Absent arterial valve leaflets are rare anomalies. On the basis of our understanding of the normal development of the arterial valves, we draw inferences that might offer clues to their morphogenesis.

We describe the findings from four human fetal autopsies with so-called “absent” arterial valvar leaflets. We then make inferences relative to these finding on the basis of our current understanding of normal development, the latter obtained by analysis of episcopic data sets from a large series of mouse embryos.

The fetuses had died between 12 and 15 weeks of gestation. In two cases, we found absence of the leaflets of the pulmonary valve, with patency of the arterial duct, but otherwise normal hearts. In a third case, there was absence of the leaflets of both arterial valves, along with a perimembranous ventricular septal defect and a “window-type” arterial duct. This fetus had a completely muscular subaortic infundibulum. The last fetus had a pulmonary dominant common arterial trunk, with absence of the truncal valvar leaflets, but again with a muscular subtruncal infundibulum. Findings from the analysis of the mouse embryos reveal that the arterial valvar leaflets are formed from the distal outflow cushions, but that the cushions have a separate function in septating the arterial roots and the proximal outflow tracts.

When interpreting the fetal findings in the light of development, we conclude that there had been normal fusion of the major outflow cushions, but failure in excavation of their peripheral margins in three of the cases. In the fourth case, however, the cushions had not only failed to excavate but had also failed to separate the arterial roots.