AGE-RELATED-CHANGES TO THE SURFACE ULTRASTRUCTURE OF THE RABBIT TEMPOROMANDIBULAR DISC

The ultrastructural characteristics of the temporomandibular disc were investigated by transmission electron microscopy in rabbits, aged fro m 7 d to 6 y. The disc is interposed between the head of the mandibula r condyle below, and the squamous temporal bone above, within the temp oromandibular joint. A single layer of flattened mesenchymal cells cov ered the articular surface of the disc during early postnatal developm ent. Gradual disappearance of this cell layer was succeeded by the dev elopment of an articular surface lamina. The first appearance of the l amina coincided with the transition from suckling to adult masticatory function. With advancing age, the lamina increased in thickness from 0.3 to 1.0 mu m in adulthood, and was well demarcated from the subsurf ace tissue. The lamina was a continuous, smooth, electron-dense layer, that reacted biochemically as proteoglycan complex. The surface lamin a may be responsible for the low frictional properties associated with articular movement and load, and the selective passage of molecules b idirectionally across the cartilage-fluid interface. Variations in lam ina thickness occurred topographically across the disc and were attrib uted to site concentration of shearing stress. Beneath the articular s urface lamina were cells surrounded by collagenous intercellular strom a. Chondrocytic, fibrocytic and fibrochondrocytic cellular phenotypes were present. Subsurface cellular debris accumulated within the rabbit disc from the commencement of mastication. Mature adult disc containe d expanses of debris, attributed to cell death in situ. Foci of minera l precipitates were found within ageing discal tissue.