Electron Microscopy of Articular Cartilage in the Young Adult Rabbit

Abstract

The appearance under the electron microscope of articular cartilage from young adult rabbits is described. In the superficial zone the cells are flattened, but show no evidence of degeneration; they possess mitochondria, cytomembrane systems, and many small cyto-plasmic vesicles of the pinocytotic type. These surface cells are surrounded by hyaline matrix containing many fine fibers; the articular surface is remarkably smooth. At a deeper level the cells are more rounded, exhibiting well-developed cytomembranes, mitochondria, Golgi vacuoles, occasional fat droplets, and branching processes closely associated with fine fibers of the surface type. These fibers merge peripherally with larger fibers, helicoidally arranged, that show the characters of adult collagen. At a deeper level, degenerating chondro-cytes are seen. Many show breaking up of the cytomembranes, together with enlargement of the Golgi vacuoles, which release their fine granular contents at the cell surface. Some cells show a different type of degeneration, the wall breaking down and the organelles being dispersed without the production of large vacuoles. In the deepest part of the cartilage, calcification is seen, commonly as rosettes of apatite crystals which are randomly scattered among large collagen fibres with complex striations. There is nothing to suggest that surface cells are degenerate or that they are worn away by friction. No evidence of cell multiplication has been observed.