The development of the pressure-bearing tendon of the bullfrog, Rana catesbeiana

The plantaris longus tendon of the bullfrog is a pressure-bearing tendon an d develops a fibrocartilage-like arrangement in the area subjected to compr essive forces. The fibrocartilage-like tissue shows some distinct aspects o f cellular and fibrillar structure and distribution as compared to the mamm alian counterparts. In this work, the development of the plantaris longus t endon was assessed by investigating some of its structural, cytochemical an d immunocytochemical aspects in developing tadpoles. The pressure-bearing r egion is structurally distinct from the tension region as early as at stage 35 of larval development. There is little extracellular matrix in both reg ions, but the former shows round mesenchymal-like cells with many processes and cell junctions, while the latter is populated by fibroblasts. As devel opment proceeds, the cells in the compression region retract the processes, loose the connections to each other, become rounded and produce abundant p roteoglycans and some collagen fibers. Progressively, their organelles beco me localized in a restricted perinuclear area and are surrounded by a const antly increasing amount of vimentin. The fibroblasts of the tension region produce mostly collagen fibrils, which are packed and aligned to each other . These cells become more elongated and show a diminished cytoplasmic area. The results allow for the conclusion that the compression region does not arise by simple hyperplasia of the peripheral layers of a normal tendon, bu t from a programmed sequence of developmental steps. This assumption is bas ed on the fact that muscle fibers are still developing when the tendon is a lready showing a differentiated compression region. We further suggest that mechanical stimulation is a secondary factor most likely associated with t he maintenance of the differentiated phenotype of this tendon.