EXPRESSION OF AVIAN PAX1 AND PAX9 IS INTRINSICALLY REGULATED IN THE PHARYNGEAL ENDODERM, BUT DEPENDS ON ENVIRONMENTAL-INFLUENCES IN THE PARAXIAL MESODERM

Pax1 and Pax9 represent a subfamily of paired-box-containing genes. In vertebrates, Pax1 and Pax9 transcripts have been found specifically i n mesodermal tissues and the pharyngeal endoderm. Pax1 expression in t he sclerotomes has been shown to be indispensable for proper formation of the axial skeleton, but expression of Pax1 in the endoderm has not been studied in detail. We have cloned the chick homologue of the mur ine Pax9 gene. Our results show that transcripts of Pax and Pax9 are f irst detectable in the prospective foregut endoderm of headfold-stage avian embryos. Endodermal expression correlates with the highly prolif erative zones of the folding foregut and evaginating pharyngeal pouche s. In later stages, Pax1 and Pax9 are expressed in overlapping but dis tinct patterns within the developing sclerotomes and limb buds. From g rafting experiments we conclude that activation of pharyngeal Pax1 and Pax9 expression is an intrinsic property of the endoderm, not requiri ng midline structures or head mesoderm. In contrast, notochord is requ ired to induce Pax1 in competent sclerotomes. Here we show that in vit ro there is a cranio-caudal gradient of inductive capacity in the noto chord. This coincides with the graded expression of Pax1 and Pax9 alon g the cranio-caudal axis in 2- to 3-day-old embryos. Furthermore, para xial head mesoderm shows no competence to express Pax1. Finally, in vi tro we find counteracting influences on notochord signaling by lateral tissues (lateral plate, intermediate mesoderm), leading to an inhibit ion of Sonic hedgehog (Shh) expression in notochord and floor plate, a s well as Pax1 and Pax9 expression in sclerotomes. Taken together, our results demonstrate that different mechanisms regulate expression of Pax1 and Pax9 in foregut and sclerotome, but suggest a common function for both genes in the two tissues that is promoting proliferation and preventing fusion of neighboring blastemas. (C) 1996 Academic Press, Inc.