Chapter 1 9 friction. Loss of 18-MEA from the hair surface by UV exposure or by chemical treatment increases the with-scales coefficient of friction.15, 18 Epicuticle: The epicuticle of mammalian hairs was first identified in 1916 by Allwörden19 as the surface membranous sacs produced when the fibers were treated with dilute chlorine water. Epicuticular sacs arise by an osmotic process. This involves the solubilization of underlying proteins by chlorine oxidation (cystine residues to cysteic acid residues) and the osmotic ingress of water through the overlying semi-permeable outer sheath (the epicuticle) that is relatively unaffected by the chlorine. There has been considerable debate as to whether the epicuticle possesses a distinct histological identity within the proteins at the immediate sub-surface of each cuticle cell. Swift and Smith determined the epicuticle to be a layer of constant 130 Å thickness covering the outer surface of each and every cuticle cell.20 In sections of hair treated with ammoniacal silver nitrate they found the epicuticle layer was stained with silver at much higher intensity than any other component in the cell. It was demonstrated the intense staining had occurred by dint of a high level of scission of cysteine-bound 18-MEA in the epicuticle to produce sulphydryl groups that had then reacted with the silver. The epicuticle remains intact after chlorine water treatment because, although its proteins contain significant amounts of cystine that undergo oxidation to cysteic acid, these proteins are also cross- linked by isodipeptide bonds. Isodipeptide cross-links, connecting the side chains of lysyl and glutamyl residues of the proteins, are produced in post-translational modifications in the hair follicle (c.f. Figure 6). Their presence renders the epicuticle insoluble to a variety of proteolytic agents that readily dissolve other hair components. When 18-MEA is specifically cleaved from the surface of mammalian keratin fibers by potassium tertiary butoxide, epicuticular membranous sacs continue to be produced with chlorine water.21 This calls into question whether or not the 130 Å layer seen in the silver-stained sections is the actual epicuticle. An alternative view by Phan et al.22 is that the