The Structure and Chemistry of Human Hair 10 epicuticle is derived from the A-layer. Their argument is that it is isodipeptide-cross-linked remnants of the whole thickness of the A-layer that form the semi-permeable sacs seen with chlorine water treatment. There seems no reason to accept this argument given that where the thickness of chlorine water-derived epicuticle has been measured it is between 50 and 140 Å.23-25 In partial support of this view is that in another epithelial-related structure, the stratum corneum, the internal surfaces of each cell are covered with an isodipeptide-rich proteinaceous layer of relatively constant thickness (153 Å).26 In these latter respects this author feels entirely justified in regarding the epicuticle of human hair and other mammalian keratin fibers to be the 130 Å thick layer seen in sections stained with ammoniacal silver.20 It is pertinent to mention that in studying isolated cuticle cells from wool Leeder found chlorine water raised sacs over the entire surfaces of the cells.27 This observation informs us that a thin isodipeptide-rich epicuticle-like layer exists on the inner surfaces of all human cuticle cells. Indeed, the so-called “inner layer” found on the inner-facing surfaces of the cuticle could be of such composition, albeit its thickness varies from 100 to 400 Å.12 A-layer: This proteinaceous sheet-like component is of relatively constant thickness (ca. 1100 Å) and is specifically located on the outer-facing aspect of each and every cuticle cell immediately Figure 6. The formation of isopeptide cross-links between proteins in the hair follicle occurs through the post-translational action of transglutaminase on the protein side chains, lysine and glutamine.