• heart;
  • looping;
  • extracellular matrix;
  • flectin;
  • laterality genes;
  • left–right;
  • CFC;
  • Pitx2;
  • Nodal;
  • asymmetry;
  • dorsal mesocardium;
  • midline;
  • plakoglobin;
  • secondary heart field;
  • chick embryo


Dextral looping of the heart is regulated on multiple levels. In humans, mutations of the genes CFC and Pitx2/RIEG result in laterality-associated cardiac anomalies. In animal models, a common read-out after the misexpression of laterality genes is heart looping direction. Missing in these studies is how laterality genes impact on downstream morphogenetic processes to coordinate heart looping. Previously, we showed that Pitx2 indirectly regulates flectin protein by regulating the timing of flectin expression in one heart field versus the other (Linask et al. [2002] Dev. Biol. 246:407–417). To address this question further we used a reported loss-of-function approach to interfere with chick CFC expression (Schlange et al. [2001] Dev. Biol. 234:376–389) and assaying for flectin expression during looping. Antisense CFC treatment results in abnormal heart looping or no looping. Our results show that regardless of the sidedness of downstream Pitx2 expression, it is the sidedness of predominant flectin protein expression in the extracellular matrix of the dorsal mesocardial folds and splanchnic mesoderm apposed to the foregut wall that is associated directly with looping direction. Thus, Pitx2 can be experimentally uncoupled from heart looping. The flectin asymmetry continues to be maintained in the secondary heart field during looping. Developmental Dynamics 228:217–230, 2003. © 2003 Wiley-Liss, Inc.