BRIDGING THE “DEAD HAIR”—“LIVE FOLLICLE” 319 ex vivo results in anagen prolongation associated with increased HM keratinocyte prolif- eration, reduced apoptosis, and TGFβ1 expression (58,59). In addition, an in vivo study showed increased hair weight and stiffness after regular employment of a caffeine- based shampoo (60). These ex vivo and in vivo observations are very well supported by the experience of consumers. However, because caffeine can also be absorbed by the HS itself (60), it is not clear whether hair fi ber quality is improved because of the impact of caffeine on the “live follicle,” “dead shaft,” or both. Davis et al. (61) reported that a hair care for- mulation containing caffeine improved HS quality, increasing hair strength and rigidity by measuring these parameters on hair tresses. This renders the possibility that caffeine acts directly on hair fi ber still more plausible. This is exactly where researchers from the “live follicle” or the “dead hair” worlds cannot close the gap. Unfortunately, at present, we do not have any well-described preclinical assays which would allow modulation of activities of the “live follicle” followed by the measurement of parameters involved in “dead hair” fi ber quality. However, taking advan- tage of new tools and techniques developed in both fi elds, such a preclinical assay could be established if experienced “live follicle” and “dead hair” researchers join forces. It is quite fulfi lling to imagine a preclinical assay in which a “live follicle” could be treated while continuously producing the HS, allowing not only parameters of the “live follicle” to be investigated (e.g., anagen prolongation and keratins expression) but also HS proper- ties (e.g., hair protein structures and hair mechanical properties) and signaling pathways involved in such modulation. CONFLICT OF INTERESTS M.B. is an employee of Monasterium Laboratory (ML) GmbH, a company owned and founded by R.P., for which N.B., G.W., C.W., and R.P. serve as consultants. ML services include the testing of hair care cosmetic ingredients in vitro, ex vivo, and in vivo, and have recently, together with TRI Princeton, opened a call for sponsors fi nancing the develop- ment of a new ex vivo preclinical assay for predicting the effects of follicle actives on hair quality. For additional information, please visit: www.monasteriumlab.com (NEWS) and www.triprinceton.org. REFERENCES (1) M. R. Schneider, R. Schmidt-Ullrich, and R. Paus, The hair follicle as a dynamic miniorgan, Curr. Biol., 19, R132–142 (2009). (2) B. Buffoli, F. Rinaldi, M. Labanca, E. Sorbellini, A. Trink, E. Guanziroli, R. Rezzani, L. F. Rodella, The human hair: from anatomy to physiology, Int. J. Dermatol., 53, 331–341 (2014). (3) G. E. Westgate, R. S. Ginger, and M. R. Green, The biology and genetics of curly hair, Exp. Dermatol., 26, 483–490 (2017). (4) R. Sennett and M. Rendl, Mesenchymal-epithelial interactions during hair follicle morphogenesis and cycling, Semin. Cell. Dev. Biol., 23, 917–927 (2012). (5) O. Duverger and M. I. Morasso, To grow or not to grow: hair morphogenesis and human genetic hair disorders. Semin. Cell. Dev. Biol., 25–26, 22–33 (2014). (6) P. Rishikaysh, K. Dev, D. Diaz, W. M. Qureshi, S. Filip, and J. Mokry, Signaling involved in hair fol- licle morphogenesis and development, Int. J. Mol. Sci., 15, 1647–1670 (2014). (7) V. A. Randall, Androgens and hair growth, Dermatol. Ther., 21, 314–328 (2008). (8) 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).

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