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Abstract Eomesodermin (Eomes) is a novel Xenopus T-domain gene. In normal development, it is expressed in mesodermal cells in a ventral-to-dorsal gradient of increasing concentration. It reaches its peak expression 1-2 hr before any other known panmesodermal gene. It is strongly inducible by normal vegetal cells and by mesoderm-inducing factors. Ectopic expression of Eomes in animal caps induces the transcription of nearly all mesodermal genes in a concentration-dependent way. Overexpression of Eomes dorsalizes ventralmesoderm, inducing gsc and changing cell fate to muscle and notochord. Blocking the function of Eomes causes gastrulation arrest and defective mesoderm-dependent gene activation. We propose that Eomes fulfills an essential function in initiating mesoderm differentiation and in determining mesodermal cell fate.
Figure 3. Eomesodermin Is Expressed in the Mesoderm of Gastrula Stage Embryos(A)C), in situ hybridization to sections cut vertically through the dorso-ventral axis of stage 10 embryos; (D), wholemount in situ (stage 10.5). Sections were probed with Eomes (A), Xbra (B), or gsc (C). Dorsal (D) and ventral (V) sides are marked. Hybridization to Xbra is relatively weak owing to the low Xbra message abundance at stage 10. The large closed arrow indicates Eomes-positive cells on the outside of the embryo. (D), wholemount hybridization with an Eomes probe, shows the gradient distribution of Eomes transcripts. Open arrows point to Eomes-positive cells at a more vegetal level than Xbra-positive cells. Small black arrows indicate the location of the dorsal lip in (A) and (B).
Figure 6. Eomes Can Induce Muscle and Notochord in Animal Caps, Activate gsc in the Ventral Equatorial Region, and Change Cell Fate(A and B) Sections through animal caps previously injected with 9 ng (A) or 18 ng (B) Eomes mRNA. Sections were stained with the 12/101 antibody for muscle (black) or with MZ15 antibody for notochord (red).(C) In situ hybridization to sections cut vertically through the dorsal-ventral axis. Embryos were injected in the ventral equatorial region at the four-cell stage with Eomes mRNA: 200 pg (C), 5 ng (D), or 18 ng (E and F). (F) shows an Eomes-induced secondary axis (arrows). As a control, Xbra mRNA was injected ventrally: 5 ng (G) or 18 ng (H). In situ hybridization to gsc transcripts (purple color) shows ectopic activation of gsc on the ventral side by Eomes, but not by Xbra. Dorsal (D) and ventral (V) sides, muscle (m), and notochord (n) are marked.
Figure 7. Gastrulation Is Arrested and Axial Differentiation Inhibited by Reduction of Eomes Function(A), arrested midgastrula; (B), severe exogastrula (stage 37) after injection of 1 ng Eomes-Engrailed mRNA; and (C), normal stage 37 tadpoles after injection of 3 ng Engrailed mRNA. (D), RNase protection analysis of stage 37 embryos injected with 1 or 6 ng Eomes-EnR compared to whole embryos (WE). (E), first column, dissected dorsal embryo halves (stage 16) that were injected with 1 ng of Eomes-EnR at the 4-cell stage. Middle column, axial structures and MyoD expression are rescued in dorsal explants by coexpression of wild-type Eomes (4 ng) with Eomes-EnR (1 ng). Last column, Eomes alone (4 ng) does not block MyoD expression in dorsal explants. Top row (Whole) shows fixed dorsal explants, middle row shows MyoD stained sections, and bottom row shows Hoescht stained sections.
