|
Fig. 1 Serrate1 and Serrate2 gene expression patterns correspond to the borders of the AVC and indicate prospective atrial and ventricular chamber myocardium. (A-D) Stage 12-18 embryos were hybridized with a Serrate1 antisense RNA probe. (A) Stage 12 embryos exhibit widespread Serrate1 expression throughout the posterior heart with the anterior boundary at the outflow tract. The posterior border stops at the presumptive atrium and sinus venosus, but may include primitive AVC tissue (white arrowhead). (B,C) Between stages 14 and 16 expression becomes restricted to the common ventricle and atria with boundaries at the AVC (white arrowheads, C). (D) At stage 18, expression is strong in the ventricles and atria, forming distinct boundaries at both the anterior and posterior AVC border (white arrowheads). (E-H) Stage 12-18 embryos were hybridized with a Serrate2 antisense RNA probe. (E,F) At early stages, 12 and 14, Serrate2 is not detected in the heart. (G) Serrate2 expression becomes apparent by stage 16 at the posterior-most region of the heart, corresponding to the distal atrium and sinus venosus (white arrowhead). (H) By stage 18, strong Serrate2 expression is observed in the atria and sinus venosus, overlapping that of Serrate1 (white arrowhead). (I-L) Stage 12-18 embryos were hybridized with the AVC marker Bmp2 for comparison with the Notch ligands. (I,J) Bmp2 is expressed in the posterior-most region of the heart, including the primitive AVC, atria and sinus venosus. The most anterior Bmp2 expression appears to correspond to the posterior border of Serrate1 expression (compare arrowheads in A,B and I,J). (J,K) Between stages 14 and 16, Bmp2 expression is restricted to the AVC. Again, Bmp2 expression appears complementary to that of Serrate1 in the primitive ventricle, and to both Serrate1 and Serrate2 in the atria (compare arrowheads in K with C,G). (L) Bmp2 mRNA persists in the AVC. (M-Q) Immunostaining against Notch2. (M) Notch2 is first detected in the linear heart tube in small patches in the ventral primitive ventricle and at the junction of the two horns of the primitive atria (white arrowheads). (N-Q) By Stage 12, Notch2 is present throughout the myocardium, where it remains at all stages examined. a, atrial region; avc, atrioventricular canal; ot, outflow tract; v, ventricular region.