The partition of p2 to form the habenula and the thalamus is controlled by the interaction between Pax6 and Shh . (A–C) Immunofluorescence for GABA, GFP, and neurofilament (NF) on sagittal sections of E14.5 embryos of indicated genotypes. (A’-C’) the blue channel of images A-C to show for NF staining. The dashed line outlines the habenula (Hb); arrows indicate the FR tract; the asterisk shows defasciculating axonal fibers in place of the FR tract. Note that the FR tract becomes enlarged and supernumerary in ShhSG/SG embryos (see inset in C’). Scale bar in (A): 200 μm (A-C’). E, embryonic day; FR, fasciculus retroflexus; GABA, gamma-aminobutyric acid; GFP, green fluorescent protein.