Electron microscopic studies on resorption of xenogeneic cartilage implants

Summary

Xenogeneic implants of devitalized cartilage were used as a model system to study the fine structural details of cartilage resorption. The implants were surrounded by a cellular infiltrate consisting of macrophages, fibroblasts, lymphocytes, and eosinophils. Throughout the observation period (2–12 weeks), macrophages formed a palisade-like layer directly opposed to the surface of the implant. The total size of the infiltrate decreased with time. In the acute phase (2 weeks), lymphocytes and eosinophils were abundant but thereafter decreased in number. Consequently, there was a marked predominance of macrophages and fibroblasts in the chronic phase (4–12 weeks). Large multinucleated cells, believed to represent polykaryons formed by fusion of macrophages, were also observed. These findings indicate that inflammatory cells are capable of resorbing cartilage without the participation of living chondrocytes. Apparently, macrophages have direct responsibility for the resorptive process. The role of the other cell types is less clear. In the early period after implantation, they may interact with the cartilage and release factors that stimulate and direct the function of the macrophages.

Fine structurally, the macrophages were characterized by a large Golgi complex and numerous coated vesicles. In contrast, phagosomes and lysosomes were few. The coated vesicles were located in the Golgi area, where they appeared to form by budding from dilated rims of stacked cisternae, and in the peripheral parts of the cells, opposing the implant. It is suggested that the coated vesicles transport degradative enzymes from the Golgi complex to the plasma membrane for release extracellularly. This conforms to the idea that cartilage resorption is a lytic rather than a phagocytic process. Out findings agree with and partly extend earlier observations on cartilage resorption in connection with inflammatory joint diseases and tumor invasion.