A highly resistant structure between the cuticle and the cortex of human hair. II. CARB, a penetration barrier

Objective We previously reported that a chemically resistant structure is present at the interface between the cuticle and the cortex of human hair. The goal of this study was to identify the position of that structure and to clarify its barrier ability. Methods Untreated, partially and completely decuticled hair fibres were characterized. The correlation between the number of cuticle layers and the penetration depth of a dye into the cortex was microscopically investigated. In addition, similar measurements were performed using hair with a denatured cell membrane complex. Results The penetration depth of the dye into the cortex showed no statistically significant correlation with the number of cuticle layers in the case of partially decuticled hair fibres (the number of cuticle layers ranged from one to four). Penetration of the dye proceeded drastically just when the last cuticle layer was lost, but denaturation of the cell membrane complex did not affect the depth of penetration. That suggested that the penetration barrier at the interface between the cuticle and the cortex is not the cell membrane complex, but rather is the resistant structure previously reported. It was observed that a thin layer structure is located beneath the endocuticle of the innermost cuticle cell layer, only in the range where the cuticle borders the cortex. It is thought that this is the structure in question. Conclusion These results demonstrate that a highly resistant structure located at the interface between the cuticle and the cortex of human hair acts as a penetration barrier. We propose that structure be named CARB, cuticle anchored resistant base.