The Layers of the Hair Medulla Hair is composed of three principal layers, each with its specific purpose. If you were to examine a cross section of a hair shaft under a microscope, you would see the innermost or center portion of the hair shaft, the medulla.
The medulla is also called the pith or marrow of the hair. It is composed of round cells, two to five rows across. Thick or coarse hair usually contains a medulla. Fine hair for the most part lacks a medulla, as does naturally blonde hair.
The purpose of the medulla has not yet been determined. Cortex The cortex of the hair is composed of fibrous protein core of elongated cells that are bonded tightly together. The cortex contains melanin pigment and is responsible for giving the hair its strength, elasticity and tenacity. It is responsible for approximately 90 percent of the total weight of the hair. This is the layer of the hair where chemical changes take place when changing the color, permanently waving, or relaxing the hair.
Cuticle The cuticle of the hair is the outermost layer. It is composed of a transparent, scale-like single layer of overlapping cells. The function of these overlapping, shingle-like scales is to protect the cortex. The cuticle is the protective barrier for the cortex and the medulla. Products with a high pH cause the cuticle layer to swell and allow liquids to penetrate into the cortex.
Products with a low pH will cause the cuticle to shrink and harden. Glossary of Terms Cortex – the second layer of the hair Cuticle – the very thin outer layer of the skin or hair Medulla – the marrow in the various bone cavities; the pith of the hair