24 JOURNAL OF COSMETIC SCIENCE 0• c. c.70•m g o g g c.O'5•Jm .50 Figure 1. Schematic diagram illustrating the form and average relevant dimensions of the cuticle in a typical Caucasian hair. Details of the central cortex of the fiber have been omitted for clarity. tudinal direction the sheets are tilted at approximately 5 o to the hair's axis and overlap each other to present a series of scale edges at the fiber surface somewhat irregularly spaced but at an average separation of about 5 pm. At the root-end, approximately ten layers of cuticle are seen in transverse section (overall thickness ca. 5 pm). The number diminishes in a tipwise direction (3) as a function of the gradual mechanical attrition of small, single-cell thickness, pieces at the surface scale edges (4). The rate of this cuticle loss, in terms of diminution in the number of layers seen in transverse section and in overall thickness, is markedly dependent upon the subject's hair toiletry habits and, importantly, upon the amount of sunlight exposure the hair receives. These are complex, inter-related factors beyond the scope of the present review. DETAILED INTERNAL STRUCTURE, CHEMISTRY, AND PHYSICAL PROPERTIES OF THE HAIR CUTICLE Most of our knowledge about cuticle structure derives from the transmission electron microscope (TEM) examination of thin sections of hairs stained with various heavy-metal

HUMAN HAIR CUTICLE 25 compounds. According to the type of stain used, some knowledge of the chemistry of the hair's microscopic components has been deduced (5). Additionally, TEM examination of sections treated with various proteolytic enzymes, coupled with chemical analyses of physically isolated cuticle treated with these same enzymes, has provided a valuable insight into the amino acid composition of the cuticle's subcomponents (6). Other methods that have aided chemical analyses of components close to the hair's cuticular surface are X-ray microanalysis (7), electron spectroscopy for chemical analysis (ESCA) (also known as X-ray photoelectron spectroscopy, or XPS) (8), and secondary ion mass spectroscopy (SIMS) (8,9). The cloning and DNA sequencing of genes is now providing useful information about the amino acid sequences of proteins present in the cuticle (10). On the other hand, it is but speculation as to which subcomponents of cuticle contain these proteins, and uncertainty remains as to their overall conformations and roles. Each cuticular sheet consists of a series of yet thinner sub-sheets and is separated from its neighbors by an even thinner laminated cell membrane complex (CMC) (cf. Figure 2). A detailed description of each subcomponent now follows. THE A-LAYER First identified in the cuticle of human hair and wool and named by Rogers (11), this proteinaceous component, located on the outer-facing aspect of each cell, is of constant thickness (ca. 110 nm) within each cell. The thickness is more or less the same both within and between different animal species. Its cystinc content, at approximately 1 in every 2.7 amino acid residues as 1/2-cystinc (7), is exceedingly high and as such would seem to fall within the class of so-called ultrahigh sulphur (UHS) proteins commonly encountered in protein analyses of mammalian keratin fibers (10,12). No accurate amino acid analysis is available for an isolated A-layer fraction. On the other hand, the proteins contained therein could well be similar or identical to one or other of the UHS proteins from cuticle, the amino acid sequences for which have been established by the DNA sequencing of the corresponding genes (10,13,14). Upper [3-layer at hair's surface • A-layer•escedgeFractured f :"" '":: ! ' :' :"' ::" :'"'"': :'" :"" , Exocuticle .... Lower :.... Cell • [3-layer • :5 '• '/' Endocuticlayer membrane. j 6, la • • ,:• ,•..,,• ii ii1•1111 ompex II II1111 (CMC) • Upper '. .... ".:.'. ':':.•: •_•_ayer Figure 2. Highly schematic diagram showing the longitudinal internal structure of the cuticle dose to a surface scale edge, with respect to the immediately underlying cuticle sheet. The scale edge represents one that has been mechanically abraded, thereby exposing the lameliar subcomponents at the hair's surface.