Signals regulating neurogenesis in the adult olfactory bulb.

Most mammalian brain cells develop from neural progenitor or stem cells that reside in the ventricular and subventricular zone. Interestingly, in rodents and primates, neurogenesis does not end when the olfactory bulb reaches adult size but rather continues throughout life (Kaplan et al., 1985; Kuhn et al., 1996; Pencea et al., 2001). For neurons at two locations in the olfactory bulb, the granule cell layer and glomerular layer, a persistent proliferative activity of progenitor cells can be observed in the subventricular zone (SVZ) of the lateral ventricles. The committed neuronal progenitor cells migrate rostrally through the remnant of the embryonic olfactory ventricle wall, the so-called rostral migratory stream (RMS), towards the olfactory bulb. Once the olfactory bulb is reached, the majority of cells disperse throughout the granule cell layer to develop into GABAergic granule cells. A small percentage, however, moves into the periglomerular region to develop into interneurons of mostly GABAergic and dopaminergic phenotype.