Rostrocaudal polarity of the tectum in birds: correlation of en gradient and topographic order in retinotectal projection.

Rostrocaudal polarity of the tectum is first detectable at embryonic day 2 (E2) in the chick by the rostrocaudal gradient of en expression. When ectopic tectum is produced by heterotopic transplantation at E2 in the diencephalon, the gradient of en expression is inverted from the host polarity by environmental influences. Here we report that the retinal fibers project to the ectopic tectum in a topographic order in accord with the inverted gradient of en expression. Moreover, if ectopic tectum was produced at E3, when the en pattern was preserved, the retinotectal projection pattern was also in accord with the en pattern, suggesting that rostrocaudal polarity of retinotectal order follows en expression patterns.