stem cellsare distinguished by their degreePerformance, which refers to their variable ability to differentiate into different cell types. Totipotent cells are the most potent of all stem cells and their definition is important to research and the field of regenerative medicine.
Definition of totipotent stem cells
There are two definitions of totipotent stem cells, reflecting the inherent technical difficulty of characterizing them.1:
- A stem cell is a single cell that, with appropriate maternal support (stricter definition), can give rise to a new organism.
- A totipotent cell is a cell capable of giving rise to all extraembryonic, body and human tissues.germline(less strict definition)
The original totipotency test was performed on mice by Tarkowski (1959).2, which isolated a single blastomere (cells formed by division of theZygote, consisting of 2 to 16 cells), placed it in an empty spacetransparent areaand monitored their development in live-born puppies.
This approach is not bulletproof; the failure ofBlastomereSupportChimeraDevelopment may indicate limitations associated with the reconstructed embryo rather than limited developmental potential of the cell in question.
For this reason, the second, less strict definition of totipotency is also widely used.
Two definitions inevitably lead to confusion, mainly because the term "totipotency" is often misused in the literature. It has been argued that this creates unnecessary ethical controversy with practical and political implications.3
Sometimes the term "totipotent" is attributed to cells that participate only in one embryonic process; However, this does not mean that they can necessarily produce an organism. Other common reasons for misclassification are assuming that expression of early embryonic markers indicates totipotency and considering partial or superficial resemblance to an embryo as evidence of totipotency.3
Totipotent stem cells differ from pluripotent cells, which can differentiate into cells from all three germ layers, and from multipotent cells, which are less potent.
Why all the clamor about totipotent stem cells?
Totipotent stem cells are unique in that they have greater potential for development compared to other stem cells. The ability to isolate and culture totipotent stem cells creates many therapeutic and research opportunities.1with reference to:
- Studying zygotic genome activation (the point at which development is controlled solely by the zygotic genome rather than the maternal genome)4
- Epigenome rewiring (treatment of pathological conditions caused by epigenetic changes)5
- Understand early embryonic development in more detail
- create human-animalThe chimera6(Theoretically, animals with human organs could help in disease modeling, drug development, and transplantation)1
Stem cells are increasingly used as model systems in research. Distinguishing between stem cell types depends on understanding the embryological pathways and the factors that define their pathways.
Source of totipotent stem cells
Ödiploid Zygotethe cell is totipotent. Stem cells also exist in later divisions of theZygote, before the rise ofTrophectoderm line(which occurs about four days after fertilization, depending on the species).
To understand the origin of totipotent stem cells, it is helpful to become familiar with the early stages of embryonic development and associated terminology.
The table below highlights the important stages of early embryonic development and shows the crucial point at which totipotent stem cells cease to exist and pluripotent cells emerge.
Main stages of early embryonic development
*In order to provide a consistent developmental timeline of mammalian embryology, the Carnegie Institution for Science developed a set of 23 stages known as "Carnegie stages," where the stages are defined by morphological development rather than directly by age or size .
**Note: The number of compaction cells varies between mammalian species
When does a totipotent stem cell change?
Totipotent cells cease to exist around the time the inner cell mass is formed, when theTrophectoderm lineIt's established.1Totipotent stem cells will divide and differentiate to give rise to cells that will develop into one of three germ layers:
| The cell | Performance | determination |
| Epiblasto | pluripotent | From the epiblast arise the three primary germ layers (ectoderm, entoderm, mesoderm), which will all formsomatic linesmore himgermline. |
| Hypoblast | Not pluripotent, not totipotent | The extraembryonic structure arises from the hypoblastprimitives Endoderm, that is, the yolk sac that provides nutrients to the embryo when the primitive placental circulation is established. |
| Trophoblast | Not pluripotent, not totipotent | The trophoblast gives rise to several extraembryonic structures that allow implantation into the uterine wall, secretes human chorionic gonadotropin to allow the secretion of progesterone from the uterus.yellow bodyand form the chorion (fetal part of the placenta). |
glossary
Blastomer: Cells created by dividing theZygote, consisting of 2 to 16 cells
Chimera:An organism composed of a mixture of different cell populations descended from more than oneZygote(either of the same species or of different species). It can be formed by various processes such as B. the mixing of early embryos or the transplantation of tissues of different developmental stages.
