Fig. 6. Slit and Robo mRNAs are expressed in the developing mouse eye. Horizontal sections through the head of mouse embryos taken at the indicated stages were hybridized with DIG-labeled riboprobes for m-slit1, m-slit2, m-slit3, m-robo1, and m-robo2. Staining for slit1 and slit2 mRNA is detected in the E13.5 and E16 ganglion cell layer of the retina. Slit2 mRNA is also seen in the lens and in the optic nerve at E13.5 and E16. Slit3 mRNA is strongly expressed in the lens at E13.5 and is still seen in the outer cell layer of the E15 lens. Robo1 is expressed in a subset of cells in the inner retinal layer at E13.5 that becomes more intense at E15. Robo2 is expressed in the inner retinal layer at E13.5 and E15, and its expression pattern is broader and more diffuse compared with robo1. Scale bar: a, c, e,g, i, 100 μm; b,d, f, h, j, 200 μm.