Initiation and progression of ossification of the posterior longitudinal ligament of the cervical spine in the hereditary spinal hyperostotic mouse (twy/twy)

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Abstract
Ossification of the posterior longitudinal ligament (OPLL) is a significantly critical pathology that can eventually cause serious myelopathy. Ossification commences in the vertebral posterior longitudinal ligaments, and intensifies and spreads with the progression of the disease, resulting in osseous projections and compression of the spinal cord. However, the paucity of histological studies the underlying mechanisms of calcification and ossification processes remain obscure. The pathological process could be simulated in the ossifying process of the ligament in mutant spinal hyperostotic mouse (twy/twy). The aim of this study is to observe that enlargement of the nucleus pulposus followed by herniation, disruption and regenerative proliferation of annulus fibrosus cartilaginous tissues participated in the initiation of ossification of the posterior longitudinal ligament of twy/twy mice. The mutant twy/twy mice (6 to 22-week-old) were used in the present study. The vertebral column was analyzed histologically and immunohistochemically. We observed that the enlargement of the nucleus pulposus followed by herniation, disruption and regenerative proliferation of annulus fibrosus cartilaginous tissues participated in the initiation of ossification of posterior longitudinal ligament of twy/twy mice. In this regards, the cells of the protruded hyperplastic annulus fibrosus invaded the longitudinal ligaments and induced neovascularization and metaplasia of primitive mesenchymal cells to osteoblasts in the spinal ligaments of twy/twy mice. Since genetic mechanisms could play a role in human OPLL, the age-related enlargement of the nucleus pulposus in the twy/twy mouse may primarily occur as a result of overproduction of mucopolysaccharide matrix material induced by certain genetic abnormalities.