FIG 44.12 Images prepared from episcopic datasets obtained from developing mouse embryos sacrificed during embryonic day 12.5 (A) Frontal section through the outflow tract, which arises at this stage above the cavity of the right ventricle Arrows show the junction between the outflow tract and the pharyngeal mesenchyme The outflow tract possesses distal, intermediate, and proximal components The cushions separating the outflow tract into aortic and pulmonary channels occupy only the intermediate and proximal parts, which have myocardial walls The distal part, at this stage, has been separated by a protrusion that has grown from the dorsal wall of the aortic sac The dotted line shows the line of fusion of the protrusion with the distal end of the major outflow cushions, which by this stage have fused with each other However, the proximal outflow cushions remain unfused The stars show the intercalated cushions, which are formed in the intermediate part of the outflow tract (B) View of the short axis through the intermediate part of the outflow tract as viewed from above and the right The interrelationships between the intercalated and major cushions are forming the primordiums of the aortic and pulmonary roots Note that that parietal parts of the major outflow cushions remain unfused in both roots, although fused centrally They will separate at right angles to the plane of fusion as the roots separate one from the other At the stage at which the roots begin to form, the outflow tract itself remains supported above the cavity of the right ventricle It is not until embryonic day 13.5 in the developing mouse that the aortic root begins its transfer to the developing left ventricle Concomitant with the transfer of the root, the initial embryonic interventricular communication becomes reoriented to form the entrance to the developing left ventricular outflow tract (Fig 44.13A) As the aortic root achieves its transfer, a component of the proximal outflow tract myocardium is similarly transferred into the left ventricle, where it forms the summit of the parietal left ventricular wall At the same time, the proximal outflow cushions themselves fuse with each other and muscularize, fusing at the same time with the crest of the muscular ventricular septum This permits the tubercles from the ventricular aspects of the atrioventricular cushions to close the persisting embryonic interventricular communication, thus completing ventricular septation (see Fig 44.13B) FIG 44.13 Images from episcopic datasets prepared from developing mouse embryos sacrificed at embryonic day 13.5 (A) and 14.5 (B) Both are sectioned in the frontal plane The images show how the aortic root is transferred to the left ventricle, with reorientation of the initial interventricular communication, taking with it the proximal outflow tract myocardium, which supports the cushions than are beginning to cavitate to form the leaflets of the aortic valve Note the stem of the right coronary artery, which is growing out from the aortic wall The parietal unfused margins of the distal outflow cushions have already begun to cavitate and separate away from the walls of the root while the root itself retains a location above the cavity of the right ventricle (see Fig 44.13A) As the cushions cavitate to form the leaflets, there is additional growth of nonmyocardial tissue from the second heart field to form the walls of the valvar sinuses This growth of the nonmyocardial tissues continues up to term in the mouse, which takes place at the end of embryonic day 18.5 It is the nonmyocardial tissues that form the valvar sinuses, with the cushions cavitating so as to form the semilunar leaflets (Fig 44.14) FIG 44.14 Images taken from episcopic datasets prepared from developing mouse embryos sacrificed at embryonic day 17.5 (A) and 18.5 (B) They show how additional growth of nonmyocardial tissues from the second heart field has formed the valvar sinuses, with the distal parts of the outflow cushions cavitating to form the semilunar valvar leaflets The images show cuts across the two coronary aortic sinuses, which retain the proximal outflow myocardium at their bases When initially transferred to the left ventricle, all three developing valvar leaflets are supported by proximal outflow tract myocardium It is only at the end of embryonic day 15.5 in the mouse that the area between the nonadjacent, or noncoronary, leaflet becomes transformed into fibrous tissue It has now been established that excessive fusion of the parietal margins of the major outflow cushions is the mechanism producing the bifoliate aortic valve with fusion of the two leaflets arising from the coronary arterial sinuses This process has been demonstrated in a colony of Syrian hamsters.18 It has also been shown that mice with knockout of the eNOS gene develop bifoliate valves with fusion of the leaflets developed from the rightward of the major cushions and the intercalated aortic cushion.19 Therefore it can reasonably be presumed that excessive fusion of the major cushions, combined with fusion with the intercalated cushion, will produce the unifoliate and unicommissural variant It is also currently known that cells derived from the neural crest populate the major cushions to form the valvar leaflets, with which the valvar sinuses are formed by tissues derived from the second heart field However, the cells from the neural crest are less abundant in the aortic intercalated cushion Evidence from other knockout mice suggests that failure of formation of the intercalated cushion produces a bifoliate valve