Fig. 6. In vivo assay to test the function of Slit1 in retina. (A-C) Design of the in vivo assay. (A) A full-length Myc-tagged Slit1 expression construct was transfected into retinal cells by in ovo electroporation. The transfected cells were identified using anti-Myc antibody (green), whereas the axons were stained with anti-neurofilament antibody (red). Because some transfected cells will be localized in the cellular GCL, immediately beneath OFL where the retinal axons travel, we will be able to determine the effect of Slit1 on axons by comparing the relative positions of the Slit1-transfected cells and the axons. (B) If Slit1 is attractive to axons, axons will preferentially overlie the Slit1-transfected cells, appearing superimposed on the cells. (C) Alternatively, if Slit1 is repulsive to axons, the axons will avoid the transfected cells, thus not becoming superimposed on the cells. The actual data of Slit1 transfection are shown in D and F. Most of the Slit1-transfected cells align with the axon bundles (blue circles). (E,G) The control GFP-transfected cells, however, appear random with regard to the position of the axons. The blue circles indicate the cells that align accurately with the axons while the white circles indicate the cells that do not align with the axons. The broken white arrows in D-G indicate the direction of axon projection towards the optic disc. Scale bar: 150 µm.