INTRODUCTION: Notochordal cells (NC) are remnant cells in the center of the intervertebral disc (IVD) originating from the notochord and present in all chordates in early embryogenesis. Strikingly, these cells co‐exist with nucleus pulposus cells (NPCs) at different ratios among different vertebrate species and the ratios change with aging. Here, we hypothesized whether there is a ratio of NC relative to NPC, which is most favorable for both cell populations in terms of cell activity and extracellular matrix (ECM) production.
METHODS: We co‐cultured porcine coccygeal NCs and bovine coccygeal NPCs at different ratios, i.e. 0%, 25%, 50%, 75% and 100%, respectively. The cells were encapsulated at a density of 4M/ml into 1.2% alginate and cultured in serum‐free medium. All co‐cultures were conducted in duplicates with culture inserts (BD). DNA content, cell activity, glycosaminoglycan content (GAG), and relative gene expression (ACAN, Col1, Col2 and r18S) were monitored for both, NCs and NPCs. The co‐cultures were tracked on day0, day7 and day14.
RESULTS: In both, NCs and NPCs, DNA content of beads was stable over culture time or even increased slightly relative to day 0 control. As for the cell metabolism the bNPCs were more strongly activated by the presence of pNCs (p = 0.035) than vice versa. We found a significant increase in GAG/DNA ratio for the 50% pNC group after 14 days of co‐culture but no effect for pNC co‐cultures. Remarkably, for pNCs, Col 2 was found to be up‐regulated by a factor of ˜100 for all of the three co‐culture ratios containing bNPCs, indicating progenitor‐like cell status of pNCs.
DISCUSSION: The current debate is on the origin and interaction of NCs and NPCs. Our data confirm stimulating interaction between these two populations in vitro.