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Fig. 3. Wnt signaling in early cardiac specification and differentiation.
(A) Cross-section of the trunk of a post-gastrulation mouse embryo
depicting how various Wnt ligands and Wnt antagonists impinge upon early
cardiac mesoderm specification. Dorsal is uppermost. Green tube represents the
notochord, the green bottom layer represents the early definitive endoderm,
blue indicates the neuroectoderm, red indicates precardiac mesoderm and yellow
represents splanchnic mesoderm. Wnt1 and Wnt3a are expressed in neuroectoderm,
while Wnt11 and others, including Wnt2a and Wnt2b (not shown), are expressed
in precardiac mesoderm. Wnt antagonists crescent and Dkk1 are expressed in the
underlying definitive endoderm and inhibit canonical Wnt signaling in the
precardiac mesoderm. (B) Studies in ES cells (yellow) show that
activation of Wnt signaling prior to, or around the same time as, cardiac
mesoderm specification results in increased cardiac differentiation of cardiac
myocytes (red), whereas later Wnt signaling activation inhibits cardiac
differentiation significantly (Naito et
al., 2006). (C) A model showing how Wnt signaling is
important for cardiac induction but is inhibitory to later cardiac
differentiation. This control could be partly due to the effects of Wnt
antagonists secreted from the underlying endoderm.
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