Hair is a common characteristic of human biology, appearing in various forms across the body. It contributes to both protection and individual appearance. Understanding its structure and growth offers insights into this aspect of human anatomy.
The Hair Shaft
The hair shaft is the visible part of the hair above the skin’s surface. Composed primarily of a tough protein called keratin, it is a non-living structure. Unlike the part embedded within the skin, the shaft does not contain nerves or blood vessels. Its primary function involves protection and thermal regulation, though its most prominent role is in contributing to an individual’s appearance.
Anatomy of the Hair Shaft
The hair shaft has three distinct layers. The outermost layer is the cuticle, which consists of overlapping, flattened cells resembling scales. This layer acts as a protective barrier, shielding the inner parts of the hair from environmental damage. Beneath the cuticle lies the cortex, the thickest layer, which provides the hair with its strength, elasticity, and color. The cortex contains melanin, the pigment responsible for hair color. The innermost layer is the medulla, a central core that may be present in thicker hair types.
How Hair Grows
The visible hair shaft originates from a specialized structure within the skin called the hair follicle. At the base of the follicle is the hair bulb, which encases the dermal papilla. The dermal papilla is supplied with blood vessels that deliver nutrients essential for cell division. As new hair cells are produced in the hair bulb, they are pushed upward and undergo keratinization, hardening and losing their nucleus to form the non-living hair shaft. Hair growth occurs in a continuous cycle involving three main phases: anagen (growth), catagen (transition), and telogen (resting), ensuring constant renewal of the visible hair.
Factors Influencing Hair Appearance
Several factors determine the visible characteristics of the hair shaft, affecting its texture, color, and overall health. Genetics play a significant role, dictating natural hair color, curl pattern, and thickness. Environmental factors, such as prolonged sun exposure or pollution, can degrade the protective cuticle layer, leading to dullness or damage. Hair care practices, including frequent washing, heat styling, and chemical treatments like dyeing or perming, also impact the hair’s appearance. These practices can alter the structural integrity of the cuticle and cortex, influencing the hair’s shine, strength, and susceptibility to breakage.