Naturally constrained intervertebral rabbit discs could be cultured for several weeks without losing cell viability. Structural integrity and matrix composition were retained. However, the organ responded to the artificial environment with a degenerative gene expression pattern and decreased metabolic rate. Therefore, the described system serves as a promising in vitro model to study disc degeneration in a whole organ.

There is a major controversy whether spinal trauma with vertebral endplate fractures can result in posttraumatic disc degeneration. Intervertebral discs, which are adjacent to burst endplates, are frequently removed and an intercorporal spondylodesis is performed. In any case, the biological effects within the discs following endplate factures are poorly elucidated to date. The aim of our investigations was therefore to establish a novel disc/endplate trauma culture model to reproducibly induce endplate fractures and investigate concurrent disc changes in vitro. This model is based on a full-organ disc/endplate culture system, which has been validated by the authors before. Intervertebral disc/endplate specimens were isolated from Burgundy rabbits and cultured in standard media (DMEM/ F12, 10%FCS). Burst endplate fractures were induced in half of the specimens with a custom-made fracture device and subsequently cultured for 9 days. The biological effects such as necrotic or apoptotic cell death and the expression of pro-apoptotic genes and other genes involved in organ degeneration, e.g. matrix metalloproteinases (MMPs) were analyzed. Cell damage was assessed by quantification of the lactate dehydrogenase (LDH) activity in the supernatant. The expression of genes involved in the cellular apoptotic pathway (caspase 3) and the pro-apoptotic proteins FasL and TNF-a were monitored. The results demonstrate that LDH levels increased significantly post trauma compared to the control and remained elevated for 3 days. Furthermore, a constant up-regulation of the caspase 3 gene in both disc compartments was present. The pro-apoptotic proteins FasL and TNF-a were up regulated predominantly in the nucleus whereas the MMP-1 and -13 transcripts (collagenases) were increased in both disc structures. From this study we can conclude that endplate burst fractures result in both necrotic and apoptotic cell death in nucleus and annulus tissue. Moreover, FasL and TNF-a expression by nucleus cells may lead to continued apoptosis induced by Fas-and TNF-a receptor bearing cells. In addition TNF-a over-expression has potentially deleterious effects on disc metabolism such as overexpression of matrix proteinases. Taken together, the short term biological response of the disc following endplate fracture exhibits characteristics, which may initiate the degeneration of the organ.

Vertebral endplate damage is associated with intervertebral disc (IVD) degeneration (DD) in vivo as confirmed by in-vitro investigations. Our aims were to further characterize the process of DD using an in vitro full-organ culture model and to elucidate whether significant endplate damage or impact loading alone is pivotal for the initiation of DD. Rabbit spinal segments (n ¼ 80) were harvested, subjected to pure axial impact loading (n ¼ 40) using a custom-made device, and cultured for 28 days. The applied threshold energy (0.76 J) induced endplate fractures in 21 specimens (group A); 19 remained intact (group B). Markers for DD (cell viability, apoptosis, necrosis, matrix remodeling, and inflammation) were monitored for 28 days post-trauma in the annulus fibrosus (AF) and nucleus pulposus and compared to non-impacted control discs. Cell viability in both groups stayed at a control level. Group A compared to group B showed enhanced lactate dehydrogenase (LDH) and caspase-3/7 activity, reduced glycosaminoglycan content, reduced aggrecan mRNA, but elevated mRNA for collagen-2, catabolic enzymes (MMP-1/-3/-13), and pro-inflammatory (TNFa, IL-6, IL-8, MCP-1) and pro-apoptotic (fas ligand, caspase-3) proteins. Group B compared to control only showed small changes in mRNA levels. Our findings demonstrate that burst endplates, but not equienergetic loading, promotes DD. ß

It is generally agreed that the mechanical environment of intervertebral disc cells plays an important role in maintaining a balanced matrix metabolism. The precise mechanism by which the signals are transduced into the cells is poorly understood. Osmotic changes in the extracellular matrix (ECM) are thought to be involved. Current in-vitro studies on this topic are mostly short-term and show conflicting data on the reaction of disc cells subjected to osmotic changes which is partially due to the heterogenous and often substantially-reduced culture systems. The aim of the study was therefore to investigate the effects of cyclic osmotic loading for 4 weeks on metabolism and matrix gene expression in a full-organ intervertebral disc culture system. Intervertebral disc/endplate units were isolated from New Zealand White Rabbits and cultured either in iso-osmotic media (335 mosmol/kg) or were diurnally exposed for 8 hours to hyper-osmotic conditions (485 mosmol/kg). Cell viability, metabolic activity, matrix composition and matrix gene expression profile (collagen types I/II and aggrecan) were monitored using Live/Dead cell viability assay, tetrazolium reduction test (WST 8), proteoglycan and DNA quantification assays and quantitative PCR. The results show that diurnal osmotic stimulation did not have significant effects on proteoglycan content, cellularity and disc cell viability after 28 days in culture. However, hyperosmolarity caused increased cell death in the early culture phase and counteracted upregulation of type I collagen gene expression in nucleus and annulus cells. Moreover, the initially decreased cellular dehydrogenase activity recovered with osmotic stimulation after 4 weeks and aggrecan gene down-regulation was delayed, although the latter was not significant according to our statistical criteria. In contrast, collagen type II did not respond to the osmotic changes and was downregulated in both groups. In conclusion, diurnal hyper-osmotic stimulation of a whole-organ disc/ endplate culture partially inhibits a matrix gene expression profile as encountered in degenerative disc disease and counteracts cellular metabolic hypo-activity. ß