Fig. 2. Ablation of Fgf8 in the cardiac crescent disrupts formation of the heart tube and outflow tract. (A-E) Ventral views of the 10-11ss Fgf8;MesP1Cre allelic series; genotypes are listed above. Heart tubes of Fgf8^C/-;MesP1Cre^/+ conditional mutants are small (red arrowheads). The mildly affected mutant heart (D) is looping, but all segments are small. Although the severely affected mutant (E) has 10 somites, its heart is the size of the 4-5ss control. Note the normal morphology of Fgf8^C/+;MesP1Cre^/+ (B) and Fgf8^C/- (C). (A'-E') Right lateral views of the E9.5 Fgf8;MesP1Cre allelic series. Superimposed black lines are the same length in each panel. The mildly affected mutant (D') has a short, narrow OFT and a small RV (black outline, red arrowhead). (E') The severely affected mutant has a single dilated ventricle, an incompletely looped heart and no OFT (red arrowheads). (F-J) Ventral views of the 7-8ss Fgf8;Isl1Cre allelic series. Fgf8^C/+;Isl1^Cre/+ (G) is normal, whereas the accruing RV/OFT of the mildly affected Fgf8^C/-;Isl1^Cre/+ mutant (I) is small (red arrowhead, this phenotype was not seen in >60 controls). (J) Severely affected Fgf8;Isl1Cre heart (red arrowheads). (F'-J') Right lateral views of the Fgf8;Isl1Cre allelic series at E9.5. Black lines in panels F'-H' are the same length and show normal flexion of the conotruncus/OFT. Mutant pharyngeal arches (pa) are hypoplastic (I',J'). The mildly affected mutant (I') has no visible RV and a short OFT lacking normal flexion (black lines); the OFT arises from the LV. Severely affected mutants (J') have incompletely looped, dilated hearts. (K,K') Fgf8;Mef2cAHFCre mutants at E9.5 and 8ss appear normal. (L) Sectioned E10.0 Fgf8^C/+;MesP1Cre^/+ control. The OFT arises from the RV (black arrows). (M) Sectioned mild E10.0 Fgf8^C/-;MesP1Cre^/+ mutant heart with a short OFT and small RV. The OFT arises abnormally from the LV (red arrow). Note the enlarged right atrium (ra, red arrowhead) and small RV. OFT cushion cellularity appears normal. (M') Sectioned, severe/dying Fgf8^C/-;MesP1Cre^/+ mutant; the single ventricle (v), short OFT, and atrium (a) are all dilated. No RV is present. (N) Sectioned E10.5 Fgf8^C/+;Isl1^Cre/+ control. The OFT arises from the RV and is beginning to septate; the conotruncal cushions are dense with mesenchymal cells (black arrowheads). (O) Sectioned, mild E10.5 Fgf8^C/-;Isl1^Cre/+ mutant with a RA enlargement, RV hypoplasia, and the OFT arising from the LV. OFT cushions are hypocellular (red arrowheads). (O') Sectioned, severe E10.5 Fgf8^C/-;Isl1^Cre/+ mutant. There is a dilated single ventricle and atrium. (P) Fgf8^C/+;Mef2cAHFCre control. (P') Sectioned, E10.5 Fgf8^C/-;Mef2cAHFCre mutants reveal normal OFT cushion cellularity and mild RV hypoplasia. (Q) Dissected heart/great vessels of a newborn Fgf8^C/+;MesP1Cre^/+ control. Great vessels are normally related (ao, aorta; pa, pulmonary artery; da, ductus arteriosus); white arrow indicates the normal RV alignment and egress via the pulmonary artery. The aorta is posterior and to the right of the pulmonary artery. (R) A newborn Fgf8^C/-;MesP1Cre^/+ mutant reveals TGA: abnormal OFT alignment/rotation with the aorta anterior, arising aberrantly from the RV (white arrow) and the pulmonary artery arising from the LV. (R') Bicuspid aortic valve (BAV) in a Fgf8^C/-;MesP1Cre^/+ newborn. (S,S') Aberrant OFT alignment/rotation in an Fgf8^C/-;Mef2cAHFCre mutant manifest as DORV with both the aorta (av, aortic valve black arrowhead) and pulmonary artery arising from the RV. (T) Newborn Fgf8^C/-;Isl1^Cre/+ mutant with Persistent Truncus Arteriosus (PTA) and right aortic arch (white arrow). rcc, right common carotid artery; lcc, left common carotid artery; lsc, left subclavian artery; TA, truncus arteriosus. (T',T'') Sectioned Fgf8^C/-;Isl1^Cre/+ mutant reveals an abnormally aligned truncal vessel completely overriding the RV; the LV egress is via a large ventricular septal defect (vsd, black arrowhead). tv, truncal valve.