Aging of collagen.

Current knowledge of the structure and the mechanism of formation of the covalent cross-links that fuse individual collagen molecules into a stable fiber is reviewed. Some of the mechanical properties of dermal connective tissue and the way in which these change with age can be correlated with the types of cross-link present in the tissue. Cross-links are routinely detected by treatment of a tissue sample with tritium-labeled borohydride and subsequent isolation and quantification of the cross-linked compound, which has been rendered radioactive by reaction with this reducing compound. After maturity, the number of detectable cross-links decreases even though the mechanical stability of the tissue increases. This anomaly is examined in the light of recent data suggesting that cross-links may be oxidized in vivo and thus become undetectable since they can no longer react with borohydride.