Fig. 1. Tangentially migrating cells from the ganglionic eminence invade the neocortex and CA fields of the hippocampus, but not the dentate gyrus. (A) A coronal section through the developing telencephalon showing the locations of the medial (M) and lateral (L) ganglionic eminences, cortex and hippocampus (Hipp.) during embryonic development. (B) Low magnification photomicrograph of the relative positions of the GE from GFP-expressing mice (GE-GFP), cortex and hippocampus in a co-culture assay. The white label in the cortex indicates the location of GE-GFP cells that have migrated from the GE to the cortex. Note the extensive invasion of the cortex and hippocampus by the tangentially migrating cells. (C,D) Analysis of the spatial distribution of E15 GE-GFP cells that have migrated into the cortex after 20 hours in vitro indicates that GFP cells are found mainly in the intermediate zone (IZ) and the marginal zone (MZ) of co-cultured cortical slices. In these zones, cells are mainly oriented tangentially. Note that some GFP cells have already invaded the cortical plate (VI, CP). Cells that enter the CP from the IZ typically have leading processes directed towards the pial surface (arrowhead, C), while those entering the CP from the MZ have leading processes directed away from the pial surface (arrowhead, D), indicating that cell bodies follow the leading process as the cells migrate into the CP. The dorsal (D) and lateral (L) aspects of the cortical slice are indicated in C. In (C,D), the GE explants are located to the right of the cortical explant shown. (E) By 36 hours in vitro, large numbers of E16 GE-GFP cells have migrated into the cortex, most of which travel through the IZ. Progressively more cells are found in the CP but they no longer show a clear radial orientation in the CP, suggesting that cells alter their trajectories of migration after entering the CP. The broken line in E indicates the edge of the slice on the ventricular side. In C-E, the red channel correspond to MAP2 immunofluorescence. (F,G) Examples of the morphology of tangentially migrating cells. The cells typically have a leading process that is 10-15 times the cell body diameter. The leading process is always tipped by a prominent growth cone (arrowheads) and often contains multiple filopodia (arrow in F). Many of the migrating cells also have branched leading processes (G), which may participate in the mechanics of altering trajectories of migration (see also Fig. 5). (H) In GE-cortex co-cultures, E16 GE-GFP cells migrate tangentially up to the most medial aspect of the telencephalon by 3 days in vitro where they accumulate in the developing hippocampus but avoid the developing dentate gyrus (DG). Cells in the marginal zone (star) of the putative CA regions (pCA) stop migrating sharply at the interface between the pCA and DG (arrow) identified as being TuJ1 negative (red channel). (I-K) The region of the developing hippocampus avoided by E16 GE-GFP cells (green channel, I) is a highly proliferative zone (delineated by the broken line in K; red channel, BrdU), which is typical of the DG anlage. (L-N) The avoidance of the developing DG by GE-GFP cells can be observed even when an explant of E16 MGE-GFP (to the right in L) is placed directly adjacent to the DG for 3 days in vitro. Higher magnification images show a marked difference in GFP cell density in DG (M) and pCA (N) regions located 250 µm away from the interface with the MGE explant. (O) GE-GFP cells do not migrate into wild-type dorsal thalamus (WT-DT) slices in a co-culture assay, indicating that GE cells are selective about their target zones of migration. All panels in this and other figures are from isochronic co-cultures, unless otherwise indicated. CP: cortical plate; SP: subplate; IZ: intermediate zone; VZ: ventricular zone; DG: developing dentate gyrus; pCA: putative CA regions; VI: cortical layer VI. Scale bars: 150 µm in C-E; 30 µm in F,G; 250 µm in H; 350 µm in I-K; 300 µm in L; 75 µm in M,N; 200 µm in O.