Germinal centers in the adult brain. Neurogenesis in the adult brain is largely confined to two germinal centers: the dentate gyrus and the subventricular zone, shown schematically (a) and in a corresponding sagittal section of the rodent brain (b). Insets in b show the position of high-resolution micrographs in c–f. In the dentate gyrus (c), newly generated cells are detected through incorporation of the thymidine analog BrdU and labeled with a green fluorophore (Cy2). These cells differentiate into mature neurons, as seen by their coexpression of the marker NeuN (red) but not S100β (blue), a marker for mature astrocytes. In contrast, cells generated in the subventricular zone (d) do not differentiate into mature neurons (red) but migrate away through the rostral migratory stream (RMS). Within the RMS (e), newly generated cells are surrounded by astrocytes (glial fibrillary acidic protein [GFAP], blue) and begin to express immature neuronal markers (polysialylated neural cell adhesion molecule [PSA-NCAM], red) as they migrate to the olfactory bulb. Upon arrival in the olfactory bulb (f), newly generated cells differentiate into mature neurons (NeuN, red), but not astrocytes (S100β, blue).