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 from 7 d to 6 y. The disc is interposed between the head of the mandibular condyle below, and the squamous temporal bone above, within the temporomandibular joint. A single layer of flattened mesenchymal cells covered the articular surface of the disc during early postnatal development. Gradual disappearance of this cell layer was succeeded by the development of an articular surface lamina. The first appearance of the lamina coincided with the transition from suckling to adult masticatory function. With advancing age, the lamina increased in thickness from 0.3 to 1.0 micron in adulthood, and was well demarcated from the subsurface tissue. The lamina was a continuous, smooth, electron-dense layer, that reacted biochemically as proteoglycan complex. The surface lamina may be responsible for the low frictional properties associated with articular movement and load, and the selective passage of molecules bidirectionally across the cartilage-fluid interface. Variations in lamina thickness occurred topographically across the disc and were attributed to site concentration of shearing stress. Beneath the articular surface lamina were cells surrounded by collagenous intercellular stroma. Chondrocytic, fibrocytic and fibrochondrocytic cellular phenotypes were present. Subsurface cellular debris accumulated within the rabbit disc from the commencement of mastication. Mature adult disc contained expanses of debris, attributed to cell death in situ. Foci of mineral precipitates were found within ageing discal tissue.