Expression of Slits in embryonic rat retina. Coronal sections of E13 (A, D,G), E15 (B, E,H), and E17 (C, F,I) rat retinas showing slit1(A–C), slit2(D–F), and slit3(G–I) expression detected with S³⁵-labeled riboprobes. Dorsal is up. The sections are counterstained with bisbenzimide. Each photo is a single exposure using both dark-field and UV fluorescence illumination. At E13 (A, D) and E15 (B,E), slit1 (A,B) and slit2 (D,E) expression is highest in the ganglion cell layer in central retina (arrows), but by E17, bothslit1 (C) and slit2(F) are expressed throughout the RGC layer (arrows). Note that slit1 expression is more restricted to the ganglion cell layer than slit2expression. Arrowheads in E andF indicate expression of slit2 at the optic disk and bounding the optic nerve (OpN).slit2 expression at E17 shows a high ventral to low dorsal graded pattern (C). slit3expression is not detected in retina at E13 (G), E15 (H), or E17 (I). Scale bars, 200 μm.