Involvement of Macrophages in Collagen Resorption

Although relatively stable in most tissues, collagen is rapidly turned over in several cases of morphogenesis (27) . During anuran metamorphosis (7), postpartum uterine involution (9), and wound healing (6), large amounts of collagen are resorbed in a short time. The enzyme responsible for collagen degradation is thought to be a collagenase (13) . Early attempts to demonstrate a collagenase in systems in which active remodeling is taking place were unsuccessful . Recently, however, Gross and his coworkers have shown that, when pieces of tissue undergoing resorption are grown on reconstituted collagen gel, they secrete a collagenase capable of lysing collagen at neutral pH and physiological temperatures (6-8) . Biochemical investigations have shown that during remodeling of tissues hydrolytic enzymes (such as acid hydrolases) increase many times (24, 26) . On the basis of the well known association of hydrolytic enzymes with lysosomes, Woessner postulated a lysosomal theory of collagen resorption (27) . According to this theory, the collagenase secreted extracellularly breaks up the long collagen fibers, which are then capable of being taken into phagocytic cells where they will be further degraded by the lysosomal enzymes . Several light microscopic investigations suggest that the osteoclast, fibroblast, foreign body giant cell, macro-