Figure 1.

In utero fate mapping labels two lineages of cogenerated neocortical neurons. A, The 2.5 kb of 5′ genomic sequence upstream of the Tbr2 translational start site (red box) was subcloned to generate the pTbr2-Cre plasmid, which fate maps Tbr2 lineage cells with mCherry and non-Tbr2 lineage cells with ZsGreen when cotransfected with the CAG-stoplight reporter plasmid. B, The four major classes of neocortical precursors include the stem cell RGCs and the three major IPC classes: aIPCs (SNPs), bIPCs, and bRGs. C, IUE at E14.5 marks Tbr2 lineage precursors and their offspring with mCherry and the non-Tbr2 lineage with ZsGreen. The majority of mCherry-expressing precursors are immunopositive for Tbr2 protein and are found concentrated away from the ventricular surface (bright red bar with plus symbol, D; bright red line, E). The mCherry⁺/Tbr2-immunonegative precursors (dark red bar with minus symbol, D; dark red line, E) are primarily found near the ventricle and likely represent cells that have begun Tbr2 gene expression but have not yet made measurable protein. The ZsGreen⁺/Tbr2-immunonegative precursors (bright green bar with minus symbol, D; bright green line, E) are found primarily within the 60 μm closest to the ventricle. F, Confocal image from E15.5 pial surface, 24 h after IUE. Nearly all of the pial-contacting basal fibers emanate from the non-Tbr2 apical precursor lineage, whereas virtually no basal fibers were derived from the Tbr2 lineage mCherry⁺ precursor population. Asterisks indicate autofluorescent red blood cells. G, H, Confocal image in P21 neocortex, following IUE at E14.5, demonstrates that all mCherry(+) and ZsGreen(+) neurons reside within layers 2 and 3. I, J, Electroporated neurons express the layer 2/3 markers Brn1 and Satb2. *p < 0.0005. **p < 0.0001.