836 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS related increase in the shrinkage temperature of collagen--that is, the temperature at which rapid shrinkage of an isolated collagen strip began. Other findings of a decreased capacity for swelling (17), an increased resistance to collagenase (18), a lowered hexosamine: collagen ratio (19), and a higher calcium-binding capacity (20) have also been reported for aged collagen. Another derreal component often studied in investigations into aging is the acid mucopolysaccharides (AMPS). These are long-chain, protein-bound polymers of high viscosity containing repeating units of hexosamine and uronic acid or galactose (21). Although their exact function is unknown, they may serve as a template for laying down fibrous connective tissue such as collagen. Upon examination of aging skin, it has been found that the total AMPS decrease, while in sun-damaged skin they increase. Likewise, the hexosamine component of AMPS isolated from aged skin decreases, and in actinic skin it increases. Nimni et al. (15) have shown a striking relation between derreal hexosamine and neutral salt-soluble collagen in aging skin. In rabbit skin a few weeks after birth there was an initial sharp increase in hexosamine with a lag in collagen formation. After four weeks, when the amount of hexosamine reached a peak and began to decline, collagen exhibited a sharp increase. After about eight weeks, the soluble collagen followed the hexosamine in a gradual, age- related decline as the insoluble fraction began to replace both the ground substance and the soluble collagen. With age, then, there is an increase in insoluble collagen and a decrease in AMPS with sun damage, there is a decrease in insoluble collagen and an increase in AMPS. Derreal Changes: Elastin It was mentioned previously that the protein elastin represented about 2% by weight of the dermis of normal skin. Elastin fibers are not present in premature infants of less than four months' gestation, but in older subjects are seen as delicate, wavy, freely branching fibers, as compared to the dense, coarse bundles of collagen. It is agreed that there is a tremendous increase in the elastin content of skin which has been chronically exposed to the sun -described previously as solar elastotic skin--so that the elastin then represents 13% by weight. It was initially noted that concomitant with this large increase in elastin was the striking decrease in collagen. Several investigators therefore offered the theory that the collagen was being converted into an elastin-

CHANGES IN HUMAN SKIN WITH AGING 837 like compound. However, Smith (22) and others have reported that this increase is due to true elastin build-up, basing their beliefs on the morphology, solubility, enzyme susceptibility, tinctorial and physical properties, and amino acid composition of the elastin formed. In quantitation of elastin from aged skin, diverse results are pre- sented. Sams and Smith (11), after chemical and physical separation of elastin from adult skin, concluded that there is a slight, quantitative increase with age. Weinstein and Boucek (23) subjected the elastin fraction to enzymatic digestion with elastase and found no increase in aged skin. Hult and Goltz (24) employed a procedure whereby they investigated the elastin content of human aged skin by two methods: the first, a microphotometric examination of orcein-stained skin sections, and the second, an enzymatic digestion. This was done to test whether there might be an increased susceptibility of elastin in aged skin to enzymatic digestion, rather than an increase in the quantity of elastin. They showed that there were indeed higher values for aged skin as opposed to young skin when the enzymatic method was used. The theory of age-related increased susceptibility might therefore appear to be true. But it would also seem that a positive answer has yet to be arrived at. EPIDERMAL CHANGES Turning now from the dermis to the epidermis of the skin, a first impression might be that the knowledge of the changes in protein and mucopolysaccharide content in the dermis with age has been gained at the expense of investigation of aging effects on the epidermis. Although some of the most apparent changes which accompany aging--skin wrinkles--are perceived in the outer layer of the skin, their true cause lies in the changes in dermal collagen and, possibly, elastin, which take place beneath the epidermis. Another common sign of aging is dry and flaking skin. At first this might seem hard to reconcile with the finding that there is an increased water content in human skin with increased age (13). However, it must be remembered that the outer horny layer of a healthy epidermis normally forms a barrier against the exchange of water, and that hydration of the skin surface is not influenced to any great extent by the water present in the deeper layers. Brief mention should also be made here of the effects that hormones can have on the aging skin. Although the role of the hormones in the body's homeostasis is all-encompassing, just one of the effects of the so-called sex hormones on the skin is presented here. In later life, and