Introduction Intervertebral disc degeneration may cause chronic low back pain. Disc degeneration is characterized by dysfunctional cells and a decrease in extra-cellular components. Bone marrow derived mononuclear cells are a heterogeneous cell population which contains mesenchymal stem cells. Transplantation of stem cells and progenitor cells may provide a new approach to treat disc degeneration, but it is unclear if transplanted cells can survive and differentiate in the non-vascularized disc.

Methods Bone marrow was collected from syngeneic Sprague-Dawley rats and mononuclear cells were isolated. The cells were labelled with a fluorescence dye (Cell Tracker Orange) and suspended in PBS. 10–20μl of the cell suspension (1–2x10⁵ cells/disc) was transplanted into coccygeal discs in 12 syngeneic rats. For each rat two discs were cell transplanted and one disc served as control. The rats were sacrificed after 0, 7, 14 or 21 days. For each time point the discs from one animal were saved for routine histological staining. The cell transplanted discs of the other animals (n=4 discs per time point) were formalin-fixed, frozen and sectioned together with the control discs. Frozen disc sections were visualized with fluorescence microscopy and the number of transplanted cells assessed. Expression of collagen II, a marker of chondrocytes and chondrocyte-like cells in the disc, was assessed in the transplanted cells using immunofluorescence technique.

Results All cell-suspension injected discs contained transplanted bone-marrow cells. The discs within each time-group demonstrated a large variation in number of detected cells. There was a decrease in detected cells at 7, 14 and 21 days compared to day 0. Transplanted cells expressed collagen II after 21 days but not after 7 and 14 days.

Discussion The results suggest that transplanted bone marrow-derived mononuclear cells can survive and differentiate within the intervertebral disc. Further studies in models of disc degeneration are warranted to investigate the regenerative potential of the disc following cell transplantation.