Transit amplification was demonstrated in the developing brain by direct visualization of intermediate progenitors undergoing division over time (Noctor et al., 2004). A similar experiment would test our model in the adult hippocampus. Cell survival impacts any lineage as it accumulates, but unfortunately the number of cells undergoing apoptosis over time is difficult to quantify due to transient expression of apoptotic markers and rapid clearance of dead cells. While some

indirect information about cumulative cell death can be achieved through BrdU survival studies, the technique provides limited information about total populations of cells (Taupin, 2007). BrdU survival experiments are least informative about slowly dividing NSC and rapidly dividing IP populations since the former are poorly labeled by BrdU, while decreased label retention in the latter can reflect either cell death or multiple divisions (Dayer et al., 2003, Encinas et al., Enzalutamide nmr 2011, Mandyam et al., 2007 and Taupin, 2007). Hence, the contributions of NSC and IP cell death during lineage accumulation and in response to environmental manipulations remain to be determined. Our own cell survival studies of neurons, which are postmitotic and thus lend themselves to BrdU survival studies, did 3-deazaneplanocin A solubility dmso not detect an effect of social isolation on survival (Figures S4A and S4B). In EEE-treated mice, in addition to observing a well-established and robust increase

of proliferation (data not shown), we also detected increased neuronal survival suggesting that decreased neuronal death contributes to the lineage gains described in this study. Our inability to detect any relationship between apoptosis and lineage expansion, within or between any of our groups, suggests that apoptosis (Figure S4D and S4F), while impacting accumulation of the lineage, is unlikely to account for the differences that we observe. An expanding stem cell compartment could allow the brain to grow a NSC reservoir during

deprived conditions such as isolation stress. This brain adaptation would then allow an augmented neurogenic response through experience-directed fate specification when the environment next became richer. An analogous type of proliferative control was recently recapitulated in a more artificial, embryonic stem cell system where extrinsic stimulation and activation of signaling pathways favored differentiation, while depleting these signals favored self-renewal (Ying et al., 2008). The signals dictating changes in the fate of the NSC lineage remain to be determined. Of particular interest are the multiple observations that neural activity is positively linked to cell division in the adult dentate gyrus (Deisseroth et al., 2004 and Tozuka et al., 2005). Environmental enrichment was previously demonstrated to increase neuronal activity in the dentate gyrus (Tashiro et al., 2007) while social isolation was recently demonstrated to decrease it (Ibi et al., 2008).