649 WHAT CAUSES CURLY HAIR? their transit through the follicle will likely be required to identify additional potential contributions of cell division, migration, and reshaping to curvature. PROTEOMIC AND PROTEIN EXPRESSION Our ability to both identify and relatively quantify individual structural proteins and their chemical modifications and associations (e.g., chemical cross-links) between hair samples is rapidly developing. The current state of knowledge from sheep model studies is that the abundance of some structural proteins differs between straight and curly wool fibers from closely related individual animals (fraternal twins). Quantitative mass spectrometric analysis demonstrated that straight wool and curly wool differed in the relative abundance of individual species from high glycine-tyrosine protein families and ultra-high sulfur protein families (46). Similarly, protein abundance differences have been observed between human scalp hair samples chosen for being very straight versus very curly (77). Proteomic analysis was able to pick out individual keratin and keratin-associated proteins that differed between the sample groups (keeping in mind that samples likely differed in other ways than just curliness). Quantitative mass spectrometry is a rapidly advancing technology, and these early results suggest it will be a powerful tool to identify the proteins of interest for understanding phenomena such as hair curliness. The main current limitation of all mass spectrometry techniques is that optimal results require multiple hairs and a lack of data on protein associations or locations, although both of these are areas of current research (78,79). Figure 2. Summary of a generic mammalian anagen phase follicle growing a curly hair, with key structures and features associated with curvature listed.

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).