REGIONAL DIFFERENCES IN CELL-SHAPE AND GAP JUNCTION EXPRESSION IN RATACHILLES-TENDON - RELATION TO FIBROCARTILAGE DIFFERENTIATION

Tendon cells have complex shapes, with many cell processes and an inti mate association with collagen fibre bundles in their extracellular ma trix. Where cells and their processes contact one another, they form g ap 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 fibrocarti lage at its calcaneal attachment and a sesamoid fibrocartilage where i t is pressed against the calcaneus just proximal to the attachment. St udies using DiI staining demonstrated typical stellate cell shape in t ransverse sections of pure tendon, with cells withdrawing their cell p rocesses and rounding up in the fibrocartilaginous zones. Coincident w ith change in shape, cells stopped expressing the gap junction protein s connexins 32 and 43, with connexin 43 disappearing earlier in the tr ansition than connexin 32. Thus, there are major differences in the ab ility of cells to communicate with one another in the phenotypically d istinct regions of tendon. Individual fibrocartilage cells must sense alterations in the extracellular matrix by cell/matrix interactions, b ut can only coordinate their behaviour via indirect cytokine and growt h 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 t ensile forces. It may be that different systems are used to sense and respond to such forces in fibrous and cartilaginous tissues.