Crocodilian bone-tendon and bone-ligament interfaces.
We investigated bone-tendon (27 sites) and bone-ligament (12 sites) interfaces in six pairs of crocodile limbs and girdles under light microscopy. These crocodilian interfaces often included a direct, unmediated insertion in which the tendon or ligament fibers inserted directly into the bone itself without fibrocartilaginous mediation. This was quite different from the usual direct insertion known in mammals and lizards. Fibrocartilaginous tissue at the bone-tendon interface is generally believed to protect tendon fibers against shear stress. Other types of insertions were found in the crocodilian epiphyses, namely, hyaline cartilage and pseudofibrocartilaginous insertions. Notably, a thick periosteum/perichondrium and subchondral layer was involved at both interfaces. The thick periosteum/perichondrium seemed to form along the epiphyseal hyaline cartilage and might function in replacement of fibrocartilaginous tissues. Crocodilian thick periosteum/perichondrium would be expected to reinforce the limb and girdle bones--especially their epiphyses, in which secondary centers of ossification are absent. The subchondral layer--a kind of fibrocartilaginous tissue--seemed to play the role of the growth plate in compensating for the absence of secondary centers of ossification. Therefore, we hypothesized that the crocodile-specific bone-tendon interfaces were the result of these specializations of bone development and growth. In crocodiles, the disadvantages of the single ossification center are effectively compensated for by specialized morphologies, including these interfaces. Specialized bone growth provides the crocodile with the largest body size of the recent reptiles and an extremely fast method of locomotion.