Migration and torsions of the conotruncus in the chick embryo heart: observational evidence and conclusions drawn from experimental intervention.

It seems difficult to escape the conclusion that conotruncal migration and torsions do occur: The amplitude, direction and timing of these movements can be accurately tracked and they can be experimentally arrested. The movements have a significant function: Associated with the partitionings, they regulate the outlet ventricular distribution at two levels, permitting an adjustment between the ampullae and conus (a function of migration) and between the proximal and distal conal segments (via torsions). Their defects result in malalignments--double outlet right ventricle and transposition of the great vessels, respectively. The mechanism of the migration is linked to the differential growth of ampullae; the right ampulla and certain areas of this ampulla appear as dominant during the embryonic period. The conus is carried by the right ampulla and passively undergoes migration. The mechanism of the torsions is not entirely clear. The proximal torsion appears to be linked to a myocardium--jelly dissociation; the distal torsion seems to be linked to an oriented myocardial growth of its wall; the torsion of the truncus is evidently passive, occurring as a consequence of the conus distal torsion.