Articular cartilage and intervertebral disc proteoglycans differ in structure: an electron microscopic study.

Articular cartilage and the intervertebral disc tissues have different material and biological properties and different patterns of aging and degeneration. To determine if the proteoglycans of these tissues differ in structure, we used the electron microscopic monolayer technique to compare baboon articular cartilage proteoglycans with baboon annulus fibrosus, transition zone, and nucleus pulposus proteoglycans. Intervertebral disc and articular cartilage proteoglycans differed significantly. Articular cartilage contained large proteoglycan aggregates formed from hyaluronic acid central filaments, multiple monomers, and large nonaggregated monomers. These molecules were identical to those of nasal cartilage, growth plate cartilage, chondrosarcomas, or menisci. In contrast, the intervertebral disc tissues contained only nonaggregated proteoglycan monomers and clusters of monomers without apparent central filaments. Intervertebral disc nonaggregated monomers were shorter and more variable in length than those from articular cartilage, and nucleus pulposus nonaggregated monomers were even shorter and more variable in length than transition zone and annulus fibrosus monomers. These observations suggest that significant differences in proteoglycan metabolism exist between articular cartilage and intervertebral disc.