THE BICOID-RELATED HOMEOPROTEIN PTX1 DEFINES THE MOST ANTERIOR DOMAINOF THE EMBRYO AND DIFFERENTIATES POSTERIOR FROM ANTERIOR LATERAL MESODERM

Ptx1 is a member of the small bicoid family of homeobox-containing gen es; it was isolated as a tissue-restricted transcription factor of the pro-opiomelanocortin gene. Its expression during mouse and chick embr yogenesis was determined by in situ hybridization in order to delineat e its putative role in development, In the head, Ptx1 expression is fi rst detected in the ectoderm-derived stomodeal epithelium at E8.0, Ini tially, expression is only present in the stomodeum and in a few cells of the rostroventral foregut endoderm, A day later, Ptx1 mRNA is dete cted in the epithelium and in a streak of mesenchyme of the first bran chial arch, but not in other arches, Ptx1 expression is maintained in all derivatives of these structures, including the epithelia of the to ngue, palate, teeth and olfactory system, and in Rathke's pouch. Expre ssion of Ptx1 in craniofacial structures is strikingly complementary t o the pattern of goosecoid expression, In addition, Ptx1 is expressed early (E6.8) in posterior and extraembryonic mesoderm, and in structur es that derive from these. The restriction of expression to the poster ior lateral plate is later evidenced by exclusive labelling of the hin dlimb but not forelimb mesenchyme. In the anterior domain of expressio n, the stomodeum was shown by fate mapping to derive from the anterior neural ridge (ANR) which represents the most anterior domain of the e mbryo. The concordance between these fate maps and the stomodeal patte rn of Ptx1 expression supports the hypothesis that Ptx1 defines a stom odeal ectomere, which lies anteriorly to the neuromeres that have been suggested to constitute units of a segmented plan directing head form ation.