Changes in the nucleus pulposus of the intervertebral disc in bipedal mice. A light and electron microscopic study

Bipedal mice were produced by clipping the forelimbs and tails of mice within one week of birth. Using light and electron microscopy, the nucleus pulposus of the lumbar intervertebral disc in the bipedal mice was compared with that in normal mice at three, six, and 12 months of age. In normal neonatal mice, the nucleus pulposus is composed of densely packed notochordal cells, which undergo degenerative changes and decrease in number with age. In the bipedal mice, degenerative changes in the nucleus pulposus were accelerated, and herniation of the nucleus pulposus occurred frequently. At the same time, active chondrocytes associated with cartilage matrix appeared in the nucleus pulposus. This sequence of morphologic changes in the nucleus pulposus of the bipedal mice resembles the age-related changes that occur in the nucleus pulposus of the human intervertebral disc. These morphologic changes can be accelerated by creating abnormal mechanical stress. Chondrocytes in the nucleus pulposus may develop from surrounding cartilaginous tissue--cartilage plates and annulus fibrosus.