JOURNAL OF COSMETIC SCIENCE 330 Mature hair is dead hair, composed of the dead remnants of cells that have been sacrifi - cially transformed into the cuticle, cortex, and medulla (63). The function of hair, defi ned by their structure, protein chemistry, and mechanical properties, is codifi ed in life to generate a phenotype that only becomes functional after death. How human hair pheno- type is determined by specifi c genetic controls remains mostly unknown. Most genetic disorders that may provide interesting clues come from human heritable diseases, such as monilethrix mentioned previously, or similar scenarios in animal models. We propose that macroscale studies of hair phenotype should form an essential complement to bio- logically driven hair research (64). SUMMARY AND CONCLUSIONS Generation of human hair is a complex process involving biology to create biomass and generate biosynthetic building blocks and chemistry to use these building blocks to cre- ate the fi nal hair shaft. These activities are compartmentalized within the hair follicle, a complex structure with many cell types coordinated into a single production unit. Mea- surement of the end product, the hair shaft, is mainly based on physics. To best serve patients, hair loss subjects, and consumers in need of better hair, the scientifi c com- munities in the diverse disciplines of biology, chemistry, and physics need to work together. When merged, a joint program has defi ned follicle energy production and mitochondrial energy production as high potential intervention points for designing new products. REFERENCES (1) P. Mohammadi, K. K. Youssef, S. Abbasalizadeh, H. Baharvand, and N. Aghdami, Human hair recon- struction: close, but yet so far, Stem Cells Dev., 25, 1767–1779 (2016). ( 2) C. Robbins, P. Mirmirani, A. G. Messenger, M. P. Birch, R. S. Youngquist, M. Tamura, T. Filloon, F. Luo, and T. L. Dawson Jr., What women want—quantifying the perception of hair amount: an analysis of hair diameter and density changes with age in Caucasian women, Br. J. Dermatol., 167, 324–332 (2012). ( 3) R. Sinclair, M. Patel, T. L. Dawson, A. Yazdabadi, L. Yip, A. Perez, and N. W. Rufaut, Hair loss in women: medical and cosmetic approaches to increase scalp hair fullness, Br. J. Dermatol., 165, 12–18 (2011). ( 4) R. M. Trüeb, Pharmacologic interventions in aging hair, Aging, 1, 121–129 (2006). (5) N. Hunt and S. McHale, The psychological impact of alopecia, Br. Med. J., 331, 951–953 (2005). (6) Statista. The statistics portal. Global hair care market size 2012–2024. Available at: https://www. statista.com/statistics/254608/global-hair-care-market-size. Accessed July 29, 2018. (7 ) J. W. Oh, J. Kloepper, E. A. Langan, Y. Kim, J. Yeo, M. J. Kim, T. C. Hsi, C. Rose, G. S. Yoon, S. J. Lee, J. Seykora, J. C. Kim, Y. K. Sung, M. Kim, R. Paus, and M. V. Plikus, A guide to studying human hair follicle cycling in vivo, J. Invest. Dermatol., 136, 34–44 (2016). (8) M. Saitoh, M. Uzuka, H. Sakamoto, and T. Kobori, “Rate of hair growth,” in Advances in Biology of Skin, W. D. Montagna and R. L. Oxford. Eds. (Pergamon Press, New York, NY, 1969), pp. 183–201. (9) F. J. Ebling and P. J. Hale, “Hormones and hair growth,” in Biochemistry and Physiology of the Skin, L. A. Goldsmith. Ed. (Oxford University Press, New York, NY, 1983), pp. 522–562. (1 0) D. J. Cottle, Australian Sheep, and Wool Handbook (Inkata Press, Melbourne, Australia, 1991). (1 1) J. P. Hogan, N. M. Elliott, and A. D. Hughes, “Maximum wool growth rates expected from Australian merino genotypes,” in Physiological and Environmental Limitations to Wool Growth, J. L. Black and P. J. Reis. Eds. (University of New England, Armidale, New South Wales, Australia, 1979), pp. 43.

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