Characteristic microstructure of curved human hair

Abstract

Vol. 43, No.3 (2009) pp.201–208 Hair appearance is affected by several factors, such as hair shape, color and optical properties relating to hair luster. For example, recently, we have found that meandering (curved) hair fibers increase with aging and cause the disordered alignment of hair fibers, resulting in a decrease in hair luster in older women. It is, therefore, important to understand the relationship between hair shape and its microstructure to control the fiber alignment and then the hair appearance. In this study, we have investigated the characteristic microstructure of curved human hair with microscopic methods and the following results were obtained: (i) The permeation behavior of fluorescein isothiocyanate into curved hair was asymmetrical. The permeation rate was faster from the convex-side but slower from the concave-side of the curved hair. This difference in permeation rate suggests structural and/or compositional differences between the convex- and concave-sides. (ii) Transmission electron microscopic (TEM) observations revealed that macrofibril morphology is different between the convex- and concave-sides. The macrofibrils in the convex-side are relatively smaller and dispersed in inter-macrofibrillar material, while those in the concave-side are relatively larger and fused together. (iii) TEM observations also suggested that the alignment structure of intermediate filaments in the convex-side is helical or twisted in each macrofibril, while that in the concave-side is approximately parallel to the fiber axis. (iv) The amino acid analyses of the convex- and concave-halves of the curved hair fibers revealed that Cys is lower and Asp, Glu and Gly are higher in the convex-half. Interestingly, these results of the TEM observation and the amino acid analysis are analogous to the difference between ortho- and para-cortical cells in wool fibers, suggesting the universal structure of curved mammalian hair.