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j. Soc. Cosmet. Chem., 34, 439-451 (December 1983) Aging patterns in collagen in vivo and in vitro B.J. RIGBY, CSIRO Division of Textile Physics, 338 Blaxland Road, Ryde, NSIV 2112, Australia. Received September 1983. Presented at the IFSCC/SCCJoint Conference on Skin, San Francisco, September 1983. INTRODUCTION The fibrous protein collagen is the major component of tissues such as tendon and skin. The other components include the proteins elastin and keratin, and the glycosaminoglycans and lipids, the whole system being irrigated by a solution containing a number of inorganic ions, notably Na*, C1 , Ca**, and K *, vitamins, and oxygen. This paper is concerned with the age-related changes which occur in these tissues and some of the factors which affect their stability at any one time. In particular, some recent work relating in vivo and in vitro aging is summarized. It concerns the role of oxygen in cross-linking reactions, and of ascorbic acid and various inorganic ions in the stabilization of collagenous fibers. The physical, mechanical, and chemical changes which take place in collagenous tissues with in vivo growth and aging are well known (1,2). The most obvious alterations are (a) a decrease in solubility or extractability and susceptibility to enzymes and acid swelling, and (b) an increase in mechanical and thermal stability. In general, these changes would be consistent with an increase in the cross-link density of the tissue, which would include collagen-collagen as well as collagen-glycosaminoglycan and other interactions. Although no direct evidence appears to exist, it is possible that such changes in connective tissue interfere not only with the mechanical functioning of the anatomical structures with which it is involved, but also with cellular function itself. The modern view is that cells and their supporting tissues function as a total dynamic system. Some of the changes associated with aging which take place in collagen in vivo have their counterparts in in vitro systems. Thus, with respect to thermal stability, solubility, and tensile mechanical properties, similar changes occur with time for both native (3-5) and reconstituted (6,7) collagen fibers. Whether such evidence shows that the chemical processes involved in the in vivo and in vitro situations are essentially the same is an important question. If they are the same, then in vitro studies become valuable adjuncts to aging studies because the environment of the tissues can be controlled. 439