Repair of Partial-Thickness Defects in Articular Cartilage: Cell Recruitment from the Synovial Membrane*

Abstract

Partial-thickness defects evolving in mature articular cartilage do not heal spontaneously. This type of defect was created in the articular cartilage of adult rabbits and Yucatan minipigs, and the effects of chondroitinase ABC or trypsin, fibrin clots, and mitogenic growth factors on the healing process were examined histologically at intervals ranging from one to forty-eight weeks.The effect of chondroitinase ABC or trypsin was examined initially. Articular cartilage contains macro-molecules, including proteoglycans, which render the surfaces of this tissue, and of partial-thickness defects within it, antiadhesive. Chondroitinase ABC digests the glycosaminoglycan chains of cartilage proteoglycans, and trypsin degrades their core proteins. To test the hypothesis that mesenchymal cells may be prevented from adhering to and migrating over the surfaces of partial-thickness defects by proteoglycans, we removed a superficial layer of these macromolecules from the surface of the defect with use of one of these enzymes. The treatment evoked an increase in the coverage of the defect surface with mesenchymal cells; when combined with the local application of a mitogenic growth factor (basic fibroblast growth factor, transforming growth factor-β1, epidermal growth factor, insulin-like growth factor-1, or growth hormone), the coverage was more extensive but mesenchymal cells did not extend into and completely fill the volume of the defect.When the surface of the defect was treated with chondroitinase ABC and the cavity of the defect was filled with a fibrin clot to furnish a matrix or scaffolding for the migration of cells therein, there was migration and proliferation of cells throughout the volume of the defect but at a low population density. Mesenchymal cells remodeled the deposited fibrin matrix, which was replaced by a loose fibrous connective tissue.When defects that had been treated with chondroitinase ABC were filled with a fibrin clot containing a mitogenic growth factor, mesenchymal cells filled the entire cavity of the defect, and the density of the cells was greatly increased, particularly when transforming growth factor-β1 was used. Histological studies revealed a continuous layer of mesenchymal cells extending from the synovial membrane across the superficial tangential zone of normal articular cartilage into the defect, indicating that the cells that were recruited for the repair process were of synovial origin. At forty-eight weeks, the entire cavity of the defect remained filled with a fibrous connective tissue.CLINICAL RELEVANCE: The partial-thickness defects created in articular cartilage in this study are analogous to the clefts and fissures manifested during the early stages of osteoarthrosis; neither heal spontaneously. If the development of early defects could be impeded or arrested by eliciting a repair response, then exacerbation of the pathological condition might be prevented. We describe a procedure for evoking the ingrowth of mesenchymal cells from the synovial membrane into such defects, where they lay down a loose fibrous connective tissue. Conditions to induce their differentiation into chondrocytes, thus promoting the formation of hyaline cartilage, must now be defined.