c-otx2 is expressed in two different phases of gastrulation and is sensitive to retinoic acid treatment in chick embryo.

We cloned the chick homologue of the mouse Otx2 gene, c-otx2, and analyzed its expression pattern during gastrulation. During mouse embryogenesis, Otx2 expression is first detected in the entire epiblast and after the formation of the primitive streak becomes confined to the most anterior region of the embryo corresponding to presumptive fore- and mid-brain. Similarly, two distinct phases of c-otx2 expression were observed in the chick. c-otx2 transcripts were first detected in the unincubated egg and up to stage XIII, in all epiblast, and forming hypoblast and mesoblast cells. During primitive streak progression, c-otx2 expression becomes progressively restricted to anterior regions and is mainly associated with Hensen's node. When the extension of the streak is maximal, transcripts are only found in Hensen's node. A second phase of c-otx2 expression starts during streak regression. c-otx2 transcripts are lost from the node and present in higher abundance in anterior neuroectoderm and mesendoderm, with the exception of forming notochord and floor plate. The first phase of expression bears strong similarity with that of c-gsc, a gene shown to be a marker for cells that have organizer activity in the chick. Therefore, we compared the expression of the two genes by double staining on the same embryo. This analysis demonstrated that c-otx2 is transcribed first and its expression in the hypoblast precedes that of c-gsc. On the other hand, c-gsc is an earlier marker of primitive streak cells. The expression domains of the two genes transiently overlap in Hensen's node and anterior mesendoderm, whereas only c-otx2 is expressed in neuroectodermal areas. The second phase of c-otx2 expression is sensitive to an early treatment with retinoic acid. This treatment abolishes c-otx2 expression in mesendoderm and restricts it to most anterior regions in the forming neural plate. In conclusion, our results suggest that c-otx2 expression is first associated with cells with an anterior mesendoderm fate and subsequently extends to anterior neuroectoderm.