Morphological characteristics of atrioventricular endocardial cushion development of the mouse embryonic heart

Abstract

Objective: To investigate the morphological characteristics of the formation and the fusion pattern of atrioventricular endocardial cushion of mouse embryonic heart. Methods: Serial sections of thirty mouse embryos during embryonic day (ED) 9 to ED13 were stained immunohistochemically with antibodies against α-smooth muscle actin (α-SMA), myosin heavy chain (MHC) and phosphorylated histone H3 (PHH3). Semithin sections and ultrathin sections of fifteen mouse embryonic hearts from ED12 to ED13 were observed by light microscope and transmission electron microscopy. Results: At ED9, the atrioventricular canal was observed between the atrium and the ventricle. The cardiac jelly layer was thickened and contained a few mesenchymal cells. At ED10, the atrioventricular endocardial cushion began to form. The inferior endocardial cushion was thicker than the superior cushion. The α-SMA and MHC positive myocardial cells neighboring with the inferior cushions extended towards the cushion. The mesenchymal cells in the cushions showed α-SMA or PHH3 negative. During ED11 to ED12, the inferior and superior cushions became symmetric and the mesenchymal cells increased obviously. Only a few of mesenchymal cells expressed α-SMA or PHH3. At ED12 and ED13, the bilateral cushions closed to each other and began to fuse. The cellular membrane touched each other and formed intercellular junctions between some mesenchymal cells. The mesenchymal cells contained rough endoplasmic reticulum and mitochondria, and few of microfilament were observed in the cells. Conclusion: The first change of atrioventricular endocardial cushion formation of mouse embryonic heart is the morphologic change of the endothelial cell from squamous to cuboidal. The formation of the two endocardial cushions is not simultaneous. During the cushion fusion, cellular junction is formed locally between mesenchymal cells which contain few of microfilament. Thus ultrastructure characteristics of the fusion of the atrioventricular endocardial cushion is different with the outflow tract ridge.