The fine structure of the fibrous zone of articular cartilage in the rat mandibular condyle.

The ultrastructure of the articular fibrous zone of the rat mandibular condyle was observed at 3 and 6 weeks of age in order to investigate the histological changes that occur during the establishment of mastication after suckling. We divided the articular fibrous zone into two layers (F1 layer and F2 layer). The F1 layer was composed of fibroblast-like cells, macrophage-like cells, and the matrix which consisted of granular or fibrillar material, loosely arranged small bundles of collagen fibrils with uniform thickness of 30-50 nm, and independent microfibrils. The F1 layer was continuous with the synovial membrane without any distinct boundary in its peripheral area. Mitotic figures could infrequently be observed in the fibroblast-like cells. The cells in the F2 layer closely resembled fibroblasts. The matrix in the F2 layer mostly consisted of densely arranged bundles of collagen fibrils which are not of uniform diameter ranging 40-100 nm. Elastic fibers occurred among the collagen fibrils. The F1 layer which was observed clearly at 3 weeks almost disappeared at 6 weeks of age. It is confirmed that the F1 layer resembling the synovial membrane exists over the condyle up to the weaning period about 3 weeks of age but it degenerates before 6 weeks of age when the masticatory function is established.