J. Soc. Cosmetic Chemists, 19, 829-840 (Dec. 9, 1968) Changes in Human Skin in the Light of Current Theories of Aging WILLIAM R. TROY, M.S.* Presented April 3, 1968, New York City Synopsis--The four major theories of aging are discussed, as is the expression of these changes in the body's cutaneous covering. An increase in the insoluble collagen fraction of aged dermis is seen, representing an over-all total collagen increase. Reports of elastin changes in the aged dermis are found to be equivocal. The best known changes in the epidermis include a decrease in the number of cell layers (in those parts of the skin that are not exposed to continual friction) and an increase in the turnover time of senile epidermis. INTRODUCTION Although the problem of aging began with the evolution of the first living organism on earth, its serious, scientific study has been neglected until only very recently. The wealth of theories proposed to explain aging, however, indicate a desire to make up for lost time. In the following discussion, the term "aging" will be definitively limited to those changes in the individual which occur over a time interval that is long in comparison with the individual's life span, and which are generally thought of as being undesirable to him, such as greying hair, drying skin, decline of mental acuity, etc. This will distinguish aging changes from changes which, although they may resemble aging, are precipitous in nature that is, they occur during a very short time interval in relation to the individual's life span. The first part of this examination of the aging problem will, then, be concerned with the major hypotheses of aging. In the second section, changes in the skin due to the aging process will be discussed. * Division of Basic Research, Revlon Research Center, Inc., 945 Zerega Ave., Bronx, N.Y. 10473. 829

830 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS WEAR-AND-TEAR THEORY The first theory to be considered is the so-called wear-and-tear theory. It postulates that the organism depreciates more and more, the longer it is alive, much like an automobile, and that after a number of years (i.e., approaching old age) the organism becomes worn out due to the continual use of its component parts. If this theory were true, the organism's life expectancy would be regulated to a great extent by the metabolic rate at which its cells function. In line with this concept, Johnson et al. (1) found that rats kept at 10øC, a tem- perature at which they had to increase their rate of metabolism to maintain their body temperature, had a life span which was considerably reduced when compared to that of rats kept at 28øC. In addition, the cold-acclimatized rats were found to die of the same diseases as the warm-acclimatized ones, however much sooner they did die. Work with rats using a completely different type of stress (2), however, has produced entirely different results. Feeding studies in which the animals were given an adequate amount of vitamins, minerals, and proteins, but a greatly reduced carbohydrate diet, have also shown a higher rate of metabolism per gram of body tissue. The rats on this controlled diet, however, were found to have extended life spans when compared with the animals fed ad libitum, so that the underfed animals lived in several cases as long as 1400 days, whereas the nonrestricted rats seldom lived longer than 965 days. It should be noted here, however, that in both of these studies the only parameter studied was life span, which is not a true indication of the aging process. The stress of sacrificing an animal by exsanguination also has the effect of shortening the life span, but has nothing to do with the aging process. Another version of this wear-and-tear theory is that senescence is caused by a summation of stresses which the organism receives throughout its lifetime. It is known, for example, that ionizing radia- tion does have a dramatic effect in shortening the life span of the exposed organism, as well as in producing aging-like effects. In his investi- gation using an irradiated group of mice, and groups which had been stressed with large doses of nitrogen mustard and typhoid vaccine, Curtis (3) found no shortening of the life span in the latter groups, but a definitely higher death rate among the irradiated. This could only mean that radiation is not the same kind of stress-producer as nitrogen mustard and typhoid vaccine. To investigate further the effect of chronic doses of certain stressors, Curtis gave large, nonlethal doses of typhoid toxin, typhoid toxoid, nitrogen mustard, and turpentine