Cell fusion is a specialized cellular event which occurs in multicellular organisms in health and
disease. Known as a phenomenon in modern science for over 100 years, cell fusion takes the mandatory
center stage in eutherians for the conception, development and physiology of organogenesis or,
in pathophysiology, during the process of oncogenesis. The ability of two or more cells to unit and
to form a new syncytial cell takes place in metazoans throughout evolution to form muscles, bones
and placentae, and, even to form a tumor. This process requires migration, recognition and adhesion
between the cells together with the fusion of their plasma membrane and rearrangement of their
cytoplasmatic and nuclear contents. Membrane fusion arise during many cellular processes, including
membrane traffic, intracellular vesicle fusion, fertilization, and infection by enveloped viruses.
Fusion allows to exchange biological materials between different membrane compartments. In order
to maintain the functional individuality of each of the intracellular compartments and of the cell itself,
membranes do not fuse easily under normal circumstances. The process is subjected to selective control
and requires the expression of special (glyco-)proteins and carbohydrates and the formation of a
phospholipid interbilayer via an hourglass-shaped structure called a “stalk”.