Fig. 2. The majority of cells migrating tangentially into the cortex are generated in the MGE. (A,B) Comparison of the migration of GFP-positive cells from LGE-GFP (A) and MGE-GFP (B) explants into co-cultured P2 cortical slices at 3 days in vitro. Note that many more GFP-positive cells migrate into the cortex from the MGE (B) compared with the LGE (A). (C-H) E14 MGE explants (C-E) or E14 LGE explants (F-H) were exposed to BrdU in vitro for 8 hours before being co-cultured with P2 cortex for 3 days without BrdU. GFP-positive cells migrating from the MGE were five times more frequently double labeled for BrdU than were GFP-positive cells migrating from the LGE (red arrowheads in C-H; see text for quantification), indicating that the majority of GE-derived cells that migrate into the cortex originate in the MGE. (I) MGE-derived cells do not proliferate after migrating to the cortex. E14 GFP-expressing MGE explants were co-cultured with P2 cortex for 48 hours and pulsed labeled with BrdU for 4 hours just before fixation. No GFP-positive cells in the cortex were labeled with BrdU, indicating that MGE-derived cells migrate into the cortex after their final mitosis. Scale bars: 600 µm in A,B; 100 µm in C-H; 120 µm in I.