BrdU/Thymidine

IF STEM CELLS are to be introduced into a tissue as a therapeutic mechanism to stimulate the repair of damaged tissue, or in a preliminary study to see merely where they go, scientists must have a way of monitoring what becomes of the stem cells postinjection. One way is to mark the DNA with a chemical that would remain in the DNA for generations of cell divisions. A marker of choice—until recent warnings—has been BrdU, a thymidine analog.

Deoxyribonucleic acid, or DNA, carries the genetic information of an organism, in most every cell (red blood cells do not contain nuclei and therefore do not carry DNA). DNA is made up of four nucleotide bases that contain nitrogenous rings: deoxyadenosine, deoxycytidine, deoxy—guanosine, and thymidine. Deoxy does not need to be included in the name of thymidine because thymidine is not found in ribonucleic acid (RNA), which contains adenosine, cytidine, and guanosine, but in lieu of thymidine, it carries an analog, uridine. BrdU is short for 5-bromo-2-deoxyuri—dine, a modified deoxyuridine that is quite similar structurally to thymidine and therefore can incorporate into replicating DNA. The process of supplying a dividing cell with BrdU that it will then incorporate into new DNA is called BrdU labeling. BrdU has a bromine in its nitrogenous ring, which can be detected by an antibody to BrdU in histo—logical stains.

The purpose of BrdU—labeling DNA is that BrdU incorporates into DNA that is actively replicating, replacing some of the thymidine that would normally be incorporated. BrdU is not naturally [Page 47]found in cells—it must be added by the researcher. Thus, any cell undergoing cell division, and therefore DNA replication, at the time of BrdU addition will take up BrdU into its DNA, and subsequent cells resulting from cell division of this first cell will also have BrdU in their DNA, although if BrdU is not reintroduced, the BrdU concentration per cell will be diluted by approximately one—half with every cell division.

Scientists can label stem cells with BrdU and introduce these labeled stem cells into new tissue. After a period of time, the tissue can be stained his—tologically for the presence of BrdU in tissue cells. Any cell that has BrdU in its DNA is presumed to have derived from the initial stem cells injected. If the laboratory has a high level of technology with sensitive equipment, scientists can trace a stem cell through several generations of division, thus determining the end fate of the injected stem cells and their progeny.

Recently, Dr. Catherine Verfaillie, who was at the University of Minnesota at the time, warned that stem cells injected into tissues could die and release their BrdU into the surrounding space. Neighboring cells could then take up this BrdU and label their DNA, confounding results that expect all BrdU—labeled cells to have arisen from the injected, labeled stem cells.

Some scientists agree, but others argue that this effect does not occur. In addition, in tissues such as the brain, where mature neurons do not divide and thus should not take up free BrdU, these neurons might still incorporate BrdU if an injury triggers novel DNA synthesis, even if not in preparation for cell division. This effect was observed by Dr. Pasko Rakic of Yale University in New Haven, Connecticut.

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