Neuregulin is necessary for the induction of AV conduction tissue. (A) Neuregulin is expressed widely in the developing brain (seen here at 48 hpf) and in the AV ring endocardium (arrowhead) from 36 hpf onwards. Inset shows a Hematoxylin and Eosin stained section of the AV canal in cross section with neuregulin staining (purple) in the AV ring endocardium. (B) neuregulin knockdown embryos fail to develop an AV conduction delay, as shown in this calcium activation map of a single cardiac cycle in a neuregulin morphant at 48 hpf. Isochronal lines (50 mseconds) obtained by fluorescence microscopy are superimposed on maximum intensity projection image. (C) neuregulin morphant embryos fail to develop an AV conduction block in the presence of terfenadine, as seen in contemporaneous recordings of atrial and ventricular contraction. (D) The morpholino effect is dose related, as seen in a histogram of the proportion of embryos that develop 2:1 AV block in the presence of terfenadine at 48 hpf. This proportion correlates with the amount of neuregulin mRNA detectable by RT-PCR from the respective embryos.