Axonal guidance of developing optic nerves in the frog. I. Anatomy of the projection from transplanted eye primordia.

Right or left eye primordia of Shumway stage 16-18 Rana pipiens embryos were transplanted to the position normally occupied by the ear, where they developed into externally normal eyes. Ganglion cell density in the retina of the transplanted eye was significantly lower than the ganglion cell density in the animal's normal eyes. No significant difference was observed between normal and transplanted inner and outer nuclear layers. In several preparations a large piece of embryonic forebrain was included in the initial eye graft. Optic nerves from the transplanted eyes penetrated this transplanted forebrain and established a dorsocaudal growth trajectory relative to the ectopic tissue's original anterior-posterior and dorsal-ventral axes. After traversing the forebrain, the transplanted optic nerve fibers penetrated the medulla where they invaraibly grew dorsocaudally to take up and maintain a dorsolateral position in the white matter of the spinal cord. The consistent dorsocaudal trajectory established by the translocated optic tract in the ectopic forebrain and in the hindbrain and spinal cord is very similar to the growth trajectory in the normal diencephalon, suggesting that dorsocaudal growth relative to the three major axes of the neural tube is an inherent property of optic nerve fibers. These results also suggest that the polarity cues which guide normal optic tract growth are not unique to the diencephalic regions of the neural tube.