Fig. 1. Transcriptional activation of Foxg1 in the retina at E14.5. X-gal staining (blue) of Foxg1^LacZ/+ embryos is used to show where Foxg1 is active. (A) Schematic of a mouse embryo head showing the horizontal plane of section; broken lines indicate the limits between which sections were taken. (B) Schematic of retina showing the locations of sections in E-K; blue circles indicate Foxg1-expressing RGCs. (C) Diagram of a flat-mounted retina, with blue shading indicating the area of transcriptional activation of Foxg1; boxed areas indicate regions from which DN (Foxg1-positive) and VT (Foxg1-negative) retinal explants were prepared for the co-culture experiments. (D) A Foxg1^LacZ/+ embryo showing X-gal staining mainly in nasal retinae. (E-K) Dorsal to ventral series of sections (locations marked in B) through a retina shown in D counterstained with Nuclear Fast Red (pink). (E) At the dorsal pole, X-gal staining is found throughout all layers of nasal and temporal retina. (F-H) Moving from dorsal to central sections, X-gal staining is present throughout nasal retina and occupies progressively less of temporal retina. (I-K) Moving through ventral sections, the X-gal-stained part of nasal retina becomes smaller and increasingly restricted to the anterior-most region of nasal retina until it disappears in the most ventral section. Scale bars: 500 µm in D; 200 µm in E-K. Abbreviations: D, dorsal; N, nasal; T, temporal; V, ventral; l, lens; rpe, retinal pigment epithelium. Broken lines in E-K indicate nasal-temporal boundary.