Submicroscopic structure of canine articular cartilage

Canine articular cartilage was studied in male dogs at age 1, 4, 5 and 8 years. Samples collected from four hip joints and two humeral joints in each age category were processed by standard methods to be examined by scanning and transmission electron microscopy. The cartilage of both joints was similar in structure. In the superficial cartilage layer of one-year-old animals, individual spindle-shaped chondrocytes in the extracellular matrix were, together with associated collagen fibrils, located parallel to the surface. When viewed by scanning electron microscopy, they were distinctly prominent above the surrounding surface. Changes in the thickness and arrangement of both the chondrosynovial membrane and intercellular matrix were apparent in the 4-, 5- and 8-year-old animals, indicating the onset or progression of an osteoarthritic process. The middle cartilage layer in young animals showed elliptical chondrocytes occurring in pairs. The voluminous cytoplasm contained a great amount of granular endoplasmic reticulum, a large Golgi complex and numerous transport vesicles. The pericellular matrix, up to 1 µm thick, was composed of aperiodic fibrils. In the old animals the pericellular matrix was absent and was replaced by thick collagen fibrils with a marked periodicity. The deep cartilage layer in young dogs included groups of three to four chondrocytes situated in a common territory. The cytoplasm contained distinct bundles of intermediary filaments. The pericellular matrix occasionally formed septa between adjoining cells. The intracellular matrix included bundles of collagen fibrils arranged in a matted structure. In the old animals aggregation of chondrocytes into groups almost disappeared. The cytoplasm showed only short cisternae of granular endoplasmic reticulum, small numbers of mitochondria and transport vesicles, frequent lipid droplets and small glycogen deposits. The intercellular matrix consisted of only short collagen fibrils with no distinct periodicity.