Hair shaft outer cuticle

The hair shaft (Figure 6.1) comprises three main structures (1) the outer cuticle responsible for the main optical and frictional properties of the fiber (2) the cortex, responsible for the bulk fiber mechanical properties such as strength and flexibility and (3) the porous medulla, which is more prominent in gray hair, but otherwise may or may not be present. Cuticle thickness varies markedly between species. Though much of our understanding of hair structural biology is derived from the study of wool, this homologous... 

What happens in a permanent wave Permanent-wave lotions are designed to penetrate the scales of the cuticle, the outer layer of the hair shaft. The lotions work because they affect the structure of the proteins that make up the hair. The amino acid cysteine, which contains an atom of sulfur, is found in human hair protein. Sulfur atoms in neighboring cysteine molecules within the hair protein form strong, covalent, disulfide (S — S) bonds. This cross-linking between cysteine molecules holds the strands of hair protein in place and affects its shape and strength. 

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