only in the cuticle of beard and scalp hair (33). Other than K85, which is expressed in both cuticle and cortex, K40 is the only cuticle protein of interest. Interestingly, a recent microstructure study found statistically fewer cuticle layers in curly hair of individuals of African genetic ancestry than those with straight hair from European and East Asian genetic ancestries (9). K1, K2, and K10 are epithelial keratins, normally playing a role in the follicle root sheathes rather than the hair shaft (34). They are, however, found in the medulla of hair fi bers (35). It is likely that these are associated with the peculiar assembly process and structure of the medulla by which, possibly random, combinations of various keratin species (trichocyte and epithelial) are brought together to form the medulla structures (36). The two differential non-keratin proteins, SBP1 and ADAMTS-like protein 3 (ALT3), had higher abundance in the straight hair samples. Punctin (also known as ALT3), when it occurs in living tissue, is an extracellular matrix protein associated with epithelial cells (37). During hair development, the extracellular material is transformed into cell-type– specifi c continuous structured cement called the cell membrane complex (22,38). Although identifi ed as signifi cantly different between the two groups, it should be noted that this is based on just one peptide identifi cation, and hence, should be considered with extreme care. Nevertheless, one intriguing possibility is that the higher abundance observed in straight hair is an indication of differences in cortical cell lengths across the cortex be- tween straight and curly hair similar to that correlated to the extent of fi ber curvature in wool (39). Finally, methanethiol oxidase is a cellular selenium-binding protein (SBP1) expressed in multiple tissues, also previously found in hair samples (40), which is likely a leftover from fi ber development. However, its function such as interacting with thiol groups to poten- tially generate formaldehyde molecules (41) may indicate an unknown role in the fi nal stages of hair maturation, which is an environment in which sulfur chemistry is critical to the function. In conclusion, this proteome analysis between two extreme groups of hair shape indicates that differences at the protein abundance level can provide useful insights into the physiochemistry underpinning differences in hair shape and the relevant hair growth processes which should inform technological interventions for hair care. However, although differences between the very curly and the very straight hair sample groups could be identifi ed, the linkage between biogeographic genetic differences and curl phenotype is currently unknown and would require further controlled investigation. REFERENCES (1) R. D. Sinclair, Healthy hair: what is it? J. Invest. Dermatol. Symp. Proc., 12(2), 2–5 (2007). (2) R. De la Mettrie, D. Saint-Leger, G. Loussouarn, A. Garcel, C. Porter, and A. Langaney, Shape variability and classifi cation of human hair: a worldwide approach, Hum. Biol., 79(3), 265–281 (2007). (3) B. Lindelof, B. Forslind, M. A. Hedblad, and U. Kaveus, Human hair form. Morphology revealed by light and scanning electron microscopy and computer aided three-dimensional reconstruction. Arch. Dermatol., 124(9), 1359–1363 (1988). (4) R. E. Chapman, “The ovine arrector pili musculature and crimp formation in wool,” in Biology of the Skin and Hair Growth, A. G. Lyne, B. F. Short. Eds. (Angus and Robertson, Sydney, Australia, 1965), pp. 201–232. (5) G. E. Westgate, R. S. Ginger, and M. R. Green, The biology and genetics of curly hair. Exp. Dermatol., 26(6), 483–490 (2017). JOURNAL OF COSMETIC SCIENCE 260

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