Japanese human hair is generally categorized as straight. However, our studies confirmed that approximately half of Japanese women have curved hair. The curved hair often causes hair concerns, because of a less ordered hair style and a decrease in hair luster. We, therefore, investigated morphological and microstructural features contributing to the curved hair shape.
Naturally straight and curved human hairs were examined using fluorescence light microscopy (FLM) and transmission electron microscopy (TEM). Human hair cortical cells were classified according to their TEM appearance. The abundances and distributions of the different cortical cell types in relation to fiber curvature were determined using TEM and a newly developed FLM method. Electron tomography was used to analyze 3D reconstruction of intermediate filament arrangements in the different cell types.
The cortex contained four classifiable cell types, two of which were always present in much greater abundance than the remaining two types. In straight hair, these cell types were arranged annularly and evenly within the cortex. In curved fibers, the cell types were bilaterally distributed approximately perpendicular to fiber curvature direction with one dominant cell type predominantly located closest to the convex fiber side and the other, closest to the concave side. Electron tomography confirmed that the dominant cell type closest to the convex fiber side contained discrete macrofibrils composed of helically arranged intermediate filaments, while the dominant cell type closest to the concave side contained larger fused macrofibrils composed of intermediate filament arrangements varying from helical to hexagonal arrays approximately parallel to the longitudinal fiber axis.
We concluded that different cortical cell types are bilaterally distributed in curved human hair and that intermediate filament arrangements differ in the different cortical cell types. These findings concur with the current hypothesis of hair curvature formation and behavior.