Characteristics and mechanism of the ossification of posterior longitudinal ligament in the tip-toe walking Yoshimura (twy) mouse.

To elucidate the mechanism of ossification of the posterior longitudinal ligament (OPLL), we examined the serial changes in the intervertebral disc of tip-toe walking Yoshimura (twy) mouse. At the age of 6 weeks, the volume of the nucleus pulposus increased in all intervertebral discs causing anterior and posterior herniation. Secondary to this herniation, the cartilagineous tissue of the annulus fibrosus was disrupted and showed regenerative proliferation with PCNA-positive cartilagineous cells. These cells were S-100 positive and the matrix was positive for chondroitin-4-sulfate proteoglycan, indicating the development of calcification. At the age of 15 weeks, the regenerative cartilagineous tissue of the annulus fibrosus reached the posterior longitudinal ligament together with neovascularization and appearance of PCNA-positive proliferating primitive mesenchymal cells. These cells were considered to be osteoblasts since they were positive for alkaline phosphatase and the matrix contained type I collagen. Using electron microscopic X-ray analysis, vesicles present in the matrix of regenerative cartilagineous cells of the annulus fibrosus were confirmed to contain calcium phosphate crystals, also indicating the development of calcification. In conclusion, our serial analysis indicates that ossification of posterior longitudinal ligament in twy mouse was triggered by enlargement of the nucleus pulposus followed by herniation, disruption and regenerative proliferation of annulus fibrosus cartilagineous tissues. Enchondral ossification of the new annulus fibrosus cartilagineous cells and membraneous ossification by primitive osteoblasts proceed to the final ossification of the posterior longitudinal ligaments.