CHEMISTRY OF HUMAN HAIR CUTICLE--Ill 299 amino sugars and pentoses in a cuticle cell membrane fraction, rich in fi-band, obtained by our papain/dithiothreitol procedures (2). Also this fraction had a very low protein content. (g) Consideration of the properties of the cuticle in the light of the composition of the cuticle subcomponents The hair cuticle is divided into two main subcomponents whose chemical compositions are quite different and which can be expected to behave differently under the influence of various cosmetic treatments. The exocuticle and A-layer laminae are oriented within each cuticle cell sheet towards the outside of the hair. Since they are highly cross-linked by cystine they will be extremely tough and resilient and fitting in their role for protecting the surface of the hair shaft from environmental exposure. On the other hand these components are likely to be relatively inelastic and susceptible to brittle fracture on bending. An indication of this brittleness to bending is that we have never observed with the scanning electron microscope cuticle scale edges bent back on themselves at the surface of virgin hair. Indeed where the edge of a cuticle cell is not adhering immediately to the underlying cell, it is usually tilted upwards by only a few degrees. Despite extensive combing either with or against the natural scale overlap the scale edges tend to break off in small pieces rather than be bent back (12). If, on the other hand, the cystine disulphide crosslinks are destroyed by oxidation (as occurs in weathered hair or to some extent on bleach- ing with hydrogen peroxide) or by reduction (as in thioglycollate perming) the exocuticle and A-layer become much more pliable. This is indicated by the fact that we sometimes see cuticle edges bent back through quite large angles when weathered, bleached or reduced hairs have been combed. It is also noteworthy that Makinson (13) believes that the oxidative softening of the wool cuticle is important in the anti-felting treatments of wool fabrics and it is this which elimi- nates the directional dependance of the coefficient of friction of the fibres. In contrast to the exocuticle, the proteins of the endocuticle contain very little cystine but relatively high concentrations of acidic and basic amino acid residues. Electron histochemical experiments indicate the existence of fairly high concen- trations of free carboxyl and amino groups in the endocuticle, but it is not possible to determine from these experiments alone what proportions of the groups are the side chains of the acidic or basic amino acid residues or the end groups of the protein chains, though we would expect the latter to be quite a small proportion. It is possible that some of the acidic and basic amino acid residues are involved in intermolecular isopeptide cross-linking (14). On the other hand since the pres- ence of isopeptide links in proteins such as hair medulla tends to confer trypsin insolubility (15, 16) we must presume that the amount of isopeptide link present in our trypsin-soluble ENDO A components of the cuticle is quite small. The exocuticle and endocuticle can be expected to behave quite differently in

300 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS water. The exocuticle, because of its high cross-link density, is unlikely to swell appreciably, but the relative absence of cross-links and the presence of ionic groups in the endocuticle will mean that it will absorb fairly large quantities of water (17). The resultant swelling may even be high enough to convert some of the endocuticular components into a gel. This could have interesting implications on the behaviour of the hair cuticle in the wet state and could perhaps contribute to the directional frictional properties of wet hair where there is a strong tendency towards felting. ACKNOWLEDGMENT We are grateful to Mr F. J. Bailey of Unilever Research Laboratory, Colworth House for undertaking amino acid analyses for us. (Received: loth April 1975) REFERENCES (1) Swift, J. A. and Bews, B. The chemistry of human hair cuticle--I: A new method for physical isolation of cuticle. J. Soc. Cosmet. Chem. 25 13 (1974). (2) Swift, J. A. and Bews, B. The chemistry of human hair cuticle--II: The isolation and amino acid analysis of the cell membranes and A-layer. J. Soc. Cosmet. Chem. 25 355 (1974). (3) Bradbury, J. H. and Ley, K. F. Separation and analysis of exocuticle and endocuticle. Aust. J. Biol. Sci. 25 1235 (1972). (4) Swift, J. A. The electron histochemistry of cystine-containing proteins in thin transverse sections of human hair. Jl R. Micros. Soc. 88 449 (1967). (5) Dobb, M. G., Murray, R. and Sikorski, J. Specific labelling of thiol groups in mammalian keratin suitable for electron microscope studies. J. Microsc. 96 285 (1972). (6) Kassenbeck, K.--private communication. (7) Stellwagen, R. H. and Cole, R. D. Chromosomal proteins. Ann. Rev. Blochem. 38 951 (1969) (8) Garrard, W. T., Pearson, W. R., Wake, S. K. and Bonner, J. Stoichiometry of chromatin proteins. Biochem. Biophys. Res. Commttn. 58 50 (1974). (9) Bradbury, J. H. The structure and chemistry of keratin fibres. Advn. Protein Chem. 27 111 (1973). (10) Swift, J. A. Unpublished observations on the application to human hair of an electron histochemical procedure devised by P. A. Kendall and E. A. Barnard, Jl R. Microsc. $oc. 81 203 (1963). Orwin, D. F. G. A polysaccharide-containing coat on keratinising cells of the Romney wool follicle. Aust. J. biol. $ci. 23 623 (1970). Swift, J. A. and Brown, A. C. The critical determination of fine changes in the surface architecture of human hair due to cosmetic treatment. J. $oc. Cosmet. Chem. 23 695 (1972). Makinson, K. R. The integration of various observations on the mechanisms of 'degra- dative' anti-felting treatments. J. Textile Inst. 64 228 (1973). Asquith, R. S., Otterburn, M. S. and Sinclair, W. J. Isopeptide crosslinks--their occurrence and importance in protein structure. Angew. Chem. 13 514 (1974). Rogers, G. E. Structural and biochemical features of the hair follicle. In: The epidermis, eds W. Montagna and W. C. Lobitz, 179 (1964) (Academic Press, New York). Harding, H. W. J. and Rogers, G. E. The occurrence of the e- (¾-glutamyl)lysine crosslink in the medulla of hair and quill. Biochem. Biophys. Acta 257 37 (1972). Leeder, J. D. and Watt, I. C. The stoichiometry of water sorption by proteins. J. CoIL Interface $ci. 48 339 (1974). (11) (12) (13) (14) (15) (16) (17)