Fig. 8. Slit1 defines the trajectory of the early retinal axons in OFL. In situ hybridization was carried out on E7 retinas with the Slit1 probe, followed by immunofluorescent staining of axons. The cells positive for the Slit1 probe appear purple in the bright-field image (B) but appear as dark spots in the fluorescent images (A,C,D,E). Note the growth cones of the elongating axons appear to project straight towards the Slit1-expressing cells (arrowheads in C,D), and the early retinal axon trajectories superimposed on the Slit1-positive cells (arrowheads in A,B). In addition, Slit1-expressing cells did not appear to have axons (marked with an asterisk in C,D), and axons are likely to be extended from the cells that are negative for Slit1 expression (marked with `?' in D). (E) Close to the optic disc, more mature axons do not align accurately with the Slit1-expressing cells. (F) Working model of the role of Slit1 and Irx4 in intra-retinal axon targeting.