Effect of conotruncal constriction on aortic-mitral valve continuity in the stage 18, 21 and 24 chick embryo.

The effect of conotruncal constriction on the development of aortic-mitral valve continuity in stage 18, 21 and 24 chick embryos was studied. A 10-0 nylon suture was tied around the conotruncus, constricting the outflow tract of the heart. The loop was removed after 4 or 24 hours or left permanently in place in 3 subgroups of the 236 experimental embryos. The embryo hearts were harvested at stages after completion of cardiac morphogenesis, fixed in end-diastole and microdissected. The distance between the mitral and aortic anuli was measured from the base of the heart with a calibrated filar micrometer eyepiece. This measurement was compared with the mitral-aortic separation in 72 normal and 132 control embryos. The mitral-aortic separation was similar among normal, control and 4- and 24-hour experimental embryo hearts. However, the mitral-aortic separation increased from 0.34 +/- 0.02 mm in normal hearts to 0.82 +/- 0.25 mm in stage 18, 1.11 +/- 0.36 mm in stage 21 and 0.75 +/- 0.33 mm in stage 24 permanent loop experimental hearts (p less than 0.01). In embryo hearts with an increased mitral-aortic separation, both great vessels arose from the right ventricle, the semilunar valves were at the same level and the ventricular septal defect was present beneath the aortic anulus. We conclude that conotruncal constriction modifies the relation of the aortic and the mitral valve. We speculate that conotruncal constriction alters the migration of mesenchymal tissue into the heart.