Patterning of the embryonic avian midbrain after experimental inversions: a polarizing activity from the isthmus.

Rostrocaudal inversions of the prospective midbrain (accompanied in some cases by other adjacent neuroepithelial zones) were performed in the neural tube of 2-day-old chick embryos in the form of autografts or quail/chick chimeras, involving both alar and basal plates. These experiments aimed to analyze causally the histogenetic rostrocaudal patterning of the midbrain, formed normally by diverse neuronal complexes. The experimental embryos were processed for study of the resulting cytoarchitecture or for study of the Engrailed-2 gene expression pattern. The exclusive inversion of the prospective midbrain produced a normal midbrain. Joint inversion of prospective midbrain plus the prospective isthmocerebellar tissue gave rise to an ectopic isthmocerebellar complex plus a symmetric double-caudal midbrain; additionally, the prospective caudal diencephalon was induced to form a polarized isthmomesencephalic phenotype in substitution of the pretectum. All these regulation processes affected both the alar and basal plates. The changes in cytoarchitectural fate were preceded by correlated changes in the gradiental expression of the gene Engrailed-2. These results suggest that the polarized structural pattern of the midbrain is regulated by graded positional information derived from the caudally adjacent isthmocerebellar domain, which is probably a source of morphogens.