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Y. Soc. Cosmetic Chemists, 18, 537-547 (Aug. 19, 1967) Keratin Replacement Aging Parameter as an NORMAN ORENTREICH, M.D.* and NANCY J. SHARP, B.A.i Presented September 20, 1966, Seminar, New York City Synopsis--The rate of nail growth diminishes in a measurable, reproducible, predictable manner with increasing age. There is a diminution in this rate of almost 40% between the ages of 25 and 95, or about 4.5 u/week for each year of age after maturation. Young men's nails grow faster than women's until middle age but grow more slowly after the seventh decade of life. The known factors that have no effect on or that alter nail growth are enumer- ated. The measurement of nail growth may be a useful method of screening cosmetic prep- arations that may influence the nail. INTRODUCTION The scarcity of techniques for measuring and evaluating phys- iologic age, both in humans and animals, has hampered the advance of the science of experimental gerontology. A useful aging parameter must fulfill the following criteria: ease of performance, valid predicta- bility in relationship to aging and longevity, reproducibility, low variability in each age range, and known relationship to disease. Studies performed on the rate of keratin replacement of hair, nails, and skin indicate that there is a definite relationship between this rate and the age of the individual. The earliest known quantitative evalua- tion of the rate of regeneration of epithelium was performed during World War I in France by Carrel and duNouy (1). While involved in the development of a better antiseptic for wounds, they developed * Orentreieh Medical Group, 909 Fifth Ave., New York, N.Y. 10021. } Orentreich Foundation for the Advancement of Science, New York, N.Y. 10021 537