A serial study on the development of the temporomandibular joint in the fetal mouse--in particular on the fibrous component in the condylar cartilage.

The development of the temporomandibular joint of 400 fetal mice at stages ranging from the 13th to the 20th day after insemination was investigated under the light, scanning (SEM) and transmission electron (TEM) microscopes. The differentiation and development of a cartilaginous tissue were observed at the supero-posterior end of the mandible at the 13 days after insemination. This tissue grew backward, upward and lateralward continuously and maintained a constant articulation with the squamosal part of the temporal bone. Seventeen days after insemination, cell layers in the condylar process and articular disc were arranged regularly. An supero- and inferno-directional cellular differentiation initiated from the subfibrous (SF) layer toward the articular spaces and cartilaginous layer was observed. The perichondrial ossification had taken place with the invasion of capillaries and the differentiation of osteoblasts in the SF layer, and was followed with a hypertrophic degeneration and endochondral ossification in the condylar process. Such a bi-directional growth of collagen and elastic fibers starting from the SF layer was also observed. Observation under SEM and TEM on the autoclaved condylar process revealed a complicated network consisted of main elastic fibers running in the sagittal direction. These fibers as well as the proteoglycan which contributes to the resilient property of the condylar cartilage and the ability to endure tensile or compressive stress from surrounding tissues during the growth and development of the mandibular condyle. The developing cartilaginous tissue was stimulated with the pressure from the masticatory muscles to initiate an active differentiation of the fibrous layer, which was invaded by the blood capillary system closely related with the subsequent endochondral ossification. These results elucidate that the development of the temporomandibular joint has closely kept relations with the functional influences from surrounding tissues, which also play an important role in regulating postnatal growth of the mandible.