Plasticity of avian mesencephalic polarity revealed by trajectories of tectofugal axons.

In normal E6 (sixth day of incubation) quail mesencephalons, circumferential axons originate in the tectum (the dorsal part of a mesencephalon), course in a ventral direction and split into three trajectories in the ventral region of the mesencephalon; two of them turned on the ipsilateral side of the mesencephalon, one rostrally and the other caudally, while the third crossed the ventral midline and turned caudally on the contralateral side. In this study, we examined whether the ventral part of the mesencephalon has plasticity in its rostrocaudal polarity formation. We transplanted quail mesencephalons at the 9-10 somite stage (E2) with reversed rostrocaudal orientation prior to axon outgrowth. In reversely transplanted mesencephalons, circumferential axons at E6 took the same trajectory pattern as that of normal embryos; one turned rostrally and the second caudally on the ipsilateral side, and the third one turned caudally on the contralateral side. These results indicate that the rostrocaudal polarity of the mesencephalon is not fixed at E2, and that it may develop under the influence of tissues surrounding the mesencephalon.