TEMPORAL AND SPATIAL ASYMMETRIES IN THE INITIAL DISTRIBUTION OF MESENCHYME CELLS IN THE ATRIOVENTRICULAR-CANAL CUSHIONS OF THE DEVELOPING CHICK HEART

Background: We investigated potential early asymmetries in the distrib ution of mesenchymal cells within the inferior and superior AV cushion s in the developing chick heart. Methods: Chick embryos stages 16-20 H H were fixed, embedded in polyacrylamide, and the cell nuclei stained with propidium iodide. Cells counts were determined within the cardiac jelly of the atrioventricular canal (AV) by laser confocal microscopy in coronal planes spanning its entire length. Results: Our data show at the different stages studied, 16-20 HH, that the inferior AV cushio n invariably contains more cells thats the superior AV cushion, In the inferior cushion, the cell distribution is bimodal, i.e., the proxima l and distal regions have more mesenchymal cells than the middle part of the AV canal, In the superior cushion, there is a increasing gradie nt of mesenchymal cells along the longitudinal axis from the atrium to the ventricle. Conclusions: Our findings reveal that the temporal and spatial characteristics of mesenchyme formation in the inferior vs. s uperior AV cushion are different, This asymmetry suggests several pote ntial hypotheses: (1) the distribution of the inducer molecule or its receptor has a distribution similar to that of mesenchymal cells, (2) the extracellular matrix has a differential composition or regionally- specific physical associations, (3) the endocardium is heterogeneous w ith respect to transformation capacity, or (4) these patterns result f rom an earlier inductive event, The potential importance of the observ ed asymmetries in the distribution of AV mesenchyme is discussed relat ive to localization patterns of molecules critical to successful cardi ac morphogenesis and remodeling. (C) 1997 Wiley-Liss, Inc.