Fig. 9

Outflow tract septation in Crocodylus HH40. Consecutive sections from proximal to distal (a–h) stained as indicated. a The interventricular septum is indicated but left and right ventricles are not completely separated, yet. Bar = 200 µm. b, c Sc and pc in the proximal OFT, showing signs of chondrification (arrows). c Foramen of Panizza (arrowhead) is present in sc, flanking the folding septum and adjacent to the NCC condensed mesenchyme (*). d Fusion of sc and the ic results in separation of both aortae by the aortic flow divider (#). The facing sinus of Valsalva (red curve) are connected with each other through the foramen of Panizza in c. Bar = 100 µm. d, e The folding septum and the condensed mesenchyme extend into the lumen between Pu and vAo. Note that some NCC condensed mesenchyme still separates the protrusion of the ventral myocardium (Myo) from the dorsal folding septum [19]. Two adjacent coronary ostia are indicated (ca). f CTNI staining demonstrates that the central mass (brown) of d–e is indeed myocardium, compare with Fig. 8e. g The central mass of myocardium (Myo) is continuous with the ventral myocardium. The connection of both aortic valve leaflets is to the left-sided free wall (#). Two further coronary ostia (ca) are indicated. h The pulmonary semilunar valve leaflets are located more distal than the aortic leaflets. ca coronary ostium, ic intercalated cushion, FS folding septum, IVS interventricular septum, LV left ventricle, Myo myocardium, pc parietal cushion, Pu pulmonary trunk, RV right ventricle, sAo right systemic aorta, sc septal cushion, vAo left visceral aorta, * NCC condensed mesenchyme, # interaortic flow divider, arrowhead foramen of Panizza, red curved line: facing sinus of Valsalva in sc, connected by FOP