The elastic system of a pressure-bearing tendon of the bullfrog Rana catesbeiana.

Elastic system components have been described in the pressure-bearing tendon of the bullfrog, Rana catesbeiana, as a result of histochemical tests and transmission and scanning electron microscopy. The tension region was shown to possess microfibril bundles, some of which exhibited central deposits of amorphous material. The use of ANS-butanol plus fluorescence microscopy enormously facilitated the identification of elastic system components in both tension and compression regions of the frog tendon. The compression region exhibited pre-elastic and mature elastic fibers, which were shown to be associated with the surface of the convoluted collagen bundles. Thin fibrils were observed in the compression region after ANS treatment. The visceral paratenon had an increased number of elastic fibers located between the collagen bundles and close to the cells. Congo red plus polarization microscopy failed to impart birefringence to the elastic fibers, but they could be identified by their intense staining and isotropic appearance against the bright background of birefringent collagen fibers. SEM demonstrated the three-dimensional aspects of the elastic fibers. They are composed of fibrils of a sinuous nature. The use of ruthenium red in the fixative allowed for the observation of an intimate association of proteoglycan granules with the microfibril bundles. The elastic components identified in the pressure-bearing tendon are assumed to be important for the tissue supramolecular organization, especially in the maintenance of the convoluted state of the collagen fibers in the compression region and their crimp morphology in the tension region. The elastic system must also play an important role in the restoration of the resting shape of the tendon after the deformation achieved during mechanical stimulation.