AbstractBackgroundThe notochord is an embryonic organ involved in forming and patterning the spinal column. The mechanism by which the notochord transforms from a continuous rod to a segmented structure excluded from the vertebrae and residing solely as the nucleus pulposus within the intervertebral disc is understudied. The current model of notochordal segmentation suggests that swelling through formation and maturation of the vertebrate cartilage squeezes the notochord cells from the vertebra.ResultsAnalysis of Collagen 10, a marker for hypertrophic differentiation, as well as evaluation of changes in cell density, reveal that the expansion of the vertebral precursor cells occurs after notochord segmentation has already taken place. We find that the bulk of the nucleus pulposus is derived from accelerated proliferation within the nucleus pulposus itself. In a model of cell proliferation, the increased proliferation at the nucleus pulposus importantly contributes to expand the nucleus pulposus area.ConclusionsOur data is consistent with the hypothesis that notochord cell proliferation contributes to the enlargement of the nucleus pulposus before the vertebra undergo hypertrophy.