Until very recently, only neurons born at one specific time point could be identified in each experimental animal.
It will be important to note here that these cell populations can be traced using relatively specific markers (Figure 1).
These markers can be easily used in conjunction with “birth-marking” labels to determine that the different subpopulations were actually born in the adult brain.
Therefore, the injection rate in conjunction with the survival time can produce large differences, and in the worst cases, cell populations that are markedly heterogeneous in terms of age may be labeled equally.
Within the framework of the experimental design, the investigator must judge what heterogeneity in terms of age can be accepted in the target cell population.
This approach has generated a large amount of data.
However, it is not a trivial matter that these comparisons between cell populations of different age, born in the adult brain, have been analyzed to date in different animals.
In recent years, the labeling of newly born cells in the adult brain has been almost overwhelmingly ruled by the use of 5-bromo-3′-deoxy-uridine (Brd U).
“The underlying principle is straightforward: a permanent marker is brought into a cell of interest at the time point of division and the later fate of this cell is studied.
The age of labeled cells is equivalent to the survival time (i.e., cells are 1 month old if the animal injected with Brd U was sacrificed 1 month after Brd U injection).
A key limitation of this method is its ability to recognize only a single pool of Brd U incorporated into the body, regardless of when and how it was administered.
This issue is particularly important in short experiments (i.e., days).