What Makes a Marker a Good Marker?

To validate if DCX truly predicts neuron recruitment throughout the canary brain, it should correlate to a high degree with established methods for studying neuron recruitment, such as labeling cells with the mitotic marker 5-bromo-2-deoxyuridine (BrdU). This is not the case, however, as we observed clear regional differences in the relationship between the two labeling methods in neurogenic and nonneurogenic brain areas in our study [Vellema et al., 2014: fig. 7B]. BrdU-labeling typifies the age of a cell, but gives no specific information about the cell’s functional state. Balthazart and Ball [2014] correctly address the fact that the functional state of a neuron of a certain age can differ in different parts of the brain, potentially affecting the relationship between BrdUand DCX-labeling. Whether the large regional differences that we observed can be explained by the methodological differences between BrdUand DCX-labelAntibodies are one of the most frequently used research tools in the comparative neurosciences. For an antibody to be useful as a molecular marker, it needs to specifically and selectively identify the object of interest. The validation process to determine if a marker is specific and selective is often laborious but is highly essential for establishing whether a new marker qualifies as a reliable tool. In a critical review of our work, Balthazart and Ball [2014] argue that the microtubule-associated protein doublecortin (DCX) qualifies as a reliable marker for quantifying adult neurogenesis in canaries, despite observations that DCX is expressed in mature neurons in both neurogenic and nonneurogenic brain regions in canaries [Vellema et al., 2014] and mammals [e.g. Verwer et al., 2007; Cai et al., 2009; Klempin et al., 2011; Kremer et al., 2013].