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Fig. 3. Endocardial signaling is required for the development of AV conduction
tissue. (A) Calcium activation map of a single representative
cardiac cycle in cloche mutant embryos at 48 hpf. Isochronal lines
(black) obtained by fluorescence microscopy are superimposed on a maximum
intensity projection of the same heart and demonstrate the failure of AV
conduction tissue development in the absence of endocardium. (B) Lack
of AV conduction block in cloche embryos compared with wild-type
siblings is demonstrated by atrial pacing (white bars) and terfenadine
exposure (gray bars). Inset of contemporaneous recordings of atrial and
ventricular contraction using intensity-time plots from regions over the
respective chambers in a terfenadine treated cloche embryo
demonstrating 1:1 AV conduction. (C,D). In situ expression patterns of
notch1b (C) and bmp4 (D) in cloche embryos at 48
hpf. Despite overstaining in the brain there is no evidence of
notch1b signal in the heart (C). In D, the black arrowhead indicates
the location of the AV ring; the white arrowhead denotes intense expression of
bmp4 at the sinus venosus.
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