The physical structure of a human hair can be seen through a microscope. It consists of three layers of dead skin cells: the cuticle, a smooth, outer covering fashioned from cells that overlap like fish scales; the cortex, a thick, middle layer made of spindle—shaped cells filled with keratin and hair color; and the medulla, a relatively narrow central column containing cube—shaped cells and pockets of air.
Hair's chemical structure cannot be seen under a microscope. Like all matter, hair is made up of tiny units called atoms. The outer part of an atom consists of even smaller particles called electrons that whirl around the atom's center region. When two or more atoms join to form molecules—combinations of atoms—they do so by exchanging or sharing electrons.
At the molecular level, hair is composed mostly of chains of keratin molecules. These chains are cross-linked by chemical bonds in much the same way that the sides of a ladder are held together by rungs. The bonds include very strong links between sulfur atoms found in cystine and much weaker electrical attractions between hydrogen atoms in other parts of the keratin molecules.
In straight hair, bonds form between atoms located at approximately the same sites on neighboring keratin chains. This creates relatively level “rungs” that allow the sides of the “ladder” to remain straight. In wavy or curly hair, molecules from different sites along adjacent keratin chains are attracted to each other, forming arching bonds that cause the sides of the keratin ladder to bend or loop around.