650 JOURNAL OF COSMETIC SCIENCE Proteomic analyses of follicles are few (80). Genomic analyses of curly versus straight hairs have identified single nucleotide polymorphism targets associated with curly hair or curl-associated mutations (27), but protein analyses at the follicle level have largely been carried out using techniques that target specific proteins and determine their location. Most advances have been made using in situ hybridization and immunohistochemical approaches (44,67,74,81–84), and few have targeted understanding curl. However, work on both human scalp and wool follicles indicates the association of K38 with increased expression on the ental side of the developing cortex in high-curl hair (44,58,74). CONCLUSION The future challenge will be to connect the many factors in hair and follicle (Figure 2) that correlate with the presence of hair curliness or the extent of curliness. This will require linking features through chemistry and biology to morphology across scales from microscopic to macroscopic and will require an experimental approach and likely correlative measurements (e.g., the three-dimensional location of protein chemistry in fiber and follicle). ACKNOWLEDGMENTS We are grateful to TRI Princeton for facilitating the ninth International Conference on Applied Hair Science in which this article had its origin. We are grateful to our AgResearch colleagues Jeffrey Plowman, Anita Grosvenor, and Evelyne Maes for discussions and suggestions that improved this manuscript. This work was supported by AgResearch through its Smart and Sustainable Biomaterials Programme (New Zealand Government Ministry of Business Innovation and Employment, Science strategic investment fund). REFERENCES (1) D. Kouremetis, Change your hair, change your life, Psychology Today, May 30, 2019, accessed September 16, 2021, https://www.psychologytoday.com/nz/blog/raging-grace/201905/change-your-hair-change-your- life. (2) B. Fink, K. Liebner, A.-K. Müller, T. Hirn, G. McKelvey, and J. Lankhof, Hair colour and skin colour together influence perceptions of age, health and attractiveness in lightly pigmented young women, Int. J. Cosmet. Sci., 40, 303–312 (2018). (3) G. L. Patzer, Psychologic and sociologic dimensions of hair: an aspect of the physical attractiveness phenomenon, Clin. Dermatol., 6, 93–101 (1988). (4) C. R. Robbins, Chemical and Physical Behavior of Human Hair. 5th Ed. (Springer, New York, NY, 2012). (5) P. Cornwell and E. Malinauskyte, Defying damage: understanding breakage in Afro-textured hair, Cosmetics and Toiletries,January30,2020,accessedSeptember16,2021,https://www.cosmeticsandtoiletries. com/research/biology/Defying-Damage-Understanding-Breakage-in-Afro-textured-Hair-567425551. html. (6) K. Molamodi, D. Fajuyigbe, P. Sewraj, J. Gichuri, B. Sijako, A. Galliano, and A. Laurent, Quantifying the impact of braiding and combing on the integrity of natural African hair, Int. J. Cosmet. Sci., 43, 321–331 (2021). (7) H. Bryant, C. Porter, and G. Yang, Curly hair: measured differences and contributions to breakage, Int. J. Dermatol., 51 (Suppl.), 8–11 (2012).

