In Vivo Assay of Cellular Proliferation

The use of bromodeoxyuridine (BrdU) incorporation into replicating DNA is a well-established and commonly used technique for identifying dividing cells in vivo and in cell culture. BrdU is a pyrimidine analog of thymidine that is incorporated into the DNA of cells in the S-phase of the cell cycle (1 –3 ). Intra-peritoneal injection of BrdU in mice results in nuclear incorporation only where DNA is being actively replicated. The loading time needed to label an S-phase cell to detectability is estimated at <0.2 h directly after injection of BrdU (4 ). As the availability of BrdU decreases, the labeling time increases to about 0.65 h, 30 min after injection. Thereafter, cells that enter the S-phase continue to become detectably labeled for about 5–6 h. BrdU labeling therefore provides a method for rapidly detecting replicating DNA.