Ultrastructure of adult human articular cartilage matrix after cryotechnical processing

The ultrastructure of adult human articular cartilage matrix is reexamined in tissue processed according to recently improved cryotechniques [Studer et al. (1995) J. Microsc., 179:321-332]. In truely vitrified tissue, a network of fine cross-banded filaments (10-15 nm in diameter) with a periodicity characteristic of collagen fibrils is seen throughout the extracellular substance, even within the pericellular compartment, which has hitherto been deemed free of such components. Proteoglycans fill the interstices between these entities as a homogeneously distributed granular mass; they do not manifest a morphologically identifiable reticular structure. Longitudinally sectioned collagen fibrils exhibit variations in thickness and kinking; they tend to align with their periodic banding in register and are frequently seen to split or fuse along their longitudinal course. The tendency of fibrils to form bundles is greater in deeper zones than in more superficial ones. A duality in the orientation of fibrils and fibril bundles is observed within the interterritorial matrix compartment: superimposed upon the well-characterized arcade-like structure formed by one subpopulation is another, more randomly arranged one. The classical concepts of matrix organization thus need to be modified and refined to encompass these findings.