DORSOVENTRAL PATTERNING OF THE AVIAN MESENCEPHALON METENCEPHALON - ROLE OF THE NOTOCHORD AND FLOOR PLATE IN SUPPRESSING ENGRAILED-2/

Transcription factors that are spatially and temporally restricted wit hin the embryo may be used for dorsoventral and rostrocaudal positiona l information during development. The Engrailed-2 (En-2) gene is expre ssed across the mesencephalon/metencephalon (mes/met) boundary in the cerebellar primordium with strong dorsolateral expression and limited expression in the floor plate. In a previous experiment we demonstrate d that, after removal of Hensen's node, embryos lacked a notochord in the head and the pattern of En-2 expression was normal rostrocaudally, but it was expanded into the ventral midline of the neural tube. This suggested that the notochord suppresses En-2 in the ventral neural tu be during normal development. To test further the ability of the notoc hord (and floor plate) to suppress En-2, we transplanted ventral midli ne tissues from HH 5-9 quail embryos beneath the rostral neural plate of HH 4-6 chick embryos. After 24 hours in culture, 90% of the embryos with quail notochord or floor plate near the mes/met of the host lack ed En-2 expression adjacent to the graft, and suppression was distance dependent. Enzymatically isolated notochords also suppressed En-2 (71 %), but the results from isolated floor plates were inconclusive. Othe r grafts served as controls and included tissues from the trunk ventra l midline, mes/met level dorsolateral neural plate, and trunk dorsolat eral neural plate/somite. Collectively, the results suggest that durin g normal development the notochord and possibly the floor plate are im portant regulators of normal En-2 expression. (C) 1995 John Wiley & So ns, Inc.