The Comparative Cytomorphology of Nucleus Pulposus Cells

INTRODUCTION The cells of the embryonic notochord play an important role in formation of the spinal column, including the intervertebral discs (IVDs) (1). These cells become trapped inside the annulus fibrosus and reside within the nucleus pulposus at birth, but are generally lost during aging and replaced by small chondrocyte-like cells. The age at which the notochordal cells are lost varies substantially among the different species commonly used in IVD studies, as does the age of onset of spontaneous disc degeneration (2). Despite these differences in cell behavior within the disc, the importance of notochordal cells in nucleus pulposus maintenance and the significance of their loss on degeneration has not been rigorously studied. Notochordal cells are typically distinguished from other cells in the IVD by their large size and the presence of cytoplasmic inclusions (2,3), thus we compared the cytomorphology of cells in the nuclei pulposi of five common laboratory species.