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Fig. 1. Transcriptional activation of the Foxg1 gene in tissues involved in RGC axon guidance in E14.5 embryos with and without Foxg1. Foxg1^lacZ/+ embryos have one functional copy of Foxg1 and are phenotypically normal. Foxg1^lacZ/Cre embryos are unable to produce Foxg1 protein. The Foxg1^lacZ allele reports transcriptional activation of the Foxg1 gene by expression of lacZ protein. (A,F,I,N) X-gal stained brains. (A,F) Foxg1^lacZ/+ brain viewed (A) dorsolaterally to show staining of the superior colliculus (sc), dorsal midline and telencephalon (tel). (F) Ventral view shows staining in the anterior part of the ventral hypothalamus (vh; circled in red). (I,N) Foxg1^lacZ/Cre brain showing (I) staining in the superior colliculus, along the dorsal midline and in the hypoplastic telencephalon (tel^*). (N) A ventral view shows that, as in Foxg1^lacZ/+ embryos, staining is restricted to anterior ventral hypothalamus. The yellow oval encloses the mutant elongated retina (er), which has remained attached to the brain. The retinal pigment epithelium (rpe) has been removed to show that lacZ staining is far stronger in the anterior (nasal) retina. (B-E,G,H,J-M,O,P) X-gal stained cryostat sections with nuclei stained red. (B-E,J-M) Sections through the eye. Both (B-E) Foxg1^lacZ/+ and (J-M) Foxg1^lacZ/Cre retinas exhibit predominantly nasal (n) lacZ staining, although in both cases a few stained cells are also detected in temporal (t) retina. J is dorsal to K. (K) Nasal staining in the medial elongation of the mutant retina. B and J are dorsal to D and K, respectively. (D,E,L,M) Higher magnifications of Foxg1^lacZ/+ and Foxg1^lacZ/Cre (D,L) temporal and (E,M) nasal retina, showing pronounced staining of the nasal RGC layer. (G,H,O,P) Sections through the ventral hypothalamus with paired arrows demarcating its midline. A population of midline cells overlying bundled axons at the optic chiasm (oc) stains strongly in both (G) Foxg1^lacZ/+ and (O) Foxg1^lacZ/Cre embryos. This dense midline staining is absent more posteriorly in (H) Foxg1^lacZ/+ and (P) Foxg1^lacZ/Cre embryos. (Q) The relationship between Foxg1 expression and the retinofugal tract. Regions expressing Foxg1 are shaded blue. Ovals represent cell bodies. The retinofugal tract comprises RGCs (red) projecting axons (black lines) towards the optic chasm and into the brain. l, lens; ot, optic tract; r, retina; avh, anterior ventral hypothalamus; on, optic nerve. Wholemounts (A,F,I,N) are shown with anterior facing downwards. Cryostat sections (B-D,J-L) are horizontal with anterior (nasal) at the bottom; (E,G,H,M,O,P) coronal sections. Scale bars: 200 µm in B,C,J,K; 50 µm in D,EG,H,L,M,O,P.

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