Age-related differences in the accumulation and size of hyaluronan in alginate culture

The alginate bead culture system has unique properties that make it possible to study the accumulation and turnover of macromolecules in two distinct matrix compartments of the cartilage matrix: the cell-associated matrix (CM) and the further removed matrix (FRM). Taking advantage of this culture system, the purpose of this study was to examine age-related changes in the metabolism of hyaluronan (HA) in these two compartments. Bovine chondrocytes, isolated from fetal, young adult, and old adult articular cartilage, were cultured in alginate beads. On Days 7 and 14 of culture, the alginate gel was solubilized, the CM and FRM were separated and macromolecules in both compartments were analyzed. When compared to the cells from fetal and old adult animals, the young adult cells proliferated at the fastest rate. Fetal cells produced a more abundant CM that was richer in proteoglycans (PGs) than the CM of young or old adult cells. With increasing age, there was an increased tendency for PG, collagen, and HA to escape incorporation into the CM and to become immobilized in the FRM. Very striking changes also were observed in the ratio of HA to PG, which increased markedly with age, and in the size of the HA molecules, which decreased markedly with age. The results suggest that the metabolism of HA in cartilage undergoes pronounced age-related changes, some of which are retained during culture in alginate gel. The findings also suggest that the previously documented age-related decrease in the size of HA in native bovine cartilage reflects, at least in part, a biochemical process occurring at the time or at least soon after the glycosaminoglycan chain is synthesized. It does not appear to simply be the result of age-related changes occurring slowly with time after synthesis, as was previously suggested to be the case for human articular cartilage.