The hair fiber is the visible portion of hair extending from the scalp. Although it appears simple, a single strand is a highly organized structure composed mostly of dead cells. Understanding its composition reveals how it achieves remarkable tensile strength, elasticity, and resistance to environmental factors. The assembly of proteins, water, lipids, and chemical cross-links dictates the health, texture, and appearance of hair.
Keratin: The Primary Protein Structure
Hair is composed of a tough, fibrous structural protein called alpha-keratin, which makes up approximately 80% of the fiber’s dry weight. This protein is synthesized within the hair follicle during keratinization, where new cells harden and die to form the hair shaft. Alpha-keratin molecules begin as long polypeptide chains, which are polymers of numerous amino acids.
These polypeptide chains immediately twist into a right-handed coil known as an alpha-helix. Two of these alpha-helices then wrap around each other in a left-handed fashion, creating a strong, rope-like structure called a coiled-coil dimer. These dimers further aggregate into larger, highly organized structures called intermediate filaments, which provide the hair with its fundamental mechanical strength.
The resilience of this protein is related to its high content of the sulfur-containing amino acid cysteine. Cysteine residues from adjacent protein chains form strong, covalent cross-links, which are the basis for the fiber’s durability. This high sulfur content contributes significantly to the hair’s overall resistance to wear and tear.
The Three Distinct Layers of the Hair Fiber
The physical structure of the hair fiber is organized into three distinct, concentric layers: the cuticle, the cortex, and the medulla. Each layer plays a specific role in maintaining the integrity and function of the strand.
The outermost layer is the cuticle, which functions as the hair’s primary protective shield. It consists of several overlapping layers of dead, flattened cells that resemble shingles on a roof, all facing toward the hair tip. A healthy, smooth cuticle reflects light, which gives the hair its natural shine, while also regulating the moisture balance of the inner structures.
Beneath the cuticle lies the cortex, which is the thickest layer and makes up the majority of the hair fiber’s mass, often encompassing up to 90% of its weight. The cortex contains the bulk of the keratin protein and is responsible for the hair’s strength, elasticity, and overall shape. This layer also houses the melanin pigments, which determine the natural color of the hair.
The innermost core of the fiber is the medulla, which is a central, soft, and somewhat disorganized region. It consists of loosely packed cells and air spaces, but its function is not fully understood and it is not always present. The medulla is often absent in fine or very light-colored hair, only appearing consistently in thicker hair types.
Essential Non-Protein Materials and Chemical Bonds
A variety of non-protein materials and chemical connections give the hair fiber its characteristic properties of flexibility and waterproofing. Water is a major component, making up approximately 10–15 percent of the hair’s total mass under typical ambient conditions. This bound water is crucial for maintaining flexibility; complete hydration can increase the fiber’s length by 10 to 12 percent.
Lipids, or fats, are an important non-protein element, accounting for only a few percent of the total mass. These include internal structural lipids and external surface lipids derived from sebum. These fats provide lubrication and create a hydrophobic barrier, which helps maintain the hair’s moisture balance and gives it a smooth feel.
The structure of the keratin is stabilized by two types of chemical bonds that link the protein chains together. Disulfide bonds are strong, covalent cross-links that form between the sulfur atoms of cysteine amino acids, giving the hair its durable shape and resistance to chemical change. Hydrogen bonds, by contrast, are weaker, temporary bonds that are easily broken by water or heat, which is why hair can be reshaped when wet or styled with hot tools.
Finally, hair contains trace amounts of various minerals and elements that are incorporated into the fiber during growth. These trace elements, which include iron, zinc, copper, and magnesium, make up a very small percentage of the hair’s composition. The presence of these trace elements reflects an individual’s internal chemistry and can vary based on diet and environment.