[Molecular mechanism of the formation and rostrocaudal polarity of the optic tectum]

Abstract

The optic tectum receives axons from the retinal ganglion cells in a topographical ordered manner. For the precise retinotectal projection, retinal and tectal compartments must be defined first, and then retinal and tectal cells are positionally specified. In chick tectal primordia, a homeobox-containing gene En, a homologue of a Drosophila segment polarity gene engrailed, is expressed in a caudal-to-rostral gradient, and transplantation and misexpression experiments have demonstrated that En confers posterior characteristics by controlling Ephrin A2 and Ephrin A5 expression. On the other hand, it has been shown that the mesmetencephalic boundary (isthmus) acts as an organizer for the tectum. We focused on paired box-containing genes Pax-2 and Pax-5, which are expressed in the isthmus. Comparison of the expression of Pax-2 and Pax-5 in the isthmic region and the results of overexpression of Pax-2 and Pax-5 in the mesencephalon and in the diencephalon suggested that Pax-2 plays a crucial role in establishment of the tectal region and that Pax-5 functions to maintain the state of tectal differentiation and rostrocaudal polarity of the tectum.