Axonal pathfinding in the absence of normal pathways and impulse activity.

Retinal axons were challenged to grow to their targets both along abnormal pathways and in the absence of impulse activity. Eye primordia were first transplanted from normal to ectopic sites in axolotl embryos. Most of the hosts were genetically eyeless, others were enucleated normal embryos. These axolotl embryos were then parabiotically joined to California newt embryos. Both operations were completed by stage 28, which is before axons have left the eye. The result of the parabiosis was a paralysis of the "eyeless" axolotl twin due to the newt's tetrodotoxin (TTX), while the newt twin remained normally active. When the axolotl twin reached early larval stage, about 1 week later, the projection from the silent transplanted retina was assessed using horseradish peroxidase (HRP) injections into the retina, after which the animals were killed and prepared histologically to reveal the presence of HRP in neuronal processes. The results from 17 such cases show normal topographic retinotectal projections: the dorsal retina projecting to the ventrolateral tectum, and the ventral retina projecting to the dorsomedial tectum. Unusual pathways were often taken to achieve these destinations. Control animals, both normal axolotl larvae developing alone and normal axolotl larvae parabiosed to newts, also showed the normal retinotectal projection patterns. These results indicate that the retinal projections in the experimental group were basically normal. Thus, fibers need neither impulse activity nor a particular pathway to navigate to their correct targets during development. Both factors can be eliminated simultaneously, yet retinal axons still find their way to the tectum and make an ordered map. This indicates that other factors, such as the chemoaffinity mechanisms proposed by Sperry (Sperry, R.W. (1963) Proc. Natl. Acad. Sci. U.S.A. 50: 703-710), may play a more major role in axonal pathfinding in this system.