The human hair shaft is a complex, keratinous structure produced by the hair follicle. Understanding its composition is foundational to grasping how hair color is determined and maintained. The hair shaft is made up of three concentric layers, and the middle layer plays a definitive role in establishing the hair’s natural shade. Studying this composition helps scientists and cosmetologists understand the mechanics of hair’s appearance.
The Cortex: Home of Hair Pigment
The cortex is the thickest of the three layers that make up the hair shaft, comprising approximately 70–90% of its total weight. This central layer is composed of long, tightly packed keratin filaments, which provide the hair with strength, elasticity, and texture. The cortex is the definitive location for the hair’s natural pigment.
Pigment is stored within the cortex in the form of melanin granules, embedded between the keratin fibers. The cortex is protected by the outermost layer, the cuticle, which shields the internal structure and controls moisture content. The innermost layer, the medulla, is a soft core of cells often absent in finer hair and does not contribute to color.
Melanin: The Chemical Basis of Color
Melanin, the pigment found within the cortex, is a natural polymer produced by specialized cells called melanocytes. These melanocytes are located in the hair bulb and synthesize melanin, injecting it into the keratin-producing cells of the growing hair shaft. The transfer of these pigment packets, called melanosomes, into the cortical cells colors the hair shaft before it emerges from the scalp.
There are two primary forms of melanin that combine to create the spectrum of human hair colors. Eumelanin is a dark, granular pigment responsible for brown and black shades. Pheomelanin is a lighter, diffuse pigment that imparts red and yellow tones. Both types are present in all hair colors, but their ratio and concentration determine the final hue.
How Pigment Density Dictates Hair Color
The final shade of human hair results from the density and specific ratio of eumelanin and pheomelanin granules within the cortex. Hair that appears black or dark brown has a high concentration of eumelanin, which is the dominant pigment globally. Conversely, blond hair results from a very low density of eumelanin, with both pigments present in small amounts.
Red hair is the least common color and is characterized by a high concentration of pheomelanin relative to eumelanin. The size, shape, and distribution of the pigment granules also contribute to the final visual effect by affecting how light is absorbed and reflected. The loss of hair color, commonly referred to as graying, occurs when the melanocytes stop producing pigment. This leaves the cortical cells colorless, and the resulting white hair mixed with remaining pigmented strands gives the appearance of gray.