Yellow body:Endocrine structure that develops after the ovum is released from an ovarian follicle
diploid:With two sets of chromosomes (a total of 46 in humans), with one member of each pair of chromosomes coming from the egg and one from the sperm. Eggs and sperm are haploid as each has 23 chromosomes.
germline:Cells that become sperm or egg cells
primitives Endoderm:Or hypoblast-derived yolk sac
Somatic lines:All cells in the body except germ cells
Trophectoderm line:Cell line that gives birth to the trophoblast of the placenta and supports the inner cell mass
Transparent area:Extracellular lining that surrounds the mammalian ovum and is vital for fertilization
Zygote: diploidCell resulting from the fusion of a sperm with an egg
Properties of totipotent stem cells
Efforts have been made to establish methods for stabilizing or generating cells with expanded developmental potential compared to established embryonic stem cell lines.
Through this work, usually in mice, the researchers have found some pieces of the totipotency puzzle.7This was achieved in part by analyzing the gene regulatory network active early in mammalian embryogenesis and by screening chemicals for their ability to modulate stem cell gene expression.
Since it is not practicable in all cases to determine totipotency in cells using the gold standard mentioned above (transfer of a blastomere to atransparent area, transfer it to a mother and see if it favors the development and birth of live young), some relevant measures and criteria have been established. These methods are based on specific features of the early stages of development:
gene expression
Gene expression should be characteristic of the appropriate developmental stage, for example:
- In mice, genes specific to the two-cell stage have been identified (Zscan4, Dux, Eifa, Zfp352, Tcstv1/3 and Tpodz1-5).1
- Stem cells must be missing key genes for pluripotency (Pou5f1, Media2,minanoghave been identified in mice)8
- 4 October, a single transcription factor, can indicate developmental stage.4 Octoberis encoded byPOU5F1and it belongs to a family of transcription factors that activate the expression of their target genes.4 Octoberit is expressed in germ cells, embryonic stem cells, and whole embryos, and expression levels vary dramatically during development. A key role for4 Octoberin embryogenesis has been identified in several species, including humans.9
mobility of the chromatin
The mobility of chromatin (the complexes of histones and DNA that condense to form chromosomes) is greater in totipotent cells than in pluripotent cells, and chromatin dynamics appear to underlie changes in cellular plasticity.10
ability to differentiate
Stem cells must have the ability to differentiatein-vitroin representative cells of the three embryonic germ layers as well as trophectoderm, primitive endoderm and their derivatives.1
Establishment of stem cells with expanded potential
It has been observed that although some cells can undergo various totipotency tests, those that have been "artificially" induced may not have a normally developing counterpart.
Therefore, a term was coined to describe cells that retain the properties of totipotent stem cells: expanded potential stem cells (EPSCs).
EPSCs were established in mice from single eight-cell blastomeres and were also transformed from mouse embryonic stem cells and induced pluripotent stem cells.11This protocol has been described in detail12and the conversion takes about 2-3 weeks.
Characterizing totipotent stem cells presents many challenges and it remains to be seen whether artificially induced stem cells can be kept in isolation.
However, given that other cell lines can be derived from EPSCs and the enormous potential for regenerative medicine in biotechnology and medicine, refining the totipotency criteria is likely to remain a high priority for the field.
References:
- Baker CL, Pera MF. (2018). Detection of totipotent stem cells.stem cell22: 25-34
- Tarkovsky, AJK (1959). Attempts to develop blastomeres isolated from mouse oocytes.Nature184: 1286-1287
- Condic, M.L. (2014). Totipotency: what it is and what it is not.Stem Cells and Development23(8):796-812
- Schier, A. F. (2007). The maternal-zygotic transition: death and birth of RNAs.Science316: 406-407
- Ferrari, A.,and another. (2019). Epigenome modifiers and metabolic rewiring: new frontiers in therapeutics.Pharmacology and Therapeutics193: 178-193
- Wu, J.,and another. (2016). Stem cells and interspecies chimeras.Nature540: 51-59
- Yang, Y,and another. (2017). Generation of pluripotent stem cells with embryonic and extraembryonic potency in vivo.The cell169: 243-257
- Scharow, A. A.,and another. (2008). identification ofpou5f1,media2, minanogtarget downstream genes with confidence by applying a novel algorithm to microarray and genome-wide chromatin immunoprecipitation data over time.BMC-Genomik9: 269
- Fogarty, N. M. E.and another. (2017). Genome editing reveals a role for OCT4 in human embryogenesis.Nature550:67-73
- Boškovic, A.,and another. (2014). Increased chromatin mobility supports totipotency and precedes pluripotency.live.genes and development28:1042-1047
- what, j.and another. (2017). Establishment of stem cells with expanded mouse potential.Nature550: 393-397
- Yang, J.,and another. (2019).in-vitroEstablishment of stem cells with expanded potential from pre-implanted mouse embryos or embryonic stem cells.logs of nature14: 350-378