Synthesis rates of aggrecans by phenotypically stable human articular chondrocytes and the immobilization of these aggrecans in large aggregates were used as variables reflecting the capability of these cells of restoring the extracellular matrix of articular cartilage in vivo in an aging population.
Human articular chondrocytes were isolated from articular cartilage obtained from 33 different donors at autopsy. The chondrocytes were cultured in gelled agarose. Synthesis of aggrecans was investigated using Na(2)(35)SO(4)as a radioactive precursor after a 2-week culture period. Electron microscopic study of aggrecan aggregates was done on the macromolecules accumulated over 3 weeks in culture by the chondrocytes obtained from eight other donors with increasing ages.
Sulfate incorporation rates into aggrecans correlated inversely with the age of the donor. The value of sulfate incorporation in aggrecans for chondrocytes obtained from mature cartilage of a 20-year-old individual in this system drops to 50% and 25% for chondrocytes obtained from 45- and 69-year-old individuals respectively. Electron microscopic study of aggrecan aggregates showed that the 'de novo' synthesized hyaluronan molecules were fully loaded with aggrecans. Mature human articular cartilage cells were found to synthesize an aggrecan aggregate which carried an average number of 11.7 to 13.1 aggrecans. Cells obtained from immature donors synthesized aggrecan aggregates of which the hyaluronan chain carried twice the amount of aggrecans. These immature human articular cartilage cells were also found to synthesize significant proportions of large aggrecan aggregates with 20 to over 100 aggrecans immobilized on a single hyaluronan chain. The proportions of these large aggrecan aggregates decreased with increasing age of the donors of the chondrocytes.
The declining aggrecan synthesis rates and the decreased capability of assembling large molecular size aggregates with increasing age in humans illustrates a progressive failure of the repair function of articular cartilage cells in humans.