Fig. 5: Pictorial timeline of the subdivision of the interventricular foramen into the peri-tricuspid inlet, the subaortic outlet, and the membranous septum.

The figure shows the lumens of the atriums, ventricles and outflow tract, and the muscular ventricular septum. Panels a–e show a cranial view, and panels f–j a caudal view. When first formed, the interventricular foramen is a sagittally oriented interventricular conduit, as visualized by the GlN-positive ring. The craniodorsal part of the foramen, from which the GlN fades away at CS16, is indicated by a thinner, hatched line. The tips of the atrial appendages are clipped in the images for CS18 and CS20 (dashed lines) to permit inspection of the atrioventricular junction and outflow tracts. At CS16, the caudal part of the foramen and GlN ring begin to expand in rightward direction, producing a direct connection between the right atrium and ventricle, which is best seen in panels f–j. Meanwhile, the cranial, subaortic part of the foramen, which is best seen in panels a–e, gradually expands craniodorsally. Comparing the arrangements at CS18 and CS20, when the septation of the outflow tract is complete, shows that the subaortic, but not the subpulmonary, ventricular outlet is surrounded by the GlN ring. The remaining connection between the right ventricular cavity and the subaortic channel is still present at CS18. It is obliterated at CS20 by the formation of the membranous septum (not itself visible). All images are also available as preset views in the corresponding 3D-PDFs.