Tendon cells have complex shapes, with many cell processes and
an intimate association with collagen fibre
bundles in their extracellular matrix. Where cells and their processes
contact one another, they form gap
junctions. In the present study, we have examined the distribution of
gap junction components in
phenotypically different regions of rat Achilles tendon. This tendon
contains a prominent enthesial
fibrocartilage at its calcaneal attachment and a sesamoid
fibrocartilage where it is pressed against the
calcaneus just proximal to the attachment. Studies using DiI
staining demonstrated typical stellate cell shape
in transverse sections of pure tendon, with cells withdrawing
their cell processes and rounding up in the
fibrocartilaginous zones. Coincident with change in shape, cells
stopped expressing the gap junction proteins
connexins 32 and 43, with connexin 43 disappearing earlier in
the transition than connexin 32. Thus, there
are major differences in the ability of cells to communicate
with one another in the phenotypically distinct
regions of tendon. Individual fibrocartilage cells must sense
alterations in the extracellular matrix by
cell/matrix interactions, but can only coordinate their
behaviour via indirect cytokine and growth factor
signalling. The tendon cells have additional possibilities — in
addition to the above, they have the potential to
communicate direct cytoplasmic signals via gap junctions.
The formation of fibrocartilage in tendons occurs
because of the presence of compressive as well as tensile forces.
It may be that different systems are used to
sense and respond to such forces in fibrous and cartilaginous tissues.