Fig. 2. Cardiac phenotypes of Pitx2 allelic combinations. (A-D) Transverse sections through 11.5 dpc hearts of wild-type and ab;ab mutants. In wild-type (+;+) and ab; ab embryos, the primary interatrial septum (PIAS) extends from the spina vestibuli to the roof of the atrium (solid arrowheads, A,C) and the pulmonary vein is located at the base of the PIAS (blunt arrow in A,C). Paired venous valves are found at the boundary of the right atrium and right sinus horn (winged arrowheads in A,C). In wild-type and ab; ab hearts, the cushions of the proximal OFT occlude the lumen, and twist in characteristic fashion, with left and right lateral lumens (arrows in B,D). Moreover, the right side of the atrium has trabeculations (pectinate muscles, pm in A and C), while the left side has none. (E-H) Transverse sections through 11.5 dpc abc^null;abc^null hearts. In abc^null;abc^null mutant embryos, the PIAS is just a stub (blunt arrowhead in F) and the pulmonary vein has an anomalous connection, emptying into the right sinus horn, or saccus reuniens (arrow, E). Also in abc^null;abc^null mutant embryos, paired venous valves are present on both the left and right (winged arrowheads in F). The mutant OFT has a large lumen with symmetrical cushions (winged arrows in F-H) and the dorsal left atrium is more extensively trabeculated than the right (pm in G). The left superior caval vein, present in wild-type and ab; ab hearts (asterisks in A and C), is absent in abc^null; abc^null hearts. (I-M) Transverse sections through 12.5 dpc hearts of wild-type and abc^null;ab^hypoc mutants. The PIAS is normal (blunt arrowheads I and K) and the pulmonary vein is located at base of PIAS (blunt arrow K) in wild-type and abc^null;ab^hypoc embryos. The distal OFT is separated into the left aortic arch and the right pulmonary trunk (winged arrows in I and K) and the proximal OFT cushions form three primordia of valve leaflets in both arterial trunks, although septation is incomplete (winged arrows, J,M). The right atrium in wild-type hearts has trabeculations (pm, I), while the left atrium has none. The dorsal left and right atria of abc^null; ab^hypoc hearts have similar degrees of trabeculation (pm, L). The superior and inferior AV cushions have fused in wild-type and abc^null; ab^hypoc hearts (asterisks, J,M), forming separate left and right AV canals. (N-P) Transverse sections through 12.5 dpc abc^null;abc^null hearts. In abc^null;abc^null mutants, the PIAS is truncated (blunt arrowheads, O) and the pulmonary vein is found within dorsal mesocardium, but caudal to the atrial wall, so emptying into the right sinus horn, or saccus reuniens (blunt arrow, N). The pulmonary vein has exits to both the left side (blunt arrow, O) and the right side (not shown) in abc^null;abc^null embryos. The abc^null;abc^null mutant distal OFT is unseptated, with malaligned arterial trunks (winged arrow, N) and the OFT cushions are malaligned (winged arrow, P). Moreover, the AV cushions have not fused, and there is a common AV junction (asterisk, P) in abc^null;abc^null embryos. The left superior caval vein shown in wild-type and abc^null; ab^hypoc hearts (asterisks in I,K,L) is absent in abc^null; abc^null hearts.

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