651 WHAT CAUSES CURLY HAIR? (8) M. L. Ryder. Hair (Institute of Biology: Studies in Biology) (Edward Arnold, London, 1973). (9) P. F. Scholander, V. Walters, R. Hock, and L. Irving, Body insulation of some arctic and tropical mammals and birds, Biol. Bull., 99, 225–236 (1950). (10) A. Thomas, D. P. Harland, S. Clerens, S. Deb-Choudhury, J. A. Vernon, G. L. Krsinic, R. J. Walls, C. D. Cornellison, J. E. Plowman, and J. M. Dyer, Interspecies comparison of morphology, ultrastructure and proteome of mammalian keratin fibers of similar diameter, J. Agric. Food Chem., 60, 2434–2446 (2012). (11) R. A. Quinlan, E. H. Bromley, and E. Pohl, A silk purse from a sow’s ear—bioinspired materials based on α-helical coiled coils, Curr. Opin. Cell Biol., 32, 131–137 (2015). (12) L. Cera, G. M. Gonzalez, Q. Liu, S. Choi, C. O. Chantre, J. Lee, R. Gabardi, M. C. Choi, K. Shin, and K. K. Parker, A bioinspired and hierarchically structured shape-memory material, Nat. Mater., 20(2), 242–249 (2021). (13) D. P. Harland, J. A. Vernon, J. L. Woods, S. Nagase, T. Itou, K. Koike, D. A. Scobie, A. J. Grosvenor, J. M. Dyer, and S. Clerens, Intrinsic curvature in wool fibres is determined by the relative length of orthocortical and paracortical cells, J. Exp. Biol., 221(Pt 6), jeb172312 (2018). (14) B. Tsang, Modelling the Mechanisms Governing Crimp in Wool (University of Canterbury, Christchurch, New Zealand, 2004). (15) B. P. Baxter, M. A. Brims, and T. B. Taylor, Description and performance of the Optical Fibre Diameter Analyser (OFDA), J. Text. Inst., 83, 507–526 (1992). (16) G. Loussouarn, A.-L. Garcel, I. Lozano, C. Collaudin, C. Porter, S. Panhard, D. Saint-Léger, and R. de La Mettrie, Worldwide diversity of hair curliness: a new method of assessment, Int. J. Dermatol., 46(Suppl. 1), 2–6 (2007). (17) J. E. Plowman and D. P. Harland, “Fibre ultrastructure,” in The Hair Fibre: Proteins, Structure and Development. J. E. Plowman, D. P. Harland, and S. Deb-Choudhury, Eds. (Springer Nature, Singapore, 2018), pp. 3–13. (18) J. W. Hearle, A critical review of the structural mechanics of wool and hair fibres, Int. J. Biol. Macromol., 27, 123–138 (2000). (19) E. Cloete, N. P. Khumalo, J. C. Van Wyk, and M. N. Ngoepe, Systems approach to human hair fibers: interdependence between physical, mechanical, biochemical and geometric properties of natural healthy hair, Front. Physiol., 10, 112 (2019). (20) Y. S. Lim, D. P. Harland, and T. L. Dawson Jr, Wanted, dead and alive why a multidisciplinary approach is needed to unlock hair treatment potential, Exp. Dermatol., 28, 517–527 (2019). (21) S. J. Gould and R. C. Lewontin, The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme, Proc. R. Soc. Lond. B, 205, 581–598 (1979). (22) E. Cadieu, M. W. Neff, P. Quignon, K. Walsh, K. Chase, H. G. Parker, B. M. Vonholdt, A. Rhue, A. Boyko, A. Byers, A. Wong, D. S. Mosher, A. G. Elkahloun, T. C. Spady, C. André, K. Gordon Lark, M. Cargill, C. D. Bustamante, R. K. Wayne, and E. A. Ostrander, Coat variation in the domestic dog is governed by variants in three genes, Science, 326(5949), 150–153 (2009). (23) E. Cloete, N. P. Khumalo, and M. N. Ngoepe, The what, why and how of curly hair: a review, Proc. Math Phys. Eng. Sci., 475(2231), 20190516 (2019). (24) S. Thibaut, O. Gaillard, P. Bouhanna, D. W. Cannell, and B. A. Bernard, Human hair shape is programmed from the bulb, Br. J. Dermatol., 152, 632–638 (2005). (25) H. Yang, R. C. Adam, Y. Ge, Z. L. Hua, and E. Fuchs, Epithelial-mesenchymal micro-niches govern stem cell lineage choices, Cell, 169, 483–496.e13 (2017). (26) G. C. Priestley, Histological studies of the skin follicle types of the rat with special reference to the structure of the Huxley layer, J. Anat., 101, 491–504 (1967). (27) G. E. Westgate, R. S. Ginger, and M. R. Green, The biology and genetics of curly hair, Exp. Dermatol., 26, 483–490 (2017). (28) S. L. Koch, M. D. Shriver, and N. G. Jablonski, Variation in human hair ultrastructure among three biogeographic populations, J. Struct. Biol., 205, 60–66 (2019).