Totipotency, is the ability of a single stem cell to give rise to all the different cell types in an organism. In other words, it is the ability of a single cell to divide and produce all the differentiated cells of an organism.
This capacity is peculiar to embryonic stem cells up to a certain degree of development. Intermediate levels are called pluripotency and multipotency, if the cell can specialize respectively in all or some of the cell types that make up the organism. The lowest level is called unipotency, whereby the cell can transform into a single cell species typical of a tissue.
Spores and zygotes are examples of totipotent cells. On the spectrum of cellular potency, totipotency represents the cell with the greatest potential for differentiation, being able to differentiate into any embryonic cell, as well as any extraembryonic cell. In contrast, pluripotent cells can only differentiate into embryonic cells.
It is possible for a fully differentiated cell to return to a state of totipotency. This conversion to totipotency is complex, not fully understood, and the subject of recent research. Research in 2011 showed that cells can differentiate not into a fully totipotent cell, but instead into a “complex cell variation” of totipotency. Stem cells that resemble totipotent blastomeres from embryos at the 2-cell stage can arise spontaneously in mouse embryonic stem cell cultures and can also be induced to arise more frequently in vitro through down-regulation of CAF-1 chromatin assembly activity.
Human development is a model that can be used to describe how totipotent cells arise. Human development begins when a spermatozoon fertilizes an egg and the resulting fertilized egg creates a single totipotent cell, a zygote. In the first few hours after fertilization, this zygote divides into identical totipotent cells, which can then develop into any of the three human germ layers (endoderm, mesoderm, or ectoderm), or into cells in the placenta (cytotrophoblast or syncytiotrophoblast). After reaching a 16-cell stage, the totipotent morula cells differentiate into cells that will eventually become the blastocyst’s inner cell mass or outer trophoblasts. About four days after fertilization and after several rounds of cell division, these totipotent cells begin to specialize. The inner cell mass, the source of embryonic stem cells, becomes pluripotent.
Research in Caenorhabditis elegans suggests that multiple mechanisms, including RNA regulation, may play a role in maintaining totipotency at different stages of development in some species. Work in zebrafish and mammals suggests an additional interaction between miRNAs and RNA-binding proteins (RBPs) in determining developmental differences.
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