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Fig. 2 (A) pitx2c is expressed in the early mesendoderm and prechordal plate, and pitx2a is expressed late in the anterior prechordal plate (polster). Expression was detected by in situ hybridization. (a,b,e,f,i,j,m,n) A full-length pitx2 probe was used to detect all pitx2 transcripts. (c,g,k,o) An isoform specific 5′-UTR probe from pitx2a. (d,h,l,p) An isoform specific 5′-UTR probe from pitx2c. (a-d) 40-50% epiboly: (a,b) pitx2 expression was observed in marginal blastomeres (arrowheads) with increased levels on the dorsal side. (c) No expression of pitx2a was detected. (d) Expression of pitx2c in marginal blastomeres (arrowhead). (e-h) 90% epiboly: (e,f) pitx2 expression in the prechordal plate. (g) No expression of pitx2a was detected. (h) pitx2c expression in the prechordal plate. (i-l) bud stage: (i,j) pitx2 expression in the polster and posterior prechordal plate. (k) pitx2a expression in the polster. (l) pitx2c expression in the polster and posterior prechordal plate. (m-p) 4-6 somites: (m,n) pitx2 expression was observed in both the polster and posterior prechordal plate. (o) pitx2a expression in the polster. (p) pitx2c expression in the polster and the posterior prechordal plate. In a,c,d,e,i,m, embryo views are lateral with dorsal to the right. In b, animal pole view is shown with dorsal to the right. In f-h, j-l and n-p, dorsoanterior views are shown with anterior at the top. Bars indicate the extent of expression in the prechordal plate along the anteroposterior axis. (B) Northern blot hybridizations using isoform-specific 5′-UTR probes on RNA isolated from staged embryos. The same blot was used in all panels. Ethidium bromide staining of 18S ribosomal RNA is shown at the bottom as a loading control.