Figure 4. Development of the transhilar radial glial scaffold is perturbed after ablation of Tbr2.

(A-A1) In E16.5 control mice, Blbp+ radial glia exit the dentate ventricular zone and migrate to the hippocampal fissure (HF) to take up residence in the dentate marginal zone. They can be seen exiting the fimbriodentate junction (FDJ) region and migrating across the hilus (A1, cell bodies are marked by arrows and associated processes by arrowheads). (B-B1) Migration of Blbp+ cells is delayed in Nestin-Cre;Tbr2^flox/flox mice and accumulation of Blbp+ cells is apparent in the fimbria and FDJ. Regions delineated by dashed white boxes are shown in higher magnification in their respective adjacent panels (A1, B1). (C-C1) By P0, Blbp+ cell migration is essentially complete in control mice, and enrichment of Blbp+ processes is apparent in the HF. (D-D1, arrows) In Nestin-Cre;Tbr2^flox/flox mice, Blbp+ cells remain concentrated in the fimbria and FDJ and only rarely can be seen migrating to the HF (arrowheads). (E–H) Gfap+ processes making up the transhilar glial scaffold are aberrant in Nestin-Cre;Tbr2^flox/flox mice. In control mice (E, G) Gfap+ processes exhibit clear radial orientation across the hilus, whereas Gfap+ processes are oriented either randomly or towards the reduced HF where Reelin+ Cajal-Retzius cells are located in Nestin-Cre;Tbr2^flox/flox mice (F, H, arrows). These processes do not exhibit the clear radial orientation typically seen in control mice (G, H, arrows). (I–L) Tbr2 ablation on E13.5-E14.5 rescues DG morphogenetic defects. Inducible conditional deletion of Tbr2 (Nestin-CreER^T2;Tbr2^flox/flox) was achieved by treating mice with tamoxifen on E13.5 and E14.5. Control mice were Nestin-CreER^T2;Tbr2^flox/+. (I–J) Consistent with the known role of Tbr2 in regulating granule cell neurogenesis, NeuroD1+ neuroblasts are reduced in the developing DG of Nestin-CreER^T2;Tbr2^flox/flox mice, resulting in a decrease in the overall size of the DG (J) at E19.5. However, the HF is approximately the same size in controls (K) and Nestin-CreER^T2;Tbr2^flox/flox mice (L). Reelin+ Cajal-Retzius cells are abundant in the dentate marginal zone adjacent to the HF in both controls (K) and Nestin-CreER^T2;Tbr2^flox/flox mice(L), and Gfap+ processes are radially oriented across the hilus in both groups (K–L, arrows) consistent with normal development of the transhilar radial scaffold. Scale bars: A = 50 μm, A1 = 30 μm, C = 75 μm, E = 100 μm, G = 30 μm, I = 50 μm.