Growth cartilage calcification and formation of bone trabeculae are late and dissociated events in the endochondral ossification of Rana catesbeiana

Endochondral ossification in the growth cartilage of long bones from the bu llfrog Rana catesbeiana was examined. In stage-46 tadpoles and 1-year-old a nimals, the hypertrophic cartilage had a smooth contact with the bone marro w and the matrix showed no calcification or endochondral bone formation. In spite of showing no aspects of calcification, the chondrocytes exhibited a lkaline phosphatase activity and some of them died by apoptosis. However, m atrix calcification and endochondral ossification were observed in 2-year-o ld bullfrogs. Calcium deposits appeared as isolated or coalesced spherical structures in the extracellular matrix of hypertrophic cartilage. Bone trab eculae were restricted to the central area at the sites where the hypertrop hic cartilage surface was exposed to the bone marrow. Cartilage matrix calc ification and the formation of bone trabeculae were not dependent on each o ther. Osteoclasts were involved in calcified matrix resorption. These resul ts demonstrate that the calcification of hypertrophic cartilage and the dep osition of bone trabeculae are late events in R. catesbeiana and do not con tribute to the development and growth of long bones in adults. These proces ses may play a role in reinforcing bony structures as the bullfrog gains we ight in adulthood. In addition, the deposition of bone trabeculae is not de pendent on cartilage matrix calcification.