The hair shaft is the visible part of hair that extends beyond the skin’s surface. It consists of non-living cells, which is why cutting hair does not cause pain. This structure offers protection and insulation.
The Primary Protein: Keratin
Keratin is the main structural protein that forms the hair shaft, making up approximately 85-90% of its dry weight. In humans, hair primarily contains alpha-keratin, which is also found in nails and the outer layer of skin.
Alpha-keratin possesses a helical, coiled structure, formed from long chains of amino acids. This arrangement gives hair its strength, resilience, and resistance to wear and tear. The coiled nature of keratin also provides elasticity, allowing it to stretch and return to its original shape.
Hair Shaft’s Distinct Layers
The hair shaft is structured into three distinct layers: the cuticle, the cortex, and the medulla. Each layer plays a specific role in the hair’s integrity and appearance.
The cuticle forms the outermost, protective layer of the hair shaft. It is composed of thin, flattened cells that overlap like shingles on a roof, creating a protective barrier. This highly keratinized layer shields the inner hair from physical and chemical damage, while also helping to regulate its water content.
Beneath the cuticle lies the cortex, the hair shaft’s thickest and most substantial layer. It is primarily made of tightly packed keratin cells. The cortex is responsible for the hair’s strength, elasticity, and texture. It also contains melanin, the pigment that determines hair color.
The innermost layer of the hair shaft is the medulla. This layer is often absent in fine or vellus hairs, but can be continuous, interrupted, or fragmented in thicker strands. The medulla consists of loosely packed, translucent cells and may contain air pockets. While its precise function is not fully understood, it is thought to contribute to the hair’s mechanical strength.
Supporting Molecules and Structural Bonds
Beyond keratin, the hair shaft incorporates other molecules and is held together by chemical bonds that contribute to its structural integrity and properties. Water makes up approximately 12-15% of the hair’s composition, playing a role in its flexibility and elasticity. Lipids, or fats, constitute about 3% of the hair’s makeup, originating in the hair bulb and contributing to its impermeability and cohesion. Trace minerals are also present, incorporated from the bloodstream and contributing to hair health.
The hair’s protein chains are held together by three types of chemical bonds: disulfide, hydrogen, and ionic bonds. These bonds are responsible for the hair’s shape, strength, and elasticity.
Disulfide bonds are strong, permanent covalent bonds that are particularly durable. They largely determine the hair’s natural shape and texture, including its curliness. Chemical treatments like perms, relaxers, bleaching, or excessive heat can break these bonds.
Hydrogen bonds are temporary and the weakest of the three bond types. They are easily broken by water and heat, reforming as the hair dries. These bonds contribute to elasticity and allow for temporary styling changes. Ionic bonds, also known as salt bridges, are temporary bonds stronger than hydrogen bonds. They contribute to the hair’s strength and can be disrupted by changes in pH levels.