The cartilage-bone interplay: structural biology of the temporomandibular joint

It is generally known that the cartilage/bone junction is mediated by the interposition of a calcified cartilage layer. This latter has been the subject of extensive research but its structure and functions are still unclear: in particular it is debated whether oxygen, nutrients and signaling molecules from the subchondral bone may cross this layer (1, 2) and contribute to the metabolism of the uncalcified cartilage or if, on the contrary, this latter is fed only by the perichondrium and/or the synovial fluid. A previous research on the growth plate has already shown that a single cartilage can have two completely different interfaces with the adjoining bones (3). In the present study the opposing surfaces of the temporo- mandibular joint have been investigated by light microscopy, by scanning electron microscopy and by high resolution micro-CT, the two latter being the only techniques allowing an unrestricted, face-on view of the mineralization front. Our first results seem to contradict the common knowledge on these tissues. The condylar surface is covered by a thin layer of fibrous tissue which become more cartilaginous with depth; this tissue appears to blend with the subchondral bone with no definite boundary. Small cartilage islets remain enclosed within the trabeculae of the underlying bone, where they are readily evidenced by all the techniques used. The mandibular fossa is covered by a similar thin fibrous layer but the underlying surface is represented by a solid thick plate of bone, followed by a few bony planes parallel to the articular surface, and finally by the cancellous bone. There is no trace of cartilage on this side of the joint, and no visible communication between the soft tissue of the articular surface and the underlying vascular spaces. The research is still underway. References (1) Arkill, KP, and Winlove, CP. Solute transport in the deep and calcified zones of articular cartilage. Osteoarthritis Cartilage 2008, 16, 708–714. (2) Oliveira Silva, M; Gregory, JL; Ansari, N; Stok, KS. Molecular Signaling Interactions and Transport at the Osteochondral Interface: A Review. Front Cell Dev Biol, 2020, 8, 750. (3) Zecca, PA; Reguzzoni, M; Borgese, M; Protasoni, M; Filibian, M; Raspanti, M. Investigating the Interfaces of the Epiphyseal Plate: An Integrated Approach of Histochemistry, Microtomography and SEM. J Anat, 243, 870-877