TLDR Japanese women's curved hair has an uneven internal structure and varying amino acid composition.
The study quantitatively investigated hair curvature in 230 Japanese women aged 10 to 70 years, finding that 53% had straight hair and 47% had curved hair with an average curl radius of 4.4 +/- 2.3 cm. Transmission electron microscopy (TEM) revealed an inhomogeneous internal structure in curved hair, with different amino acid compositions between the outer and inner regions. These findings were similar to the differences observed between ortho- and paracortical cells in wool fibers, suggesting a universal structure for curved mammalian hair.
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