| |
Fig. 4. Endogenous Xnrs are expressed at the right time and place to function as mesoderm inducers. (A) Time course of gene expression analyzed by RT-PCR at various developmental stages (Nieuwkoop and Faber, 1994). The mesoderm inducers Xnr1, Xnr2 and Xnr4 start zygotic expression at the same time as Siamois and Xnr3 (which are expressed immediately after midblastula and are direct targets of β-catenin regulation). ODC is used as a loading control. (B) Dissections of embryos at stage 9 showing that Xnr1, Xnr2 and Xnr4 are expressed in the endoderm and at higher levels dorsally than ventrally. Vg1 is expressed uniformly in the vegetal pole. (C-F) Xnr1 in situ hybridizations of blastula stage embryos showing a gradient of expression in endoderm. (C) Stage 8 embryo showing a few nuclei stained in the dorsal vegetal mass (arrowhead). (D) Stage 8.5 blastula embryo in which Xnr1 expression has expanded into neighboring vegetal cells. (E) Stage 9 blastula embryo displaying graded Xnr1 expression throughout the embryonic endoderm. (F) External view of a stage 9 embryo cleared in Murray solution in order to visualize Xnr1 staining in the vegetal hemisphere. In this embryo, the ventral side, with its more pigmented animal cap, can be clearly distinguished from the less pigmented dorsal side. Note that Xnr1 expression on the dorsal side is of longer duration, in addition to reaching higher levels than in ventral endoderm. |
