Only a limited number of human cells can fuse to form a multinucleated syncytium. Cell fusion occurs as part of the differentiation of some cell types, including myotubes in muscle and osteoclasts in remodeling bone. In the differentiation of the human placenta, mononuclear cytotrophoblasts aggregate and fuse to form endocrinologically active, non-proliferative, multinucleated syncytia. These syncytia allow the exchange of nutrients and gases between the maternal and fetal circulation. Alteration of syncytial formation during pregnancy affects fetal growth and the outcome of the pregnancy. Here, we demonstrate the role of annexin A5 (AnxA5) in syncytial formation by cellular delivery of recombinant AnxA5 and RNA interference. By a variety of co-immunoprecipitation, immunolocalization and proximity experiments, we show that a pool of AnxA5 organizes at the inner-leaflet of the plasma membrane in the vicinity of a molecular complex that includes E-Cadherin, α-Catenin and β-Catenin, three proteins previously shown to form adherens junctions implicated in cell fusion. A combination of knockdown and reconstitution experiments with AnxA5, with or without the ability to self-assemble in 2D-arrays, demonstrate that this AnxA5 2D-network mediates E-Cadherin mobility in the plasmalemma that triggers human trophoblasts aggregation and thereby cell fusion. The cell fusion process consists of the formation of multinucleated syncytia by the mixing of cellular membrane components and cell contents from two or more cells. This complex phenomenon occurs in fertilization, placen-tation, fetal development, skeletal muscle formation and bone homeostasis 1-4. Cell fusion processes consist of three distinct stages 5 , the competence, commitment and full fusion stage. The competence stage is characterized by the loss of cellular proliferation and the differentiation into fusion-competent cells. This includes cell migration , morphological changes and secretion or response to extracellular signals such as growth factors, cytokines and hormones 5. The commitment stage describes the recognition of fusion partners, followed by the cellular adhesion and inter-cellular communication. This leads to activation, expression or assembly of the fusogenic machinery and to the synchronization of fusion-competent cells through the exchange of fusogenic signals. These two first stages are a prerequisite to promote the cell fusion with fusion pore formation between aggregated cells and the mixing of cellular content 6. Several proteins, protein macrocomplexes and cellular signaling pathways have been reported to trigger trophoblast fusion 5. Tight junction (e.g. ZO-1), adherens junction (e.g. cadherins) and gap junction (e.g. connexins) proteins have been shown to play a fundamental role during the commitment stage of trophoblast fusion 5. E-cadherin is a transmembrane protein that mediates mononuclear cell aggregation and adherens junction formation between fusion-competent cells essential for cell fusion 7. The E-cadherin extra-cellular N-terminal domain generates cellular adhesion by clustering with homotypic and heretotypic cadherins through the neighboring cell. This cellular adhesion stabilizes the cell membrane and allows polarization to the future fusion area. This triggers the clustering of fusogenic proteins or proteins initiating trophoblast fusion at the right time and the right place to the plasma membrane 5. Gap junctions are responsible for communication between adjacent cells and are composed of connexins. Gap junction channels allow the exchange of small molecules , second messengers and fusogenic signals facilitating cellular coordination, spatial compartmentalization