Hair is not dead skin, though the visible portion of hair is composed of non-living material. This common misconception often arises because hair, like the outermost layer of skin, is primarily made of a robust protein called keratin. Understanding its distinct biology clarifies its behavior and how best to care for it.
The Living and Non-Living Components of Hair
Hair is comprised of two main structures: the hair follicle and the hair shaft. The hair follicle is the living part, embedded within the skin’s dermal layer. It contains cells actively involved in hair production and growth, supplied by blood vessels that deliver essential nutrients.
The hair shaft, the visible strand above the skin, is entirely non-living. It consists of hardened, dead cells primarily made of keratin protein. As these cells are pushed upward from the follicle, they undergo keratinization, losing their nucleus and cytoplasm to become densely packed with keratin. This transformation results in a durable, filamentous structure.
The hair shaft itself has three distinct layers: the cuticle, cortex, and medulla. The outermost layer, the cuticle, is made of overlapping, flat cells, similar to shingles on a roof, which protect the inner layers. Beneath the cuticle lies the cortex, the thickest layer, which contains the majority of the hair’s keratin and melanin pigments that determine hair color. Some thicker hairs also contain a central core called the medulla.
How Hair Grows
Hair growth originates within the living hair follicle, specifically from a region called the hair bulb. Inside the hair bulb, specialized cells continuously divide and multiply. As new cells are produced, they push older cells upwards through the follicle. This upward movement initiates the process of keratinization, where cells harden and become filled with keratin, forming the hair shaft.
The hair growth process occurs in a cyclical manner, characterized by three main phases: anagen, catagen, and telogen. The anagen phase is the active growth period, during which hair cells rapidly divide and the hair shaft elongates. This phase can last for several years, influencing the potential length hair can achieve. For scalp hair, the anagen phase typically ranges from two to seven years.
Following the anagen phase, the hair enters the catagen phase, a brief transitional period lasting a few weeks. During this stage, hair growth slows, and the hair follicle shrinks, detaching from its blood supply. The final stage is the telogen phase, a resting period that can last for several months, where the hair does not grow. At the end of this phase, the old hair is shed, and the follicle re-enters the anagen phase to begin growing a new hair.
Implications for Hair Care
Given that the visible hair shaft is non-living, it cannot repair itself like living tissues. Once hair emerges from the scalp, its cells are biologically inert, lacking the ability to regenerate or heal damage. This fundamental reality shapes effective hair care, which focuses on maintaining the integrity of the existing hair shaft and protecting it from further damage.
Damage to hair, such as split ends or breakage, is permanent and cannot be genuinely reversed by products. Products like conditioners and serums work by coating the hair shaft, smoothing down the cuticle, and providing a temporary protective layer. This external conditioning helps to improve the hair’s appearance, manageability, and feel, making it appear healthier and shinier. The goal is to minimize external stressors that can lead to physical degradation of the keratin structure.
Proper hair care emphasizes gentle handling, protection from environmental factors like UV radiation, and careful use of styling tools. While hair itself cannot heal, maintaining a healthy scalp and providing essential nutrients through diet can support the living hair follicles in producing strong new hair. The visible hair relies on protective measures to preserve its structure and aesthetic qualities.