Hair appears deceptively simple, but under a microscope, it reveals a highly organized and complex biological fiber. The visible portion, known as the hair shaft, is a non-living structure composed of hardened cells that serve a protective and aesthetic function. To understand the durability and variety of human hair, one must look closely at its intricate internal structure. A single strand involves distinct physical layers, a specific chemical composition, and a geometry that determines its final texture.
The Hair Shaft’s Three Distinct Layers
The hair shaft is organized into three concentric layers, each contributing a unique functional property. The outermost layer is the cuticle, which acts as the primary defense mechanism against physical and environmental damage. This layer consists of several overlapping, flattened cells that resemble the shingles on a roof, typically numbering between six and ten layers deep.
When the cuticle is healthy and lies flat, the hair reflects light uniformly, giving it a smooth and glossy appearance. Damage from chemical processes or excessive heat can cause these scale-like cells to lift and chip away, reducing the protective barrier and leading to a dull or frizzy texture. The cuticle’s integrity shields the central components from degradation.
Beneath the cuticle lies the cortex, which makes up the bulk of the hair shaft, accounting for approximately 75 to 90 percent of the fiber’s mass. This middle layer is responsible for the hair’s mechanical properties, including its strength, elasticity, and ability to hold shape. The cortex is composed of elongated, spindle-shaped cells packed tightly together, providing tensile strength.
The innermost layer is the medulla, a central core running through the hair shaft like a pith. It is not universally present, often being absent in finer or vellus hairs. When present, it consists of loosely arranged, transparent cells separated by air spaces. Its precise function remains a topic of scientific discussion, but its presence is more common in thicker hair types.
The Chemical Building Blocks of Hair
The structural components of the hair shaft are primarily constructed from proteins and pigments. Keratin is the main protein, making up roughly 95% of the fiber’s composition. This fibrous protein is classified as “hard keratin” and provides resilience and resistance to wear and tear.
Keratin chains are long polymers built from amino acids; cysteine is a particularly abundant amino acid in hair. Sulfur atoms within two adjacent cysteine molecules form strong, covalent connections known as disulfide bonds. These durable bonds stabilize the three-dimensional structure of the hair, significantly contributing to its strength.
The natural coloration of hair is determined by melanin, a pigment produced by specialized cells in the hair follicle. There are two primary types of melanin: eumelanin and pheomelanin. Eumelanin provides dark colors, ranging from black to brown.
Pheomelanin is responsible for lighter hues, contributing red and yellow tones. The specific color of an individual’s hair is determined by the ratio, concentration, and distribution of these two pigment types within the cortex. As people age, melanin production decreases, and air becomes trapped within the hair shaft, leading to the appearance of white or grey hair.
How Cross-Sectional Shape Determines Texture
The texture of hair, whether straight, wavy, or curly, is determined by the shape of the hair follicle, which dictates the cross-section of the emerging hair shaft. This geometry influences how the hair fiber is structured internally and how it behaves once it exits the skin. The shape is established while the cells are still in the hair bulb, long before the strand is visible.
A hair shaft that is nearly perfectly round in cross-section will grow straight. This uniform shape allows the keratin fibers within the cortex to align evenly, resulting in a smooth, straight strand that lies flat. The diameter of these round shafts can vary, but symmetry determines the lack of curl.
As the hair shaft’s cross-section becomes more flattened, or elliptical, the hair develops a noticeable curve. A slightly oval cross-section typically produces wavy hair, causing the strand to form gentle bends. This asymmetry forces the internal keratin structures to align unevenly, promoting wave formation.
Hair that is distinctly curly or coiled grows from follicles that produce a significantly flattened or asymmetrical, kidney-shaped cross-section. This highly elliptical shape creates tension and different growth rates on opposite sides of the hair shaft, causing the hair to spiral into tight curls or coils. The degree of flattening correlates directly with the tightness of the curl pattern.