Objective To investigate the origin of α-SMA positive cells in the outflow tract ridge of the embyonic mouse heart and to explore the ultrastructure change of the mesenchymal cells during the ridges fusion. Methods Sections of embryonic day 10(E10d) to E14d mouse embryonic hearts were stained by immunohistochemistry assay with monoclonal antibodies against α-smooth muscle actin (α-SMA), α-sarcomeric actin(α-SCA) and in situ hybridization method with PlexinA2 probe. The outflow tract ridges fusion was observed by transmission electron microscopy at E12.5d. Results From E10d to E11d, PlexinA2 positive cells were seen in the neural tube with mesenchymes around it, the aortic sac and aortic arch. These cells also migrated into the outflow tract ridge along the aortic sac wall and part of them expressed α-SMA. At E12d, PlexinA2 was expressed in the spinal ganglia, the pharyngeal mesenchyme, the aortopulmonary septum and the ascending aorta and pulmonary trunk. The septum showed α-SMA strongly positive. But only a few of α-SMA positive cells were observed in the ascending aorta and pulmonary trunk. At E12.5d, two clusters of condensed mesenchymal cells in the outflow tract ridges formed and began to express PlexinA2 weakly and α-SMA strongly. When the ridges began to fuse, the endothelial cells formed a cellular seam, which rapidly broke into pieces and disappeared and were replaced by the sparsed mesenchymal cells containing a few of microfilaments. Two clusters of condensed mesenchymal cells gradully moved to merge. It was noted that some mesenchymal cells contained plenty of microfilament bundles, mitochondria and focal contacts between them. Some mesenchymal cells only had a few of microfilaments and plasma membrane fusion was seen between them. Conclusion α-SMA positive cells in the outflow tract cushion may be derived from cardiac neural crest. The endothelial cells might participate in the fusion of the outflow tract ridges by endothelial-mesenchymal trans