Dynamic state of collagen: pathways of collagen degradation in vivo and their possible role in regulation of collagen mass.

There is now evidence that collagen turnover in normal tissues can occur at rapid rates. In skin of adult rats, the mean rate lies between 3 and 5%/day, but rates of greater than 10%/day have been reported in some tissues, such as lung and periodontal ligament. The major sites of this degradation are still uncertain. Intracellular degradation, which occurs within minutes of synthesis, may be extensive, but its magnitude varies between tissues. This process may have evolved to allow the adaptive advantages of rapid turnover without compromising the supportive role of fibrillar collagen. There is now abundant evidence that changes in collagen degradation play an important role in the regulation of collagen mass. Increased breakdown of extracellular collagens may be required whether the mass is increased or decreased. However, preliminary evidence suggests that the degradation of newly synthesized collagen may be up- or down-regulated, depending on the direction of the change in content.