Fig. 1. Wnt-8c signaling regulates heart looping in the chick embryo. (A)
Wnt-8c expression in a HH stage 5 chick embryo. Transcripts are
observed throughout the entire length of the primitive streak and
symmetrically in the presumptive mesoderm cells lateral to the streak. (B)
Detail of the same embryo, where Wnt-8c expression in the node is
restricted to the right side (indicated by top arrow). (C) At HH stages 5+ to
6, Nodal becomes restricted laterally and anteriorly to the left side
of Hensen's node. (D) A close-up of the Hensen's node area of the embryo in C,
showing left-specific expression of Nodal mRNA in the node (arrow), a
pattern complementary to that of Wnt-8c in the node (compare B with
D). (E) At the same stage, ß-catenin transcripts appear to be
symmetrically expressed (arrows). (F) Scheme showing the implantation of
Wnt-8c cell pellets to the right side of Hensen's node of a HH stage
4 chick embryo. The syringe indicates the area to the left of the node where
the Axin adenovirus is injected at the same stage. (G,H) 24-36 hours
after implantation of Wnt-8c cells to the right side of the node,
specimens were processed for scanning electron microscopy. Normal heart
looping (G) is altered after implantation of Wnt-8c cells. Like
right-sided ectopic expression of Nodal (Levin et al.,
1995; Levin et al.,
1997), right-sided
Wnt-8c cell implants induce bilaterally symmetric hearts (not shown)
or reversed heart looping (H). (I) Injection of an adenovirus expressing the
Axin protein to the left of the node of a HH stage 4 chick embryo (see F) also
results in reversed heart looping (I). White semicircular arrows in (G-I)
indicate direction of heart looping. In this and the following figures, WT
indicates wild-type or normal embryos, which are the controls mentioned in the
Materials and Methods section.
