THE SPATIAL AND TEMPORAL DYNAMICS OF SAX1 (CHOX3) HOMEOBOX GENE-EXPRESSION IN THE CHICKS SPINAL-CORD

Sax1 (previously CHox3) is a chicken homeobox gene belonging to the sa me homeobox gene family as the Drosophila NK1 and the honeybee HHO gen es. Sax1 transcripts are present from stage 2 H&H until at least 5 day s of embryonic development. However, specific localization of Sax1 tra nscripts could not be detected by in situ hybridization prior to stage 8-, when Sax1 transcripts are specifically localized in the neural pl ate, posterior to the hindbrain. From stages 8- to 15 H&H, Sax1 contin ues to be expressed only in the spinal part of the neural plate. The a nterior border of Sax1 expression was found to be always in the transv erse plane separating the youngest somite from the yet unsegmented mes odermal plate and to regress with similar dynamics to that of the segr egation of the somites from the mesodermal plate. The posterior border of Sax1 expression coincides with the posterior end of the neural pla te. In order:to study a possible regulation of Sax1 expression by ifs neighboring tissues, several embryonic manipulation experiments were p erformed. These manipulations included: removal of somites, mesodermal plate or notochord and transplantation of a young ectopic notochord i n the vicinity of the neural plate or transplantation of neural plate sections into the extraembryonic area. The results of these experiment s revealed that the induction of the neural plate by the mesoderm has already occurred in full primitive streak embryos, after which Sax1 is autonomously regulated within the spinal part of the neural plate.