Age‐related changes in articular cartilage proteoglycans: Electron microscopic studies

Biochemical and biophysical studies have demonstrated that proteoglycan monomers from immature and adult articular cartilage differ in composition and size. To investigate the structural basis of age-related differences in articular cartilage proteoglycan monomers and aggregates, we isolated and purified high buoyant density proteoglycans from the articular cartilages of 2- to 3-month-old calves and 18-month-old steers. The molecular architecture and dimensions of the proteoglycans were examined using the electron microscope monolayer method. Aggregated and nonaggregated monomers from calf cartilage were longer and less variable in length than the corresponding monomers from steer articular cartilage. Calf monomer lengths had unimodal frequency distributions whereas nonaggregated steer monomer lengths had a bimodal distribution. These observations were confirmed by acrylamide-agarose electrophoresis, which demonstrated that the samples contained only one species of proteoglycan monomer in calf but two species in steer. In addition, calf aggregated monomers had longer thin segments indicating that calf and steer monomers differed in structure as well as in size. Steer proteoglycan aggregates were shorter and had fewer monomers than those from calf. These observations demonstrate the existence of significant age-related structural differences in articular cartilage proteoglycans and form the basis for future study of the mechanisms responsible for these differences.