VITRIFIED ARTICULAR-CARTILAGE REVEALS NOVEL ULTRA-STRUCTURAL FEATURESRESPECTING EXTRACELLULAR-MATRIX ARCHITECTURE

The quality of cryosections prepared from high pressure frozen bovine articular cartilage has been recently evaluated by systematic electron diffraction analysis, and vitrification found to be zone-dependent. T he lower radial layer was optimally frozen throughout the entire secti on thickness (150 mu m), whereas in the upper radial, transitional and superficial layers this was achieved down to a depth of only approxim ately 5-50 mu m. These differences were found to correlate proportiona lly with proteoglycan concentration and inversely with water content. In the current investigation, extracellular matrix ultrastructure was examined in high pressure frozen material (derived from the lower radi al zone of young adult bovine articular cartilage), by both cryoelectr on microscopy of cryosections and by conventional transmission electro n microscopy of freeze-substituted and embedded samples. Several novel features were revealed, in particular, the existence of a fine filame ntous network; this consisted of elements 10-15 nm in diameter and wit h a regular cross-banded structure similar to that characterising coll agen fibrils. These filaments were encountered throughout the entire e xtracellular space, even within the pericellular region, which is gene rally believed to be free of filamentous or fibrillar components. The proteoglycan-rich interfibrillar/filamentous space manifested a fine g ranular appearance, there being no evidence of the reticular network p reviously seen in suboptimally frozen material.