['Changes in gene expression profile and cell fusion of mesenchymal stem cells (MSC) and nucleus pulposus cells (NPC) after coculture were analyzed.', 'To investigate the mechanisms of the interaction between NPC and MSC such us differentiation, stimulatory effect, and cell fusion.', 'Introduction of exogenous cells to supplement and replenish intervertebral disc cell population offers a potential approach to treat intervertebral disc degeneration (IDD). Recent evidences showed that intradiscal injection of MSC effectively alter the course of IDD in vivo, and the regenerative potential may result from up-regulated extracellular matrix protein synthesis mediated by MSC and NPC interaction.', 'Using a double labeling cell system and flow activated cell sorting, we quantitatively analyzed changes in the gene expression profile of human male MSC and female NPC after coculture in a 3-dimensional system that allows short distance paracrine interactions typical of the nucleus pulposus. Furthermore, we analyzed for cell fusion in the cell interaction by fluorescence in situ hybridization (FISH) for X and Y chromosomes, using a 3-dimensional culture system to allow cell-to-cell interactions conducive to cell fusion.', 'Two weeks of coculture cell interaction in a 3-dimensional environment induces a change in MSCs towards a more chondrogenic gene expression profile indicating MSC differentiation, and NPC gene expression changes in matrix and chondrogenic genes demonstrating only a modest trophic effect of MSC on NPC. Moreover, FISH analysis demonstrated that cell fusion is not responsible for MSC plasticity in the interaction with NPCs.'] Do coculture of bone marrow mesenchymal stem cells and nucleus pulposus cells modulate gene expression profile without cell fusion?