Morphological analyses in fragility of pili torti with Björnstad syndrome.

Pili torti is an extremely rare hair phenotype characterized by short length of hairs with hair shafts being easily broken. However, the mechanism of fragility in pili torti is unclear. In this study, we examined the underlying morphological features responsible for pili torti formation using transmission electron microscopy (TEM). We used pili torti samples from a patient with Björnstad syndrome and normal hairs from a healthy subject as a comparison. The macroscopic morphological features of the samples agreed with the results of a previous study showing that pili torti is twisted, flattened, thin and with partial trichorrhexis. Young's modulus of the samples was lower than that of normal hairs. Because the cross-sectional area of the pili torti samples was also smaller than that of normal hairs, it was clarified that the tensile strength of pili torti is 2.1-times lower than that of normal hair. Assessment of morphological features by TEM showed that the cuticle layers of the samples had wavy shapes with different thicknesses. Additionally, the cortex in the samples showed loose keratin intermediate filaments (IF). Our results suggested that these abnormalities in pili torti had already occurred below the infundibulum. Thus, the weakness of pili torti in tensile strength is thought to result from loose IF because of dysformation of disulfide bonds.