Fig. 3. Depletion of Xena/XVASP function interferes with the elongation but not with the guidance of retinal ganglion cell axons. Retinal ganglion cell (RGC) axon growth and guidance were analyzed in serial coronal cryostat sections at stage 39 (A-H) and stage 42 (J,K). RGCs expressed the control constructs GAP-GFP (A-D) or RFP (J), or the combined FP4-Mito-GFP+RFP constructs (E-H,K). (I) Drawing representing the stereotypical route followed by RGC axons during development and indicating the regions illustrated in A-H,J,K. (A-H) Arrowheads outline individual axons. (A,B) Axons of control RGCs exit the eye through the optic nerve head (onh; arrow in A). (B) Close-up of A. (C,D) The fibers navigate contralaterally across the optic chiasm, and cross the ventral (VOT), the medial (MOT) and the dorsal (DOT) optic tract to reach the tectum. (E-H) At stage 39, axons of RGCs expressing FP4-Mito initiate (arrowheads in E) and develop normally inside the optic nerve head (F and arrow in E), but only very few axons were detected at the level of the optic chiasm (G) and of the optic tract (H). (J,K) At stage 42, as in controls, the majority of RGC axon terminals of neurons expressing FP4-Mito had reached the tectum. on, optic nerve; onh, optic nerve head; RGC, retinal ganglion cell; DOT/MOT/VOT, dorsal/medial/ventral optic tract, respectively. Scale bar: 120 µm in A,D,E,J,K; 60 µm in B,C; 40 µm in F-H.