundational in cosmetic science, guiding the development of products designed to protect the hair fiber from environmental stressors and chemical treatments. The hair shaft is an architecturally complex structure built primarily from specialized proteins.
The Three-Layered Architecture
The hair shaft is organized into three distinct, concentric layers that provide structural integrity and protection.
The outermost layer is the cuticle, which functions as the hair’s primary defense mechanism against physical and chemical damage. This layer consists of several overlapping, flattened cells that are arranged like shingles on a roof, pointing toward the tip of the hair strand. When the cuticle is smooth and intact, it minimizes moisture loss from the inner layers and contributes to the hair’s reflective shine and smoothness.
Beneath the protective cuticle lies the cortex, which comprises the greatest bulk of the hair shaft, often making up around 90% of its mass. This middle layer is composed of elongated, tightly packed cells that are responsible for the hair’s mechanical properties, including its resilience, strength, and elasticity. The cortex houses the majority of the chemical components that dictate the hair’s physical characteristics, such as its natural color pigment.
The innermost layer is the medulla, a central core that is not present in all hair types. Fine or vellus hair, for instance, may completely lack this structure, while it is typically visible in thicker, coarser hair. When present, the medulla is often described as a soft, porous channel composed of a honeycomb-like network of cells and air spaces. Its precise biological function is not fully understood, but it is known to contain a higher concentration of certain lipids than the surrounding cortex.
The Core Building Block: Keratin
The vast majority of the hair shaft, exceeding 90% of its total dry weight, is composed of a fibrous structural protein called alpha-keratin. This protein is synthesized within the hair follicle and gives hair its remarkable mechanical properties. Keratin is a family of proteins that assemble into complex structures to form the hair fiber.
The fundamental structure of hair keratin begins with polypeptide chains twisted into a right-handed coil known as an alpha-helix. Two of these helical chains then wind around each other to form a left-handed structure called a coiled-coil dimer. These dimers further aggregate and polymerize to create the robust intermediate filaments that pack tightly within the cells of the cortex.
The immense strength of hair is largely conferred by covalent chemical cross-links called disulfide bonds, which form between the sulfur atoms of the amino acid cysteine. Hair keratin is classified as hard alpha-keratin because of its high cysteine content, which allows for a dense network of these stabilizing bonds. This chemical density provides the hair with rigidity and resistance to external forces compared to the soft keratin found in the outer layers of the skin.
Pigments, Lipids, and Essential Moisture
While protein forms the structure of the hair shaft, other components are responsible for its visual characteristics and flexibility.
Pigments
The natural color of hair is determined by melanin, a pigment stored within the cortical cells. The two primary forms are eumelanin, which produces black and brown shades, and pheomelanin, which is responsible for red and yellow tones. The specific ratio and total amount of these pigments dictate the final hair color.
Lipids
Lipids, or fats, constitute an important fraction of the hair shaft, typically ranging from 1% to 9% of its total mass. These lipids are present both internally and externally, playing roles in structural integrity and surface protection. Structural lipids, such as ceramides and free fatty acids, are found within the cuticle cells, where they act as a cement to keep the overlapping scales sealed.
On the surface, lipids derived from sebaceous glands, called sebum, coat the hair shaft. This external layer provides a natural waterproofing and lubricating effect. Sebum helps maintain smoothness and contributes to the hair’s shine by creating a smooth surface for light reflection.
Moisture
Water is an integral component of the hair shaft, accounting for approximately 10% to 15% of its total weight under normal conditions. This moisture is held within the keratin structure and is responsible for the hair’s flexibility and elasticity. When the water content drops below this range, the hair can become dry, brittle, and significantly more prone to breakage.