CHANGES IN HUMAN SKIN WITH AGING 839 known. However, one might be tempted to postulate a feedback con- trol mechanism operative in the epidermis. As was mentioned, the adult and senile skin show increased desquamation, possibly because of increased keratinization, modification of dermal vascularity, or insuffi- ciency of epidermal glandular secretion. This cell loss may act as a trigger to signal an increase in the replacement of cells. As cell loss is accelerated, then, so also is cell replacement. While there are but few techniques presently available for the treatment of aging skin, the ultimate method will probably involve the use of agents which will be able to act directly on the controlling mech- anisms of the body's cells--the genes--either to correct errors (which, as we have seen, can evolve with time), or to initiate some desired ac- tivity (such as induction of a specific enzyme). When man is able to achieve this kind of control, there will be practically no limit to his ability to regulate the functioning of his body even to stemming the tide of aging. (Received April 17, 1968) REFERENCES (1) Johnson, H. D., Kintner, L. D., and Kibler, H. H., Effects of 48øF and 83øF on lon- gevity and pathology of male rats, J. Gerontol., 18, 29 (1961). (2) McCay, C. M., Magnard, L. A., Sperling, H., and Barnes, L. L., Retarded growth, lifespan, ultimate body size and age changes in the albino rat after feeding diets re- stricted in calories, J. Nutr., 18, 1 (1939). (3) Curtis, H. J., Biological mechanisms underlying the aging process, Science, 141, 686 (1963). (4) Albert, M.D., X-irradiation induced mitotic abnormalities in mouse liver regenerating after CC14 injury. I. Total body irradiation. J. Nat. Cancer Inst., 20, 309 (1958). (5) Stevenson, K. G., and Curtis, H. J., Chromosome aberrations in irradiated and nitrogen mustard treated mice, Radiation Res., 15, 774 (1961). (6) Bjorksten, J., Chemical causes of the aging process, Proc. Sci. Sect. Toilet Goo&, Assoc., 41, 32 (1964). (7) Verzar, F., The aging of collagen, $ci. Am., 208, 104 (1963). (8) Harman, D., The free radical theory of aging: effect of age on serum copper levels, J. Gerontol., 20, 151 (1965). (9) Norins, A. L., Free radical formation in the skin following exposure to ultraviolet light, J. Invest. Dermatol., 39, 445 (1962). (10) Lorincz, A. L., Physiology of the aging skin, [llinois Med. J., 117, 59 (1960). (11) Sams, W. M., Jr., and Smith, J. G., Jr., Alterations in human dermal fibrous connective tissue with age and chronic sun .damage, in Montagna, W., Advances in Biology of the Skin--Volume VY--Aging, Pergamon Press, New York, 1965, pp. 199-210. (12) Rothman, S., Water and electrolytes, in Physiology and Biochemistry of the Skin, The University of Chicago Press, Chicago, 1961, pp. 493-514. (13) Flesch, P., The chemistry of the aging skin, J. Soc. Cosmetic Chemists, 6, 377 (1955). (14) Smith, J. G., Jr., and Finlayson, G. R., Dermal connective tissue alterations with age and chronic sun damage, Ibid., 16, 527 (1965).

84O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (15) (16) (17) (18) (19) (20) (22) (23) (24) (25) (26) (27) (28) (29) Nimni, M. E., deGuia, E., and Bavetta, L. A., Collagen, hexosamine and tensile strength of rabbit skin during aging, J. Invest. Dermatol. 47, 156 (1966). Rasmussen, D. M., Wakim, K. G., and Winkelmann, R. K., Isotonic and isometric thermal contraction of human dermis. II. Age-related changes, Ibid., 45,341 (1904). Kohn, R. R., and Rollerson, E., Relationship of age to swelling properties of human diaphragm tendon in acid and alkaline solutions, J. Gerontol. 15,241 (1958). Keech, M. K., The effect of collagenase on the fixed and unfixed skin lesions of morphoea: An electronmicroscopic study, J. ?athol. Bacteriol., 77, 351 (1959). Houck, J. C., DeAngelo, L., and Jacob, R. A., Connective tissue. III. Dermal chemical response to age, ?roc. Soc. Exp. Biol. Med., 107, 280 (1961). Verzar, F., The aging of connective tissue, Gerontologia, 1,363 (1957). Smith, J. G., Jr., Davidson, E. A., and Taylor, R. W., Human cutaneous acid muco- polysaccharides: The effects of age and chronic sun damage, in Montagna, W., Ad- vances in Biology of the Skin--Volume VI Aging, Pergamon Press, New York, 1965, pp. 211-8. Smith, J. G., Jr., The derreal elastoses, Arch. Dermatol., 88,382 (1963). Weinstein, G. D., and Boucek, R. J., Collagen and elastin of human dermis, J. Invest. Dermatol., $5,227 (1960). Hult, A.M., and Goltz, R. W., The measurement of elastin in hmnan skin and its quantity in relation to age, Ibid., 44, 408 (1965). Weinstein, G. D., Autoradiographic studies of turnover time and protein synthesis in pig epidermis, Ibid., 44, 413 (1965). Storey, W. F., and Le Blond, C. P., Measurement of rate of proliferation of epidermis and associated structures, Ann. N.Y. Acad. Sci., 55, 537 (1951). Bertalanffy, F. D., Mitotic activity and renewal rate of sebaceous gland cells in rat, Anat. Record, 129, 231 (1957). Bertalanffy, F. D., Pusey, V., and Abbott, M.D., Mitotic rates of rat epidermis, Arch. Dermatol., 92, 91 (1965). Thur/nger, J. M., and Katzberg, A. A., The effect of age on mitosis in the human epidermis, J. [•vest. Dermatol., $$, 35 (1959).