E10.5 FVB/N but not mix and C57BL/6 KLF2−/− atrioventricular endocardial cushions are hypoplastic and disorganized.

<p>(<b>A</b>) and (<b>B</b>) are micrographs of FVB/N WT heart; (<b>C</b>) and (<b>D</b>) are micrographs of FVB/N KLF2−/− heart. The light micrographs, A and C (magnification 100X), show the structure of the E10.5 heart including the atrial (At) and the ventricular chambers (V) and B and D show the endocardial cushion regions that are in the boxes in A and C respectively, magnified at 200X. The red dashed lines indicate the positions of the hypoplastic FVB/N KLF2−/− AV cushions in D, compared to normal AV cushions in H and L. (<b>G</b>) and (<b>H</b>) are micrographs of Tie2-cre KLF2−/−, and (<b>E</b>) and (<b>F</b>) are WT littermate controls without Tie2-cre; all are in the FVB/N background. (<b>I</b>) and (<b>J</b>) are micrographs of mix WT; (<b>K</b>) and (<b>L</b>) are micrographs of mix KLF2−/− hearts. (<b>M</b>) and (<b>N</b>) are micrographs of C57BL/6 WT; (<b>O</b>) and (<b>P</b>) are micrographs of C57BL/6 KLF2−/− hearts. (<b>Q</b>) Bar chart representing the number of mesenchymal cells/mm² in the endocardial cushion tissue associated with the AV canal. Counts were performed of all cells in a single central section from E10.5 WT or KLF2−/− hearts in all three genetic backgrounds and for Tie2-cre KLF2−/−. Student’s t-test indicates that the number of mesenchymal cells is decreased in FVB/N KLF2−/−, Tie2-cre KLF2−/− and C57BL/6 KLF2−/−, compared to WT. Mes: Mesenchymal cells; Endo: Endothelial cells; Ery: Erythroid cells; Myo: Myocardium. n = 3–5 hearts for histological staining, and n = 3 hearts for mesenchymal cell counts. WT and KO embryos for each background are somite matched. Asterisks* indicate AV endocardial cushion region. Arrowheads show atrial septum that is forming in A, E, I, K and M; the atrial septum is absent in C, G and O.