AbstractMany of the therapeutic interventions for intervertebral disc degeneration attempt to repopulate the nucleus pulposus (NP) tissue; however, NP cells are heterogeneous and not well characterized. To address this, we have investigated the morphology, extracellular gene and protein expression, and apoptosis changes in NP explants cultured in vitro with or without chondrogenic reagents for different periods. We also compared the susceptibility of the explants to different treatments by comparing: treatment of NP explants with GDF5 protein, transfection of NP explants with GDF5 plasmid, and infection of NP explants with GDF5 adenovirus vector. We found that expression levels of two of the major extracellular proteins found in NP tissue, that is, collagen II and aggrecan, could be maintained in an NP explant culture model with a chondrogenic medium up to 7 days, and were significantly higher than that of fresh NP tissue after 14 days of culture in vitro, whether or not chondrogenic medium was used. In addition, the NP explant responded to treatment with growth factor, and could be infected by virus and transfected by plasmid for further evaluation of growth factor gene therapy. NP explant culture could therefore provide an easy in vitro culture model to characterize NP cells and evaluate potential therapeutic reagents. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27: 814–819, 2009