Study design: Coculture of human nucleus pulposus (NP) cells and mesenchymal stem cells (MSCs) using a noncontact system.

Objective: To investigate the interaction between NP cells and MSCs through paracrine stimulation.

Summary of background data: Cell-based therapies have a potential role in the treatment of intervertebral disc degeneration. Upregulating the viability of NP cells and differentiating MSCs into NP-like cells are potential alternatives to achieve viable cells.

Methods: Culture plates and inserts were used to coculture MSCs and NP cells without direct contact or exchange of cellular components. Cellular proliferation and RNA expression of selected genes were then evaluated after coculture.

Results: Coculturing slightly promoted the proliferation of MSCs, and expression of collagen I and Fas-associated death domain protein significantly decreased. MSCs, which initially expressed no collagen II, started to show collagen II expression after coculturing; the expression level was highest when the cells were cultured with a higher number of NP cells. On the converse, proliferation of NP cells significantly rose even after cocultured with a few MSCs. Increasing the number of cocultured MSCs did not further enhance proliferation of NP cells. Expression of aggrecan in the NP cells significantly increased when the cells were cultured with a higher number of MSCs.

Conclusion: The results showed a possible mechanism of interaction between MSCs and NP cells mediated by secreted factors. The most significant effect on NP cells was enhancement of cellular proliferation when they were cocultured with even a small number of MSCs. To differentiate MSCs into NP-like cells with heightened collagen II expression, MSCs must be in an environment containing numerous NP cells.