The Structure and Chemistry of Human Hair 12 Exocuticle: This lamina occupies approximately 50% of the bulk of each cuticle cell and at its outward-facing edge it smoothly contours the A-layer (c.f. Figure 4). It forms a highly irregular boundary on its inner-facing edge with the adjacent endocuticle, in places sometimes almost to the exclusion of the endocuticle. It is relatively unaffected by digestion with pronase or trypsin29 but is removed fairly rapidly with a mixture of papain and dithiothreitol.28 Its relatively high level of staining in sections treated with ammoniacal silver indicates it contains a high level of cystine, albeit not as high as in the A-layer. Its proteins are estimated to contain approximately 1 in 5 of its amino acid residues as ½ cystines, with approximately 1 in 10 resides each of serine and proline.28 Again this makes for a mechanically tough and compact structure highly cross-linked by cystine disulphide bonds, but as judged by the speed of dissolution with papain-dithiothreitol reagent, probably not cross- linked by isodipeptide bonds. Endocuticle: At approximately 35% of the cellular contents of the cuticle, this highly irregular sheet-like structure occupies the remainder of each cuticle cell. It forms a convoluted boundary with the exocuticle from which there are often local excursions into its bulk. A less irregular boundary is formed with the inner layer on the inner-facing aspect of each cell. The endocuticle is generally believed to contain the cellular debris pushed to one side of each cell as the sulphur-enriched exocuticle and A-layers were laid down. Except for local incursions of exocuticular material into the bulk of the endocuticle, there is little or no staining with ammoniacal silver indicating an absence of cystine (c.f. Figure 4). On the other hand the structure is positively stained when sections are treated with phosphotungstic acid (PTA) (c.f. Figure 7). That this is due to the presence of free amino groups is indicated by the fact that PTA staining is eliminated when the initial fibers are reacted with dinitrofluorobenzene.31 The PTA stain solution contains the trivalent phosphotungstate anion (PW 12 O 40 )3- and possesses a self-pH of 1.8. Under these conditions protein amino groups in the hair sections are protonated and electrostatically bind the phosphotungstate