STRUCTURAL ASPECTS OF KERATIN FIBRES 431 Microfil Cuticle -- Cell membranes Nuclear remnants Figure 1. Schematic diagram of human hair structure. the cell membrane complex (18), and has been shown to reduce the rate of penetration of dyes (19) and acids (20) into the fibre. It was first isolated by Lindberg, Philip and Gral•n (21), who called it 'epicuticle', and later shown by other workers (22) to be almost entirely proteinaceous and to have a high content of cystine. However, it has now been demonstrated in this laboratory that the 'epicuticle' of human hair is probably a portion of a unit cell membrane and adhering material (vide infra). The exocuticle and the underlying endocuticle, which comprise the main bulk of the cuticle, are differentiated from one another on the basis of differences in their intensity of staining with a variety of electron stains, e.g. osmium tetroxide, silver nitrate, and dodecatungstophosphoric acid, and also on the ground of chemical reactivity. There is some evidence that the exocuticle itself is complex with an outer cystine-rich layer termed exo- cuticle 'a' (23). The endocuticle is digested by enzymes (24) and hence is

432 JOURNAL OF TIlE SOCIETY OF COSMETIC CHEMISTS considered to have a low content of cross-linking cystine. The cuticle is more resistant to diffusion of reagents than the cortex (25) and the shape of cuticle cells varies with the source of fibres. It is possible to separate cuticle cells, cortical cells, and the cell membrane complex by shaking wool fibres in formic acid (26). The acid rapidly disrupts and dissolves at least part of the cell membrane complex, setting free the individual cells. Amino acid analyses of the separated, whole cuticle show that it contains con- siderably larger amounts of cystine, proline, serine, valine, and glycine than the fibre as a whole (27-29). Cuticular protein is relatively amorphous and shows neither orientation nor crystalline formation. The schematic diagram of Fig. 3 indicates the generally accepted nomenclature for the various submicroscopic components of human hair cuticle cells. Hair surface 'a'LAYER -- Exocuticle Endocuticle Membrane (cell I) Cement or 8-band Membrane (cell 2) Portion of adjacenf cuticle cell Figure 3. Schematic diagram of the various components of human hair cuticle cells. The cell membrane complex The cell membrane complex, which originates from the fusion of two unit cell membranes, one from each of the adjacent cuticle or cortical cells, has been observed by electron microscopy of transverse sections by Rogers (30, 31) and other workers. The whole structure is about 30 nm thick and consists of a unit cell membrane, which probably contains protein and a bimolecular lipid layer, then a fairly thick layer of dense material called 'intercellular cement' or fi-band and a second unit cell membrane. The intercellular cement is easily digestible by trypsin (32) and its composition has been the subject of much speculation (33). The cell membrane complex may be extracted at least partially and possibly entirely by formic acid and certain milder reagents. Amino acid analyses of the extract show that this