Cellular Kinetics within the Mandibular Joint

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Abstract
Fifty-eight Sprague-Dawley rats were injected with tritiated thymidine at 4 weeks of age and sacrificed at various intervals up to 180 hr. after injection to investigate changes in the cell behavior with time in the different structures of the mandibular joint. The tissues were processed for histologic and radioautographic analysis. A definite difference in DNA duplication between cell populations within the squamosomandibular joint was demonstrated. A greater labeling index in the embryonic zone was observed, less pronounced in the fibrocartilage of the fossa and the fibrous zone of the condylar cartilage, and almost negligible in the biconcave portion of the articular disc. The cells within the embryonic zone revealed a cell cycle of 100 hr. average duration. The S1, G2, M and G1 phases average 7, 1 1/2, 1 1/2 and 90 hr. respectively. After division approximately 50% of the cells remained in the progenitor pool to maintain the reserve cell population while the remaining cells specialize and become chondroblasts. Following differentiation, the labeled chondrocytes retain their labeling suggesting a continuous G1 phase during their migration within the condylar cartilage. These cells are finally released into the medullary cavity after an average lifespan of 5 to 7 days. The renewal rate of cells in the articular zone of the condylar cartilage is comparatively low and apparently matched to maintain the articular surface and not associated with condylar growth. The labeled cells are mainly derived within the articular layer except a small contribution from the embryonic zone. The labeling indices of the fibrocartilage of the fossa and the fibrous zone of the condylar cartilage were in evidence of cell renewal at their articular surfaces.