How are new neurons identified?

New neurons can be identified by injecting a compound known as bromodeoxyuridine (BrdU) into the brain of an animal. While all cells will get exposed to BrdU, they do not react with this compound. Only DNA of newly generated cells that are in S phase can incorporate BrdU.  

Immunohistochemistry is then employed to bind a fluorescent label to the BrdU incorporated in the DNA. This is followed by fluorescent microscopy that enables researchers to visualize the presence of BrdU, and consequently, identify new neurons in brain tissue.