832 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS To relate this back to the mutation theory should not be too diffi- cult now. If a mutation caused by irradiation occurred, and a part of a chromosome were broken off, then the enzymes whose manufacture was involved with that portion of the chromosome would be affected. The practical problem, however, of how to detect possible mutations of this kind arises. Many of the somatic cells of the body do not usually form a second generation and so these chromosomal abnormal- ities would not be manifest in the daughter cells. Albert (4), however, has devised a technique of dosing mice with carbon tetrachloride and thus destroying about 60% of the normally nondividing liver cells. He reasoned that the ability of this organ to regenerate lost portions would then provide a method of recording chromosome abnormalities in second generation cells. Using this technique, Stevenson and Curtis (5) found a steady increase in chromosome aberrations as a function of increasing age the number of mutations reached a peak value of 22% of all the liver cells when the animals were 12 months old. The cells of the body which divide frequently, however, present quite a different situation than do the infrequently dividing cells. In the former types of cells, a mutation can arise much more easily and does not need radiation insult for its cause. In the process of replication of the nuclear DNA, if a wrong base lined up opposite its complementary half and were incorporated into the DNA, this would first cause mis- reading of the genetic code and then substitution of the wrong amino acid in the enzyme protein structure. In many cases, the substitution of one wrong amino acid in an enzyme leads to its inactivation, since much of an enzyme's activity depends upon its stereochemistry. While it is also true that in the frequently dividing body cells replication errors will tend to be eliminated by the process of natural selection, several faulty enzymes may be synthesized before this elimination takes place these enzymes will then be unavailable to mediate the reactions for which they were originally intended. Mutations, therefore, of both nondividing and dividing cells of the body play a very real role in the process of senescence, since they affect the fundamental control of cell function. CROSS-LINKAGE THEORY A third theory of aging concerns the possible cross linking of various of the body's most important molecules as it grows older. Bjorksten (6), one of the most vigorous proponents of this theory, has postulated

CHANGES IN HUMAN SKIN WITH AGING that the ester linkages which are present within the three strands of the triple helical molecule of collagen may shift to other strands within the same molecule. These linkages, he says, can also form between two different molecules of collagen and, as the body ages and this process continues, the collagen becomes more and more rigid. Verzar (7) has described in detail how this process may occur in collagen. In addition to this molecule, however, Bjorksten extends his theory to include other cellular proteins and nucleic acids. Arranged as they are in long, linear strands, he feels that DNA and RNA are perfect targets for cross linking by agents in the cell which are capable of such action. He lists the dibasic acids formed in the tricarboxylic acid cycle, quinones, aldehydes, silicon, and various metals as possible cross-linking agents. One can imagine the consequences if two strands of DNA became linked in such a way that cellular replication or guided protein synthesis could not take place. The manufacture of faulty enzymes has already been suggested above in the explanation of mutation theories such useless, giant molecules might well have a clogging effect on the cell's machinery if not used, but instead are allowed to accumulate. FREE-RADICAL THEORY A fourth theory which is directly connected to the cross-linkage proposal is the free-radical theory of aging. It states that the degen- erative changes which occur in old age are due to the accumulation of free radicals in the body which may also act as cross-linking agents. Harman (8) believes that two of the more common free radicals involved are the hydroxyl radical (OH) and the hydroperoxide radical (H0.o), which can be formed by the interaction between oxidative enzymes and oxygen, by the enzymatic degradation of hydrogen peroxide, or by radiation. Norins (9) has also observed the production of free radicals in the skin following exposure to ultraviolet light. To investigate his own theory, Harman included in the diet of a special breed of short-lived mice such compounds as 2-mercaptoethylamine and cysteine, which would react rapidly with free radicals. He then studied the life spans of these mice as compared to those receiving a normal diet. He found that the half-survival time of the animals on the special diet was ex- tended to 10 months, as compared to the $ months of the control animals. Consideration of Theories The preceding review of four of the most popular theories of aging indicates that there is no want for opinions on this topic. There does