Fig. 1.

Architectural homology in the arterial pole of two widely diverged species. In all three images, cranial is to the top. (A) Detail of the chick pulmonary outflow, gestational day 11, sectioned longitudinally and labeled with MF20 (myocardium—red) and anti-SMA (smooth muscle—green). The pulmonary outflow valves are surrounded and supported by a collar of myocardium. Smooth muscle known to be derived from the secondary heart field (SHF) can be seen at the lumenal face of the myocardial collar, extending caudally from the base of the arterial trunk to the level of the valves (arrowheads). Arrows show distal outflow tract smooth muscle known to be neural crest derived. The smooth muscle in the remnant of the aorticopulmonary septum (aps) is also neural crest derived. PI, pulmonary infundibulum; PT, pulmonary trunk. Scale bar=250 μm (image adapted from Waldo et al. 2005). (B) Detail of the adult zebrafish arterial pole sectioned longitudinally and labeled with MF-20 (red) and anti-MLCK (smooth muscle—green). DAPI (blue) labels nuclei. White arrowheads show that, as in the chick, the valves are supported by a collar of myocardium (the conus arteriosus) and that smooth muscle extends caudally from the base of the bulbus arteriosus (BA) to the level of the valve leaflets. Scale bar=100 μm. (C) Similar longitudinal section of adult zebrafish stained with elastic trichrome, showing an abundance of fibrous and elastic proteins (arrowheads) in the “fibrous ring” connecting the ventricle (V) with the bulbus arteriosus (BA). Scale bar=100 μm.