Fig. 4. Dll1 mutants show increased early neurogenesis and reduced Notch signaling. Dark-field and fluorescence photomicroscopic images of coronal hemi-sections of control (left side) and Dll1 mutant (right side) telencephalons at E10.5 (A-E') and E11.5 (F,F') studied using ³⁵S-labeled radioactive in situ hybridization (A,A') and immunofluorescence (B-F'). Dll1 mutants show reduced Hes5 expression (A,A'), which is indicative of decreased Notch signaling, thinning of the VZ [with reduced RC2 expression (F,F')], and an expansion of the mantle zone that expresses DLX2 (B,B'), ß-III-tubulin (C,C') and GABA (D,D'). These neurons may not have differentiated completely based on the low level of MAP2 expression particularly in the presumptive MGE (arrowhead in E,E'). Similar to the Mash1 mutants, GABA expression appears in maturing neurons before MAP2 expression (compare D,D',E,E'). ctx, cortex; LGE, lateral ganglionic eminence; LV, lateral ventricle; MGE, medial ganglionic eminence; MZ, mantle zone; OE, olfactory epithelium; VZ, ventricular zone. Scale bars: 100 µm.