Adult neurogenesis produces a large pool of new granule cells in the dentate gyrus

Knowing the rate of addition of new granule cells to the adult dentate gyru s is critical to understanding the function of adult neurogenesis. Despite the large number of studies of neurogenesis in the adult dentate gyrus, bas ic questions about the magnitude of this phenomenon have never been address ed. The S-phase marker bromodeoxyuridine (BrdU) has been extensively used i n recent studies of adult neurogenesis, but it has been carefully tested on ly in the embryonic brain. Here, we show that a high dose of BrdU (300 mg/k g) is a specific, quantitative, and nontoxic marker of dividing cells in th e adult rat dentate gyrus, whereas lower doses label only a fraction of the S-phase cells. By using this high dose of BrdU along with a second S-phase marker, [H-3]thymidine, we found that young adult rats have 9,400 dividing cells proliferating with a cell cycle time of 25 hours, which would genera te 9,000 new cells each day, or more than 250,000 per month. Within 5-12 da ys of BrdU injection, a substantial pool of immature granule neurons, 50% o f all BrdU-labeled cells in the dentate gyrus, could be identified with neu ron-specific antibodies TuJ1 and TUC-4. This number of new granule neurons generated each month is 6% of the total size of the granule cell population and 30-60% of the size of the afferent and efferent populations (West et a l. [1991] Anat Rec 231:482-497; Mulders et al. [1997] J Comp Neurol 385:83- 94). The large number of the adult-generated granule cells supports the ide a that these new neurons play an important role in hippocampal function. Pu blished 2001 Wiley-Liss, Inc.(dagger).