How Hair Pigment Works
Specialized cells called melanocytes reside around the follicle from which hair grows. These cells manufacture melanin, a water insoluble biopolymer produced by the amino acid tyrosine, and they disperse it into the cortex of developing strands of hair as they emerge from the follicle. Melanin is the substance responsible for pigment in our skin and hair, and it comes in three forms. Eumelanin is responsible for brown and black hair, and pheomelanin yields red hues. Mixtures of the types of pigments, as well as the concentration and distribution of the melanin particles result in varying shades of hair color present in nature.
Melanin exists as granules that are dispersed throughout the cortex in human hair, amongst the proteins, water molecules, and fatty acids. These granules have differing geometry and distribution, depending upon which type they are. Eumelanin is generally evenly distributed throughout the cortex, has an oval shape, and has sharply defined edges. Pheomelanin is more randomly distributed in the cortex and also is more irregularly shaped, often having both an oval shaped end and a rod-like end. The presence of these particles in the cortex may contribute to some of the mechanical properties of the hair strand, such as elasticity and strength. Another important role of melanin is to act as a protectant from environmental damage by absorbing ultraviolet radiation. The structure of the polymer enables it to absorb UV energy and release it as heat via a photochemical process, thereby preventing it from attacking and damaging the protein structures of the hair.
Graying Hair
Once melanocytes are no longer manufacturing melanin, they die. Their absence sometimes causes the follicle to change shape slightly, which results in a slightly different shape for subsequent hairs that grow from that follicle. This is important, because the geometry of a hair strand is a significant contributing factor to the curl pattern, or lack thereof, for that hair. This explains why the texture of some gray hairs often varies significantly from the rest of the hair. This can certainly be perplexing and frustrating.
One notable feature of gray hair that can contribute to its having different properties from pigmented hair is the presence of a medulla. Most animal fur contains a central hollow core called the medulla, which provides insulation for the mammal. In contrast, this feature is generally missing in human hair, but sometimes appears in white strands. This presence of this hollow core may change the physical properties of the hair, making it more wiry and unruly.
An additional explanation for variations in the properties of white hair is found in the absence of melanin particles from the cortex. This changes the overall structure of the region, which may affect properties of the hair, such as curl pattern, elasticity, or strength. Also, the cuticle of gray hair strands is often tighter than pigmented hair, making it more difficult to process chemically as well. This is especially relevant when attempting to hide gray hair with color. Gray hair typically needs a longer processing time or a pre-treatment to open the cuticle to allow penetration of the artificial dyes.
Perhaps the most significant feature of white hair is its vulnerability to damage from the sun. The absence of its natural sunscreen, melanin, leaves hair highly susceptible to mechanical degradation from UV radiation. This can lead to broken hairs, split ends, frizz, excessive tangling, and cuticle damage. The surface of hair can even lose its natural hydrophobic protection and become hydrophilic, allowing far too much water to penetrate into the hair, causing irrevocable damage. White hair is also vulnerable to yellowing from the sun due to oxidation.
