Fig. 4.

Endocardial Smad4 is required for EMT in the endocardial cushion. (A–J) Atrioventricular cushion explants from wild-type (A, C, E, G, and I) and Tie2:Cre⁺; Smad4^fl/− (B, D, F, H, and J) embryos. Mesenchymal cells were abundant in wild-type explants (A) but rare in mutant explants (B). Average numbers of invasive mesenchymal cells in wild-type explants (C: 102.8 ± 10.2 cells) was significantly greater (P = 3.8 × 10⁻⁷) than in mutant explants (D: 8.2 ± 4.4)(n = 5 each). Mesenchymal cells from control explants expressed Pecam (G and I: gray arrowheads), consistent with endocardial origin, and Sma (E and I; gray arrowheads), indicative of a mesenchymal fate. Cells from mutant explants did not express Sma (F and J).