Figure 5.

(A) Camera lucida plotting of ES cell descendants from three sections of chimera 5 indicates a continuous laminar allocation restricted to the deep layers (V and VI) of the entire cerebral cortex. Labeled cells in the upper cortical plate were much scarcer and more sporadically distributed. (B) Double labeling with antibody against glial fibrillary acidic protein indicated that astrocytes (arrowhead) were only a small percentage of this group of lacZ+ cells located above the white matter (WM). (C) Tissue blocks of the deep and the upper cerebral cortices were picked individually from histological specimens and processed for PCR detection of the lacZ gene (200-bp signal). The endogenous γ-enteric actin gene (SMGA, 100-bp signal) also was amplified to ascertain the successful harvest of DNA from histologic specimens. Results of one representative set of samples are shown here. In a larger series of samples (n = 43), 72% of the deep cortical samples and 11.6% of the upper cortical samples were lacZ-positive. The correlation between X-Gal histochemical expression and the presence of the lacZ gene was statistically significant (P < 0.005 in χ² analysis; df = 1). (D) Comparative studies indicated that the three-layered general cortex expanded and propelled the archicortex (A: hippocampus, H) and the paleocortex (P: piriform cortex, P) toward the medial wall of telencephalon during evolution. It also has been postulated that neuroblasts of the dorsal ventricular ridge (DVR) located in the dorsal striatum (S) migrated and incorporated into the general cortex in ancestral mammals to result in the modern six-layered isocortex (4). One possible explanation of the coexistence of radial and laminar clones in the mouse cerebral cortex is that these two phylogenetically distinct populations of cells remain ontogenetically segregated and are allocated differently. (D is modeled after ref. 4). (Bar = 100 μm.)