Fig. 2. Cardiac morphology and histology of embryos mutant for Tbx20. (A-I) External morphology of embryos and isolated hearts. (A-D) E8.5 heart regions. The wild-type heart (A,C) shows looping, the mutant heart (B,D) remains linear. (E) At E9.5, Tbx20 mutants are substantially smaller than their wild-type littermates, and exhibit bleeding and edema. Somites are smaller and irregularly organized, as is the neural tube. The first branchial arch is formed. Boxes indicate the heart regions. (F-I) Isolated hearts of E9.5 embryos. (F,H) Wild-type hearts show formation of chambers with right (rv) and left ventricle (lv) and an atrium (a), and of the atrioventricular canal (avc) and outflow tract (oft). (G,I) Mutant hearts feature an outflow tract, a primitive ventricle (pv) and an inflow tract from anterior to posterior. Anterior is up. Views are from the left-lateral side (A,B,E-G), or from the ventral side (C,D,H,I). (J-Q) Histological analysis of E9.5 embryonic hearts by Hematoxylin and Eosin staining of paraffin sections. Differentiation of cardiac tissue in endocardium (e), myocardium (m), cardiac jelly (cj) and endocardial cushion tissue (ect) can be seen in wild-type (J-L) and mutant hearts (M-Q). Trabeculated myocardium (tm) is only formed in the wild-type heart. Sections are sagittal, with anterior up and ventral to the left (J,K,M,N), or transverse, with right up and ventral to the left (L,O,P,Q). Transverse sections of the mutant heart are at the level of the inflow tract (O), primitive ventricle (P) and outflow tract (Q). pc, pericardium; pcv, pericardial cavity; peo, proepicardial organ. Genotypes are indicated in the figure.