Articular cartilage repair in the avian quail-chick chimera model.

The natural history of the cells which participate in the repair of defects in articular cartilage was studied in the chick-quail chimeric system. This system involves interspecific grafting of quail limb rudiments onto the vascularized chick embryo chorioallantoic membrane (CAM) where the grafts revascularized and grow. Differences in cell morphology make it possible to clearly differentiate cells which subsequently arise from the grafted elements, from those which subsequently originate in the host (chick) tissues and migrate into the grafts. Herein, lesions were produced in quail femurs just prior to the time they were grafted to the chick CAM. The lesions were of two types: those restricted to the proliferative and hypertrophic zones of the cartilaginous epiphyses, and those which penetrated through the cartilage into the subchrondral marrow space. Epiphyses cut transversely at the level of the proliferative and hypertrophic cells failed to resurface with hyaline cartilage or fibrocartilage. Cavities produced in these areas also failed to repair. However, lesions which penetrated through the epiphyses into the medullary spaces filled with calcified bone trabeculae, and all the osteoblasts and osteocytes in the repair bone had nuclei of the graft (quail) type. The observations suggested that the cells resident in articular cartilage have little ability to effect healing, and that the new bone was derived from osteoprogenitor cells in the quail marrow stroma which moved with capillaries into the lesions. Indirect evidence is presented to support the concept that chondrocytes surviving ossification of the epiphyses can also contribute to the osteoprogenitor cell pool. A fibrocartilage metaplasia was not observed at the joint surface in this system, presumably due to the absence of biomechanical influences.