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