Degradation of the type II collagen network early in the degeneration of cartilage

Abstract

Type II collagen has been characterized as being present in joint tissues subjected to compression rather than tension. The purposes of this study were to investigate the distribution and ultrastructural changes in the type II collagen fibrillar network in the early stage of degeneration of cartilages. Softened cartilage which represents the early stage of degeneration was obtained from the tibial surfaces of a cow. Normal control cartilage was obtained from the femoral condyles in the same joint. Nomarski light microscopy and transmission electron microscopy (TEM) were performed respectively for structural and ultrastructural studies. Confocal light micrscopy was applied for imaging the staining of collagen type II in normal and softened matrices by immunohistochemical studies. The results demonstrated that in normal cartilage the extracellular matrix was characterized by a fine ”ground-glass-like” amorphous texture. This reflects the fact that individual fibril elements in their repeatedly interconnecting configuration present a fineness of structure that is beyond the limits of optical resolution. The normal matrix observed by TEM was shown to have a pseudo-randam fibrillar architecture. The extracellular matrix in the softened cartilage was characterized by a distinct fibrous texture strongly aligned in the radial direction. This optically resolved structural feature of the softened matrix reflects a tendency for individual collagen flbrils to aggregate into relatively large fibrillar bundles strongly aligned in the radial direction as observed under TEM. The type II collagen-labeled images obtained from the normal matrix exhibited a fine texture. By contrast, staining of the softened matrix exhibited a relatively coarse granularity with a distinctly radial texture in the deeper zones. It was concluded that the degeneration of cartilage begins with degeneration of the extracellular matrix, and the decrease in collagen fibrillar interactions is mainly caused by the loss of cross-linking in type II collagen. This is the first report to correlate ultrastructural changes of the fibrillar network with confocal imaging of the type II collagen distribution in the process of cartilage degeneration.