Optic nerve tansection in adult rats results in the death of -50% of the axotomized retinal on cells (RGCs) by 1 week and nearly 90% by 2 weeks after injury. RGC Lang. ROCs were retrogradely labeled with Fluorogold (Fluorochrome, Englewood, CO; 2% in 0.9% NaCl containing 10%o dimethyl sulfoxide) applied to the surface of both SC, as described for 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (diI) (11,12). For the experiments in which a correlation between RGC survival and axonal regrowth was investigated (as described below), horseradish peroxidase (HRP; Boehringer Mannheim) was applied to the distal tip of the PN graft, "1 cm from the eye, 6 weeks after the graft was attached to the ocular stump ofthe ON (13).ON Transection. One week after Fluorogold application, the left ON was transected 0.5 mm from the eye (1). In some animals, a segment of autologous sciatic nerve was attached to the ocular stump of the transected ON to test the capacity of the RGCs to regenerate and extend their axons (13).Injection Procedure. Anesthetized animals received single injections 3 or 6 days before or 0-10 days after ON transection. Multiple injections were given on postoperative days 0, 3, 7, and 10 for the animals without PN grafts and on days 0, 3, and 7 for the animals with PN grafts. Intraocular injections were made with a 10-A4 Hamilton syringe fitted with a 26-gauge needle whose tip was inserted into the vitreous space by an anterior or posterior approach. For the anterior approach, a drop of 2% lidocaine (Xylocaine) was applied to the conjunctiva, and the needle was inserted through the cornea-sclerajunction and advanced into the vitreous chamber, avoiding direct contact with the retina. By this approach, the needle usually pierced the margins of the iris and could damage the surface of the lens. After injection, Polysporin ointment was applied to the puncture site. For the posterior approach, the needle was inserted through the sclera and retina at the time of ON transection; this route avoided direct injury to the iris or lens. All experiments to determine the range of effective times for BDNF and control injections, as

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In order to visualize the organization and the morphology of developing retinal axons in the trout (Oncorhynchus mykiss), HRP or DiI crystals were applied on the central part of the retina at different stages ranging from 21 days postfertilization (stage 27) to one month post-hatching (stage 36). Retinal axons and arborizations were observed on tectal whole mounts with a light microscope. The earliest stage investigated (stage 27) showed two groups of axons entering the tectum by its ventro-rostral part and extending in a dorso-caudal direction. As the tectum grows, these two groups separate to outline the dorsal and the ventral borders of the tectum. At three weeks post-hatching (stage 35) we observed three distinct brachia: the dorsal and ventral fascicles, and a small group in the middle that we called the intermediate fascicle. At hatching (stage 30), retinal axons start to arborize in the centre of the tectum. During the first month post-hatching, these axons migrate dorso-caudally and exhibit various morphologies. Until two weeks post-hatching (stage 34), they sprout a few long side branches, bearing numerous filopodial growth cones, in a phase of exploratory growth towards their target site. At stage 36, four types of terminal arborizations can be identified on the basis of their tangential and radial location in the tectum, and on their gross morphology. Three of these arbor types are already present at earlier stages and undergo refinements in their shape--reduction in their branching axes, loss of branches that are behind the terminal arborization, and the sprouting of more numerous branches at their extremities. These findings confirm that the widely branched arborizations are transient during development.