Structural studies of the fibrillar architecture of normal and softened bovine articular cartilage

Abstract

Articular cartilage functions successfully as a compression load-bearing tissue by virtue of the functional interplay between a 3-dimensional structure of collagen fibrils and the entrapped water-swollen proteoglycan molecules. Crucial to this entrapment process is a mechanism or set of mechanisms that maintain the collagen fibrils in a finely divided interconnected configuration that immobilises the macro-molecular proteoglycan complexes. Any loss of interconnectivity in the collagen network that might reduce the constraints on the swelling tendency of the proteoglycan domains will lead to a lower matrix stiffness. There are some structural similarities between this less stiff or abnormally softened cartilage and the degenerative osteoarthritic matrix, although ultrastructural studies to date are somewhat limited. The primary objective of the research reported in this thesis was to investigate the fibrillar architecture in the general matrix of both the normal and abnormally softened cartilage matrices. The fibrillar architectures of the normal and abnormally softened general matrices were compared using Nomarski light microscopy (LM), transmission electron microscopy (TEM) with combined stereoscopic reconstruction, and scanning electron microscopy (SEM). As reported earlier by Broom (1984b), a pseudo-random network developed from an overall radial arrangement of collagen fibrils is the most fundamental ultrastructural characteristic of the normal general matrix. By contrast, this present investigation has shown that the most distinctive feature of the softened matrix is the presence of parallel and relatively unentwined fibrils, strongly aligned in the radial direction. A structural model illustrating the transformation from the normal to the softened matrix is proposed based on the important property of lateral interconnectivity in the fibrils which involves both entwinement and non-entwinement based interactions. The distribution of proteoglycans in the normal and the softened matrix was compared. The distribution of Type II collagen was investigated using immunohistochemical staining combined with confocal imaging. It is concluded that the Type II fibrils do persist in the altered matrix thus adding further experimental support for the proposed transformation model. The swelling behaviour of the general matrix of both normal and abnormally softened articular cartilage was compared by subjecting tissue specimens under different modes of constraint to a high swelling bathing solution of distilled water and comparing structural changes imaged at the macroscopic, microscopic and ultramicroscopic levels of resolution. Near-zero swelling was observed in the isolated normal general matrix with minimal structural change. By contrast, the similarly isolated softened general matrix exhibited large-scale swelling in both the transverse and radial directions. This difference in dimensional stability was attributed to fundamentally different levels of fibril interconnectivity between the two matrices. The structural transformation model was further developed to accommodate fibrillar rearrangements associated with the large-scale swelling in the radial and transverse directions in the softened general matrix.