Plasticity in mesencephalic and retinal polarity formation in avian embryos

Polarity and its plasticity in the avian mesencephalon and the retina are reviewed. Rostrocaudal polarity of the tectum (dorsal mesencephalon) is revealed by the gradient of engrailed gene expression, by its cytoarchitecture, and by positional specification of tectal cells which retinal fibers read to find their targets. In the ventral mesencephalon, the polarity is revealed by directional growth of tectofugal axons. To examine the plasticity in rostrocaudal polarity, rostrocaudal orientation of the mesencephalon was surgically reversed on E2. Rostrocaudal polarity of the reversed mesencephalon was all adjusted to the host pattern. This suggests that the mesencephalic polarity is not fixed on E2 and is established under environmental influences. Retinal polarity is revealed by the distribution of certain molecules, by retinal structures, and by the retinotectal projection pattern. Retinal structures and retinotectal projection pattern were kept as in the original if retinal primordium was rotated or partially excised after stages 12-14 of Hamburger and Hamilton. The location of an optic fissure and the retinal fiber pattern were adjusted to those of normal eyes when the rotation was performed before stage 12, therefore the retinal polarity was supposed to be plastic before stage 12 and to be fixed around the stages 12-14 under environmental influences.