COMPLEMENT PROTEINS ARE PRESENT IN DEVELOPING ENDOCHONDRAL BONE AND MAY MEDIATE CARTILAGE CELL-DEATH AND VASCULARIZATION

Normal endochondral bone formation follows a temporal sequence: immatu re or resting chondrocytes move away from the resting zone, proliferat e, flatten, become arranged into columns, and finally become hypertrop hic, disintegrate, and are replaced by bone. The mechanisms that guide this process are incompletely understood, but they include programmed cell death, a stage important in development and some disease process es. Using immunofluorescence we have studied the distribution of vario us complement proteins to examine the hypothesis that this sequence of events, particularly cell disintegration and matrix dissolution, are complement mediated, The results of these studies show that complement proteins C3 and Factor B are distributed uniformly in the resting and proliferating zones. Properdin is localized in the resting and hypert rophic zone but not in the proliferating zone. Complement proteins C5 and C9 are localized exclusively in the hypertrophic zones. This anato mically segregated pattern of distribution suggests that complement pr oteins may be important in cartilage-bone transformation and that the alternate pathway is involved. (C) 1996 Academic Press, Inc.