Hair fibers are composed of many layers with the three most commonly discussed layers being the medulla, the cortex, and the cuticle.
The central medulla is the only living layer of the hair fiber and contributes to the thickness of each hair fiber. The medulla does not extend the entire length of the hair fiber and with age, shortens and thins, leading to fine hairs.
The cortex is the middle layer composed of keratins and melanin and maintains the integrity of the hair fiber from root to tip. The cuticle contains melanin, a pigment that gives hair it’s color. A separate article describes how melanin can make hair colors from blonde to red to black. The cortex is what holds the negative charge of hair and allows the cuticle to adhere. The cortex is important for maintaining the moisture content of the hair fiber.
The cuticle, commonly targeted by hair products, is the outermost layer of the hair fiber. Under high magnification the scales of the cuticle resembles overlapping shingles on a roof. This layer is susceptible to everyday elements such as heat, sun, and chemicals. These added injuries cause mechanical damage and can alter the pH of the hair. These changes can alter the cuticle causing the individual scales to swell, soften, and open. This allows increased friction which complicates the hair fibers with tangling, knotting, reduced slip of the hair, and increased breakage.
Image of cuticle: Healthy on the left and Damaged on the right.