Cell Fusion

• Until recently, cells were thought to be integral and discrete components of tissues, and their state was determined by cell differentiation.
• However, under some conditions, stem cells or their progeny can fuse with cells of other types, mixing cytoplasmic and even genetic material of different (heterotypic) origins.
• Cell fusion is an important cellular process in which several uninuclear cells (cells with a single nucleus) combine to form a multinuclear cell, known as a syncytium.
• Syncytia are commonly found in placenta, bone, cartilage, muscle and multi-nucleated giant cells.
• Fusion of one or more cells can occur during development as well as in mature individuals.
• Cell fusion occurs during differentiation of muscle, bone and trophoblast cells, during embryogenesis, and during morphogenesis.
• Cell fusion is a necessary event in the maturation of cells so that they maintain their specific functions throughout growth.
• The fusion of heterotypic cells could be of central importance for development, repair of tissues and the pathogenesis of the disease.

Types of Cell Fusion

There are two different types of cell fusion that can occur. These two types include homotypic and heterotypic cell fusion.

Homotypic cell fusion

• It occurs between cells of the same type.
• An example of this would be osteoclasts or myofibers being fusing together with their respective type of cells.
• Whenever the two nuclei merge a synkaryon is produced.
• Cell fusion normally occurs with nuclear fusion, but in the absence of nuclear fusion, the cell would be described as a binucleated heterokaryon. A heterokaryon is the melding of two or more cells into one and it may reproduce itself for several generations.
• If two of the same type of cells fuse, but their nuclei do not fuse, then the resulting cell is called a syncytium.

Heterotypic cell fusion

• It occurs between cells of different types, making it the exact opposite of homotypic cell fusion. The result of this fusion is also a synkaryon produced by the merging of the nuclei, and a binucleated heterokaryon in the absence of nuclear fusion.
• An example of this would be Bone Marrow Derived Cells (BMDCs) being fused with parenchymatous organs.

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Significance of Cell Fusion

• Cell fusion can cause a change in the phenotype and/or the function of cells. Cell fusion could, therefore, explain the transdifferentiation of committed somatic cells.
• Cell fusion can reverse or repair the injury to tissues. In this context, cell fusion facilitates the regeneration of tissues.
• Cell fusion can promote the transmission of viruses. Fusion of cells of individuals of disparate species might explain the transmission of viruses between species and the genesis of new pathogens.
• Cell fusion and nuclear fusion that occur in the formation of synkaryons can allow the re-sorting and recombination of chromosomal DNA. Fusion of nuclei of an immature cell, such as a stem cell, and a mature cell might generate proliferation and malignancy.