Time-dependent segmentation of BrdU-signal leads to late detection problems in studies using BrdU as cell label or proliferation marker.

Bromodeoxyuridine incorporates into DNA during mitosis. A long-term stability of the incorporated BrdU is important for the recovery of BrdU-labeled cells. For testing the stability of BrdU incorporation into DNA we pulse-labeled mesenchymal stem cells with BrdU and observed these cells in vitro over 4 weeks. During this time the BrdU-signal was permanently decreasing. Starting with cells containing evenly stained BrdU-nuclei, so-called filled cells, already 3 days after BrdU removal we detected cells containing so-called segmented and punctated BrdU-signals. The number of those labeled cells continuously increased over time. Interestingly, the loss of BrdU in the nucleus was accompanied by an increasing labeling of the cytosol. Further, we injected BrdU intraperitoneally into rats after ischemia and detected BrdU-positive cells in the hippocampus 3 and 23 days after the last BrdU injection. While after 3 days most of the BrdU-positive cells in the hippocampus displayed a filled BrdU-signal, 23 days after BrdU removal an increased number of segmented and punctated BrdU-positive nuclei was detected. The gradual degradation of the BrdU-signal was not caused by cell death. The consequence of this BrdU degradation would be an underestimation of cell proliferation and an overestimation of cell death of newly generated cells.