Morphological and functional interrelationships of articular cartilage matrices

Abstract

The pericellular, territorial and interterritorial matrices of canine tibial cartilage have been identified ultrastructurally on the basis of their collagen fibre density and organisation, proteoglycan distribution and their structural response to experimentally applied compressive loads. In addition, a discrete pericellular capsule composed of fine, faintly banded fibrils is described which surrounds and encloses the pericellular matrix and chondrocytes of the middle and deep layers but not of the superficial layer. It is suggested that the fine fibrils which comprise this pericellular capsule represent some of the new minor collagen species recently localised in a similar position in hyaline cartilages. The densely compacted cupola which forms the articular pole of the capsule is frequently penetrated by a clearly defined pericellular channel, consistently orientated in the direction of the articular surface. Membrane-bound vesicles are observed in the pericellular matrix, within the lumen of the pericellular channel and accumulated in the territorial matrix immediately beyond the pericellular channel. The constancy of this distribution pattern strongly suggests a flow of material through the pericellular channel from the pericellular matrix to the territorial matrix and beyond, possibly in response to minute pressure gradients generated during compressive deformation of the non-distensible capsule. Furthermore, it is suggested that the random dispersal and subsequent rupture of matrix vesicles may represent a mechanism whereby chondrocytes, with limited mobility, could exercise homeostatic control over the cartilage matrix at some distance from the cell. Chondrocytes in the deeper layers of canine tibial cartilage are each surrounded by three distinct compartments, a pericellular matrix and capsule, a territorial matrix and an interterritorial matrix. The response of each of these concentric compartments to experimental load suggests that they function synergistically to produce an integrated, biological, hydro-elastic suspension system capable of resisting physiological compression.