The Functional Significance of Cell Clusters in the Notochordal Nucleus Pulposus: Survival and Signaling in the Canine Intervertebral Disc

Study Design. Cell viability was assessed in relation to cell clustering, and mechanisms of cell–cell signaling in the clusters were investigated. Objectives. To explore the functional role of cell clustering in the notochordal nucleus pulposus. Summary of Background Data. The intervertebral disc of some species contains residual cells from the embryonic notochord. These cells form large three-dimensional clusters in the young, healthy disc but are replaced by chondrocyte-like cells during aging and degeneration. Methods. Forty nucleus pulposi of adult dog lumbar intervertebral discs were isolated, and were left untreated, mechanically disrupted through a syringe, or enzymatically digested. The presence of functional gap junctions was determined by the fluorescence recovery after photobleaching method. Cell viability was also assessed over 20 days in vitro. Results. The cell clusters were interconnected via functional gap junctions. Mechanical disruption of the tissue had little effect on long-term cell viability, but enzymatic disruption of the tissue had a substantial negative impact on cell survival. Conclusions. These results demonstrate that the notochordal cells in adult dog nucleus pulposi are able to communicate via cytoplasmic signals and that such communications may influence the functionality of these cells in the young disc.