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Fig. 2. Cardiac progenitors move on directed trajectories, which are controlled
by Wnt3a. (A) Long-term video microscopy followed by image
processing revealed movement trajectories; still images at the time points
indicated in hours (h) are shown. (B) Corresponding bright-field
images. The last panel shows the location of GFP cells in the bilateral heart
fields, indicated by white outline (for the corresponding movie, see Movie 1
in the supplementary material). (C) vMHC in situ hybridisation of an
HH11 embryo after live imaging shows normal heart morphogenesis and cardiac
specific gene expression. (D) Paraffin transverse section of the embryo
shown in C. (E,F) In situ hybridisation shows (E) Fgf8 and (F)
Wnt3a transcripts are expressed in the HH3 primitive streak. (G) A weak
signal for Wnt11 was detected in the node of an HH4 embryo (arrowhead). The
contrast was enhanced and saturation of the magenta colour was increased to
visualise this signal. (H) Illustration of Wnt3a-expressing cells
(yellow dot) grafted into HH3 embryos to challenge the movement of
GFP-labelled cardiac progenitors. (I) Long-term video microscopy
followed by image processing revealed the movement trajectories of
GFP-labelled cardiac progenitors in the presence of Wnt3a-expressing cells
(yellow dot). Still images at the time points indicated show a wider
trajectory. (J) Corresponding bright-field images show that most cells
failed to reach the midline and remained in the area opaca and lateral
mesoderm (see Movie 2 in the supplementary material). (K,L) Many
embryos developed cardia bifida as shown by vMHC in situ hybridisation: whole
mount (K) and section (L) of an embryo after imaging. (M,N)
Paraffin sections of an HH10 embryo implanted with a control cell pellet (M)
or with a Wnt3a cell pellet to induce cardia bifida (N). Black arrowheads
indicate the endoderm and red lines indicate the foregut pocket; h, heart.
(O) Graph showing the percentage of embryos with cardia bifida under
different conditions. The embryo treatment is indicated below each bar. LNCX
ccp, control cell pellet; ep, electoporation; cp, cell pellet. (P) HH5
embryo with Wnt3a-expressing cells (arrowhead) implanted at HH3 (as shown in
H) and hybridised with a probe detecting Fgf8 transcripts.
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