SIGNALS FROM THE NOTOCHORD AND FLOOR PLATE REGULATE THE REGION-SPECIFIC EXPRESSION OF 2 PAX GENES IN THE DEVELOPING SPINAL-CORD

Members of the paired box (Pax) gene family are expressed in discrete regions of the developing central nervous system, suggesting a role in neural patterning. In this study, we describe the isolation of the ch icken homologues of Pax-3 and Pax-6. Both genes are very highly conser ved and share extensive homology with the mouse Pax-3 and Pax-6 genes. Pax-3 is expressed in the primitive streak and in two bands of cells at the lateral extremity of the neural plate. ID the spinal cord, Pax- 6 is expressed later than Pax-3 with the first detectable expression p receding closure of the neural tube. When the neural tube closes, tran scripts of both genes become dorsoventrally restricted in the undiffer entiated mitotic neuroepithelium. We show that the removal of the noto chord, or implantation of an additional notochord, dramatically alter the dorsoventral (DV) expression patterns of Par-3 and Pax-6. These ma nipulations suggest that signals from the notochord and floor plate re gulate the establishment of the dorsoventrally restricted expression d omains of Pax-3 and Pax-6 in the spinal cord. The rapid changes to Pax gene expression that occur in neural progenitor cells following the g rafting of an ectopic notochord suggest that changes to Pax gene expre ssion are an early effect of the notochord on spinal cord patterning.