Cell types and evidences for traumatic cell death in a pressure-bearing tendon of Rana catesbeiana

Tendon fibrocartilages appear in areas subjected to compressive forces. The bullfrog plantaris longus tendon was shown to be subjected to compression and to develop a modified region which differs from fibrocartilage in many respects. Ultrastructural analyses of the compression region of the bullfrog tendon demonstrated the existence of typical fibroblasts in the fibrous areas and large cells with abundant cytoplasm filled with intermediate type filaments. This large cell type has organelles restricted to a small perinuclear area or dispersed in the network of intermediate type filaments. Other cells were also found and exhibited less abundant deposition of intermediate filaments, showing an organization intermediate between fibroblasts and typical cells from the compression region. These intermediate type cells are closely associated with collagen bundles while the large cells seemed to have no connection with the fibrous components, but are immersed in a glycosaminoglycan-rich extracellular matrix. Aspects of cell death in association with extracellular matrix disruption were observed in some instances and it is likely that these are associated with traumatic stimulation of the tendon, especially when it is submitted to the sudden and strong mechanical loading expected to occur during jumping. Since the damage occurred mainly in cells of the intermediate type, it is assumed that accumulating intermediate type filaments is a protective mechanism against compressive forces to which this tendon is subjected.