Fig. 2. Developmental time-course of migration from the ventricular zone to the dentate gyrus. Intraventricular injections of nlslacZ retrovirus were performed in E18 rats and the migratory pattern of labeled cells was analyzed after varying amounts of time. Coronal sections of hippocampus were processed for X-gal histochemistry to detect ß-gal activity and then counterstained with nuclear Fast Red. For reference, the fimbria is marked with a green asterisk. Each experiment was performed on embryos from at least three different litters. (A) At E19, 24 hours after retroviral injection, labeled cells (blue) can be seen in the ventricular zone (open arrowhead). Additionally, a few cells can be seen to have left the ventricular zone (arrowhead). (B) Forty-eight hours after retroviral injection (E20), labeled cells are still seen to populate the ventricular zone. Additionally, cells can be seen above the fimbria in the dentate migratory stream (arrowheads), but have not yet entered the dentate gyrus. (C) 72 hours after retroviral injection (E21), labeled cells have migrated into the dentate gyrus and can be found in the germinal center in the dentate hilus. In addition, the first labeled cells migrate into the granule cell layer (arrow). (D) 96 hours after injection (P1), fewer labeled cells can be seen in the migratory stream; however, many labeled cells are now seen in the dentate hilus and populating the granule cell layer (arrows). (E) At E21, the morphology of labeled cells within the ventricular zone can be seen as they migrate above the fimbria and the migrating cells extended from the VZ into the dentate gyrus. The pattern and extent of migrating cells agreed with the pattern seen in the nlslacZ experiments. (F-H) By P30, cells had migrated to their final position and developed morphologies characteristic of mature differentiated cells. In the dentate granule cell layer (G, boxed area in F), many granule cells were labeled (G), and their highly branched dendritic trees and axonal arbors extended into the molecular layer and hilus, respectively. Additionally, in the hilus (H, boxed area in F), glia were also seen, confirming that cells labeled at the ventricular zone that migrate to the dentate gyrus do not have a homogenous fate. Scale bars: in A, 200 µm for A-D; in E, 75 µm; in F, 50 µm for F; in H, 50 µm for G,H.