Figure 6

MBNL1 is expressed in the AVC endocardium prior to EMT. Sagittal sections of H&H stage 15 (A) and stage 16 (B) chick embryos were hybridized with MBNL1 antisense probes. The regions indicated by red boxes are shown in higher magnification (A’, B’). Filled arrowheads indicate positively staining endocardial cells in the atrioventricular canal (AVC). Open arrowheads indicate non-expressing endocardial cells in the atrium. Positive cells in the atria and ventricles (Vent) are blood cells, which are smaller, morphologically distinct, and luminal to the endocardial lining of the chambers. (C) MBNL1 transcript levels in the AVC were measured by real-time RT-PCR. Error bars denote 95% confidence intervals. An asterisk indicates a significant difference from the transcript level at stage 14 (P ≤ 0.05). Transcript levels did not differ significantly between stages 15, 16, 18, and 23 (P = 0.12 to 0.42).