To study how adult hippocampal neurogenesis might originate from the proliferation of stem or progenitor cells in vivo, we have used transgenic mice expressing green fluorescent protein (GFP) under the nestin promoter to identify these cells. Having described an astrocyte-like type 1 cell with low proliferative activity, a characteristic morphology, vascular end feet, and passive electrophysiological properties, we focused here on the large population of nestin-GFP-expressing type 2 cells, which lack all these features. Type 2 cells were highly proliferative and showed signs suggestive of their involvement in the neuronal lineage. They could be subclassified by the absence (type 2a) or presence (type 2b) of a coexpression of the early neuronal marker doublecortin. A third type of proliferating cells was doublecortin positive but nestin-GFP negative (type 3). We believe that type 2a, 2b, and 3 cells mirror a marker progression during earliest neuronal development. This view is supported by the increasing coexpression of the early granule cell-specific marker Prox-1. The low proliferative activity of type 1 cells showed little change over time or under "neurogenic interventions," such as a challenge by environmental complexity (ENR) or voluntary physical activity (RUN). However, RUN led to a significant increase of type 2 cells labeled with the proliferation marker bromodeoxyuridine (BrdU). ENR did not cause increased cell proliferation or an increased number of BrdU-labeled type 2 cells, but both ENR and RUN resulted in more newly generated cells lacking nestin-GFP immunoreactivity and expressing Prox-1. These findings allow us to break down what was broadly perceived as "proliferation" in earlier experiments into the relative contribution of several cell types, representing the earliest steps of neuronal development.

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In adult hippocampal neurogenesis, new neurons appear to originate from a cell with astrocytic properties expressing glial fibrillary acidic protein (GFAP). Also, new astrocytes are generated in the adult dentate gyrus. Whereas the putative astrocyte-like progenitor cells are consistently S-100beta-negative, many new astrocytes are S-100beta-positive. Thus, it is unclear whether the GFAP-positive progenitor cells are astrocytes in a general sense or rather neural progenitor cells with certain astrocytic characteristics. We therefore investigated the development of GFAP-expressing cells in the context of adult hippocampal neurogenesis. Proliferating cells could be either GFAP-positive or doublecortin-positive (DCX), but never both, indicating two independent populations of dividing cells in the glial and neuronal lineages. Two distinct populations of cells with astroglial properties were detected-one expressing GFAP, the other co-expressing GFAP and S-100beta. We never found S-100beta-cells to be in S-phase. No overlap between neuronal and glial markers was seen at any time point. Thus, astrogenesis occurred in parallel and to some degree independent of adult neurogenesis. The uninterrupted GFAP expression in this lineage, and neuronal markers in the other lineage, argue against a late common precursor for neurogenesis and gliogenesis in the adult hippocampus. Very few newly generated microglia and no new oligodendrocytes were detected. Environmental enrichment and voluntary wheel running-two experimental paradigms with robust stimulatory effects on adult hippocampal neurogenesis-affected hippocampal astrogenesis differentially: Running, but not enrichment, strongly induced net astrogenesis (GFAP/S-100beta), but also GFAP-positive S-100beta-negative cells, which thus appear to be a transiently amplifiable intermediate population within the glial lineage.

Distinct types of relay neurons in the hindbrain process somatosensory or viscerosensory information. How neurons choose between these two fates is unclear. We show here that the homeobox gene Lbx1 is essential for imposing a somatosensory fate on relay neurons in the hindbrain. In Lbx1 mutant mice, viscerosensory relay neurons are specified at the expense of somatosensory relay neurons. Thus Lbx1 expression distinguishes between the somatosensory or viscerosensory fate of relay neurons.