Structure and histochemistry of a pressure-bearing tendon of the frog

The plantaris longus tendon of the bullfrog Rana catesbeiana has been structurally and histochemically characterized. The tendon has been shown to wrap around the distal tibio-fibular joint, extending into an aponeurosis. The region under the joint exhibits a large cartilaginous pad. In contrast to the mammalian flexor digitorum profundus tendons, the compression region of the frog tendon is not typically fibrocartilaginous but presents a unique arrangement of convoluted collagen bundles and fibers associated with a large amount of glycosaminoglycans. The sulfated nature of the glycosaminoglycans in the compressed region was determined through enzyme susceptibility as well as through CEC procedures. Cells of the modified zone exhibited morphological variation, from fibroblastic to condrocyte-like. Polarized light analysis revealed an intricate array of collagen bundles which crossed the tendon in different directions and connected the tension and compression regions. The ability of the tendon to resist compressive forces is assumed to be due to the large amount of glycosaminoglycans in the modified region as well as to the complex arrangement of collagen bundles in the tendon as a whole.