A strand of hair may appear to be a simple, inert fiber, yet it is a complex biological structure. The hair we see on the surface is a non-living protein filament, constantly manufactured within a sophisticated, living organ beneath the skin. This structure is primarily composed of a hardened protein called keratin. The intricate arrangement of this protein and the mechanisms that produce it determine the hair’s strength, texture, and color.
The Follicle and the Mechanism of Growth
Hair growth originates in the hair follicle, a small, tube-like organ embedded deep within the dermis of the skin. This follicle is the dynamic factory responsible for producing the hair fiber and is the only living part of the hair structure. At the base of the follicle is the hair bulb, which houses the hair matrix, a region of rapidly dividing cells.
The hair bulb surrounds the dermal papilla, a structure composed of connective tissue containing a network of tiny blood vessels and nerve endings. The dermal papilla acts as the nutrient and signaling hub, supplying the matrix cells with the blood and oxygen necessary for cellular division and growth. As the matrix cells multiply, they are pushed upward, beginning the process of keratinization where they die and harden into the hair shaft.
The life of a hair strand is a cyclical process involving three main phases. The Anagen phase is the active growth period, where cells in the hair matrix rapidly divide, pushing the hair out of the follicle. For scalp hair, this phase is the longest, typically lasting between two and seven years, with about 90% of hair in this stage at any given time.
Following active growth is the short Catagen phase, a transitional period lasting only a few weeks. During catagen, the hair follicle shrinks, and the lower part detaches from the dermal papilla, signaling the cessation of growth. The final stage is the Telogen phase, a resting period that lasts for approximately three months. The hair remains anchored in the follicle until it is eventually shed, making way for a new hair to begin the cycle.
The Three Layers of the Visible Hair Strand
The hair shaft that extends above the scalp is a non-living, highly structured collection of keratinized cells. This visible portion is organized into three distinct concentric layers, each contributing to the fiber’s strength and appearance.
The outermost layer is the Cuticle, which consists of several overlapping layers of flat, dead cells that resemble shingles on a roof. These translucent cells are arranged in a downward, protective formation. Their primary function is to shield the inner layers of the hair from physical and chemical damage. When the cuticle layers lie flat and smooth, the hair reflects light efficiently, resulting in a shiny appearance.
Beneath the cuticle lies the Cortex, which constitutes the main bulk of the hair strand, typically accounting for up to 90% of its total mass. The cortex is composed of long, tightly packed bundles of fibrous keratin proteins. These proteins are responsible for providing the hair with its elasticity, tensile strength, and flexibility. The chemical bonds within this keratin structure allow the hair to stretch and return to its original shape without breaking.
The innermost region is the Medulla, a central core that is not present in every hair type, particularly fine or naturally blonde hair. When present, the medulla is a soft, porous structure that may contain air spaces. Its exact biological function in human hair remains largely undetermined. The structural integrity and physical properties of the hair fiber are primarily dictated by the composition of the cortex and the surrounding cuticle.
What Determines Hair Color and Texture
The unique characteristics of color and texture are determined by processes that occur within the hair follicle before the strand emerges from the scalp. Hair color is a result of the amount and type of melanin pigment incorporated into the hair fiber’s cortex.
Specialized cells called melanocytes, located near the hair bulb, produce two primary types of melanin. Eumelanin is responsible for shades of brown and black, while pheomelanin provides red and yellow tones. The specific ratio and concentration of these two pigments within the cortex dictates the full spectrum of natural hair color. Hair turns gray or white when melanocytes stop producing pigment, leaving the strand unpigmented.
The texture of a hair strand—whether straight, wavy, or curly—is determined by the shape of the hair follicle itself. A perfectly round hair follicle typically produces a straight hair strand, as the fiber grows out symmetrically. As the follicle opening becomes more oval or flattened, the hair fiber is forced to grow out in a curved or helical shape, resulting in wavy or curly hair. The degree of the follicle’s asymmetry directly correlates with the tightness of the resulting curl pattern.