Hair, a ubiquitous feature, is more intricate than its visible appearance suggests. It serves various functions, from protecting the skull from sun exposure to trapping dust particles. This complex biological structure is dynamic, undergoing continuous cycles of growth and renewal. Understanding its components reveals the sophisticated biology behind each strand.
Anatomy of the Hair Shaft
The hair shaft, visible above the skin, is a non-living structure composed of three primary layers. The outermost layer, the cuticle, consists of overlapping, scale-like cells that protect inner components and contribute to the hair’s shine and smoothness. Beneath the cuticle lies the cortex, which constitutes the bulk of the hair shaft. This middle layer is primarily made of keratin proteins, giving hair its strength, elasticity, and texture, and it also contains melanin, the pigment responsible for hair color. The innermost layer, the medulla, is a soft, often hollow core that may be absent in fine hair.
The Hair Follicle and Cell Production
Hair production originates within the hair follicle, a specialized structure anchored in the skin, where the hair bulb surrounds the dermal papilla at its base. The dermal papilla contains blood vessels that supply essential nutrients and oxygen to the growing hair. Surrounding the dermal papilla within the hair bulb is the hair matrix, a region of rapidly dividing cells, primarily keratinocytes. These keratinocytes proliferate and undergo keratinization to form the hair shaft’s structural layers. Melanocytes, also located in the hair matrix, synthesize melanin pigments and transfer them to the keratinocytes, determining the hair’s color.
The Hair Growth Cycle
Hair growth occurs in a cyclical pattern involving distinct phases. The anagen phase is the active growth period, during which hair follicles produce new cells rapidly, leading to hair lengthening; this phase can last from two to eight years for scalp hair, with 85% to 90% of hairs in this stage. Following anagen is the catagen phase, a short transitional period lasting two to three weeks, during which hair growth slows, and the hair follicle shrinks, detaching from its blood supply. The final stage is the telogen phase, a resting period that lasts for two to three months. Hairs do not grow during telogen but remain in the follicle before being shed, allowing new hair growth to begin.