Syncytium is a term used to describe those multinucleated cells that arise through cell fusion. These "cells" are a kind of "cytoplasmic mass" that contains several nuclei enclosed in the same cell membrane.
Syncytia can be seen in almost all kingdoms of life: animals, plants, fungi, and archaea. For example, during the embryonic development of animals, in the plants of the Podostemaceae family and in the development of the spores of all fungi, syncytial stages are observed.
Formation of a syncytium after a wound in the epithelial tissue of a fruit fly (Drosophila Melanogaster) (Source: See page for author Via Wikimedia Commons)
However, in animals and plants the formation of syncytia can be induced by some type of pathogen. In animals, measles, HIV and other viruses tend to induce syncytia in tissues, which is why they are said to be "syncytial" pathogens.
Researchers have observed these "abnormal" formations during the course of experiments with animal cell cultures arranged in monolayers infected by viral cultures of the Paramyxovirus, Lentivirus, Cronavirus and Herpevirus families.
In plants, nematodes of the genera Globodera and Heterodera induce the formation of syncytia. These species of pathogens attack plants that are important for human agriculture.
Researchers from various areas consider it important to deepen the study of multinucleated structures such as syncytia, since they are important for basic research, and even for the discussion of a reform in current cell theory.
characteristics
Regardless of the factor that stimulates cell fusion, syncytia are cytoplasmic masses with several nuclei inside. The formation of this type of structure is very common in the life cycle of eukaryotic organisms.
The word "syncytium" comes from the Greek "syn", which means "together" and "kytos", which means "receptacle", "vessel" or "deposit". Therefore, biologists characterize syncytia as "multinucleated masses of protoplasm that are the product of cell fusion."
In some research, a distinction is made between the term "syncytium", "plasmodium" and the term "coenocyte", since despite the fact that they are all structures where a cell contains several nuclei inside, they all have different origins.
Plasmodia
Plasmodia are continuous cytoplasmic masses with several nuclei inside. However, each nucleus governs the activity of the surrounding cytoplasm; this cytoplasmic territory dominated by each nucleus is known as “energetic”.
The origin of plasmodia has to do with successive divisions of the nucleus, accompanied by an increase in the mass of the cytoplasm, but without it dividing into new cells, each separated by its own plasma membrane.
Cenocytes
Coenocytes, on the other hand, originate from various nuclear division events without cytokinesis (cell separation) taking place, while syncytia originate clearly from the fusion of one or more nucleated cells, which lose part of its plasma membrane.
At the origin of the syncytia, the cells -initially individual- emit extensions that fuse with those of other cells to establish a great network, without any limit that separates each of those that gave rise to them.
Syncytial theory
The syncytial theory of the origin of metazoans (animals) proposes that metazoans originated from ciliated protozoa. This was suggested because of the similarities observed between "modern" ciliates and acellomed flatworms.
Both types of organisms share characteristics such as size, shape, type of symmetry, position of the mouth, and the presence of superficial cilia. Therefore, the theory exposes the transition from a multinucleated ciliated protist to a flatworm of the group of the acellomates.
The syncytial theory also establishes the possibility that flatworms were the first metazoans. However, these organisms possess a cellular interior with a single nucleus and not in the form of a syncytium, like the ciliated protozoa.
This theory does not explain how cnidarians or ctenophores (groups considered more primitive than flatworms) and other more advanced groups derived from ciliates, so it does not currently have many defenders.
Examples
In plants
The formation of syncytia is frequent in the development of the endosperm of the seeds of almost all higher plants.
During the fertilization of the ovum in angiosperms a process of double fertilization occurs, since one of the nuclei of the pollen grain fuses with the two polar nuclei of the embryo sac to form a cell with three nuclei and the other fuses with the nucleus of the ovum.
Development of the female gametophyte and embryo in Arabidopsis. (a) Schematic showing ontogeny of the female gametophyte and early development of the embryo and endosperm. (Source: DPC Via Wikimedia Commons)
The cell of the first fusion will give rise to the endosperm from which the seed will feed once germinated.
In the genus Utricularia, the development of the embryo sac occurs through the fusion of the micropillary haustorium of the endosperm with the placental nutritive cells. This fusion forms a multinucleated structure called "sporophytic placental tissue."
In mushrooms
In all organisms of the Fungi kingdom, a process called “somatogamy” or “talogamy” occurs, before the formation of spores, which consists of the union of two undifferentiated somatic cells to produce a syncytium.
This fertilization is typical in groups of fungi such as basidiomycetes, some ascomycetes and phycomycetes.
In fungi that are considered "primitive", flagellated gametes usually occur. These gametes, in general, depend on an aqueous medium to move to the other sex cell and thus be able to fertilize it.
In contrast, somatogamy does not produce gametangia, nor specialized cells for reproduction and, therefore, do not depend on the presence of a specific environment for their reproduction.
In animals
During the embryonic development of animals, a syncytium is formed, which is called syncytiotrophoblast, which is a mass of cytoplasms that will form the outermost layer of the trophoblast and that works in the connection between the embryo and maternal tissue.
This layer of cells is formed by the fusion of embryonic cells that lose the cell membrane. It is located inside the epithelium, in the endometrial stroma, during the entire development of the mammalian embryo.
It is responsible for carrying out the gas and nutritive exchange with the mother of the embryo; it is also the site where hormones important for the proper development of the fetus are produced.
The syncytiotrophoblast is a great example of syncytia, since this layer of cells does not increase in size or volume due to any type of cell division. The growth of this layer only occurs by the migration and fusion of cells from the cytotrophoblast.
References
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- Hickman, CP (1939). Zoology. In Proceedings of the Indiana Academy of Science (Vol. 49, pp. 199-201).
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- Schols, D., Pauwels, R., Baba, M., Desmyter, J., & De Clercq, E. (1989). Syncytium formation and destruction of bystander CD4 + cells cocultured with T cells persistently infected with human immunodeficiency virus as demonstrated by flow cytometry. Journal of general virology, 70 (9), 2397-2408.
- Watkins, BA, Crowley, R., Davis, AE, Louie, AT, & Reitz Jr, MS (1997). Syncytium formation induced by human immunodeficiency virus type 1 isolates correlates with affinity for CD4. Journal of general virology, 78 (10), 2513-2522.