1. Developmental Biology
Download icon

Totipotency: A developmental insurance policy

  1. Nestor Saiz
  2. Anna-Katerina Hadjantonakis  Is a corresponding author
  1. Memorial Sloan Kettering Cancer Center, United States
Insight
  • Cited 0
  • Views 823
  • Annotations
Cite this article as: eLife 2017;6:e26260 doi: 10.7554/eLife.26260

Abstract

Why does a totipotent state linger within the inner cell mass of mouse embryos?

Main text

The very first decisions in the life of a mammal are made even before the embryo implants into the womb. During this time, as the number of cells in the embryo increases from one to two to four and so on, the cells start to specialize to form distinct lineages. The first choice a cell faces is whether to join a cell population called the inner cell mass and become part of the embryo, or to join the trophectoderm lineage and become part of the placenta.

The biology of this cell fate decision has been a subject of intrigue and experimental pursuit for over half a century. Building on landmark work by the late Krystof Tarkowski, Martin Johnson and others (Johnson and Ziomek, 1981; Tarkowski and Wróblewska, 1967), recent studies have demonstrated the importance of the Hippo signaling pathway – a pathway well known for regulating cell growth and death – in this process (reviewed in Sasaki, 2017). These studies have established how the polarity and position of a cell either cause activation of the Hippo pathway in the inner cells of the embryo, or inhibit it in the outer cells of the embryo to promote the expression of genes encoding a trophectoderm identity.

Previous attempts to determine the exact timing of when cells commit to either the inner cell mass (ICM) or the trophectoderm (TE) lineage yielded somewhat conflicting results. Now, in eLife, Janet Rossant and colleagues – including Eszter Posfai of the Hospital for Sick Children in Toronto as first author – report how they have used the thread and needle of fluorescent reporters and single-cell transcriptomics to stitch together classic and recent findings on this topic (Posfai et al., 2017).

A transcription factor called CDX2 has a central role in triggering the TE transcriptional program. The expression of CDX2 in cells that go on to become part of the TE relies on a complex called TEAD-YAP, which is activated by inhibition of the Hippo pathway in the outer cells of the embryo (Nishioka et al., 2009). Posfai et al. used a CDX2-GFP fusion (McDole and Zheng, 2012) to sort CDX2-positive and CDX2-negative cells, followed by single-cell RNA sequencing, to determine how the TE and ICM transcriptional programs became established as the embryo developed from the 16-cell stage to the 32-cell stage.

These data raise the question of what the progressive stabilization of cell fate might tell us about commitment to either lineage. Could the expression of TE genes restrict cells to a TE fate even when challenged experimentally (i.e., when placed in a new context)? In the assays used to test these questions, either single cells have to be implanted into genetically-distinct host embryos to generate a chimera, or an embryo needs to be rebuilt from isolated cells of one particular type (inner or outer). The contribution of daughter cells to the resulting embryo will reveal details about lineage commitment in the parental cells. With these techniques, the labs of Tarkowski, Johnson and Rossant previously established that both the inner and outer cells remain totipotent – that is, they can give rise to the ICM and TE lineages – until the 16-cell stage, with some inner cells remaining totipotent until the 32-cell stage (Suwińska et al., 2008; Rossant and Vijh, 1980; Ziomek et al., 1982).

Posfai et al. perform a contemporary version of these classic experiments using the CDX2-GFP fluorescent genetic marker, rather than cell position, to discriminate between prospective TE and ICM cells. They were able to precisely match cell fate commitment to the relevant gene expression profile of individual cells for both (GFP-positive and GFP-negative) populations at successive stages of development. This allowed them to confirm previous results and to paint a detailed picture of the molecular players that are potentially involved in stabilizing these two cell fates. For cells expressing CDX2, cell fate is basically sealed soon after blastocyst formation (at the 32-cell stage), and they are unable to give rise to the ICM. ICM cells, on the other hand, delay their commitment by an additional cell cycle, up until some point between the 32- and 64-cell stages (Figure 1). Posfai et al. confirm these results by genetically and pharmacologically modulating the activity of the Hippo pathway, which connects apico-basal cell polarity (or the lack of it in ICM cells) to gene expression.

Cell differentiation in mammalian embryos.

Cells that develop into the trophectoderm (TE) express the transcription factor CDX2 (denoted here as CDX2+) and commit to their cell fate at around the 32-cell stage. Cells that will develop into the inner cell mass (ICM) keep their options open and only commit to their cell fate at a time between the 32- and 64-cell stage; these cells do not express CDX2 (denoted here as CDX2-). As embryos grow from the 32- to the 64-cell stage, ICM cells start to differentiate into two new cell lineages: the embryonic epiblast (future fetus) and the extra-embryonic primitive endoderm (future yolk sac).

This raises the question of why commitment to the ICM lineage takes place later during development. Perhaps it is no coincidence that ICM cells gradually begin to make their next cell fate choice during this time window. It is at this time that ICM cells make a decision to become epiblast (future fetus) versus primitive endoderm (future yolk sac). Therefore, the ICM may not be a cell fate per se, but rather a transitory state that lasts only until all the cells in the embryo have been allocated to one of the three lineages that make up the blastocyst. An asynchrony in making these early fate decisions could therefore reflect a developmental insurance policy: a strategy to guarantee that enough cells differentiate for each of the cell types that lay the foundation for all embryonic and extra-embryonic tissues (Saiz et al., 2016).

References

    1. Tarkowski AK
    2. Wróblewska J
    (1967)
    Development of blastomeres of mouse eggs isolated at the 4- and 8-cell stage
    Journal of Embryology and Experimental Morphology 18:155–180.

Article and author information

Author details

  1. Nestor Saiz

    Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, United States
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-0637-791X
  2. Anna-Katerina Hadjantonakis

    Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, United States
    For correspondence
    Hadjanta@mskcc.org
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-7580-5124

Publication history

  1. Version of Record published: March 28, 2017 (version 1)

Copyright

© 2017, Saiz et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 823
    Page views
  • 140
    Downloads
  • 0
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Download citations (links to download the citations from this article in formats compatible with various reference manager tools)

Open citations (links to open the citations from this article in various online reference manager services)

Further reading

    1. Developmental Biology
    Georges Raad et al.
    Research Article Updated

    Obesity is a growing societal scourge. Recent studies have uncovered that paternal excessive weight induced by an unbalanced diet affects the metabolic health of offspring. These reports mainly employed single-generation male exposure. However, the consequences of multigenerational unbalanced diet feeding on the metabolic health of progeny remain largely unknown. Here, we show that maintaining paternal Western diet feeding for five consecutive generations in mice induces an enhancement in fat mass and related metabolic diseases over generations. Strikingly, chow-diet-fed progenies from these multigenerational Western-diet-fed males develop a ‘healthy’ overweight phenotype characterized by normal glucose metabolism and without fatty liver that persists for four subsequent generations. Mechanistically, sperm RNA microinjection experiments into zygotes suggest that sperm RNAs are sufficient for establishment but not for long-term maintenance of epigenetic inheritance of metabolic pathologies. Progressive and permanent metabolic deregulation induced by successive paternal Western-diet-fed generations may contribute to the worldwide epidemic of metabolic diseases.

    1. Developmental Biology
    2. Stem Cells and Regenerative Medicine
    Jack F Cazet et al.
    Research Article Updated

    During whole-body regeneration, a bisection injury can trigger two different types of regeneration. To understand the transcriptional regulation underlying this adaptive response, we characterized transcript abundance and chromatin accessibility during oral and aboral regeneration in the cnidarian Hydra vulgaris. We found that the initial response to amputation at both wound sites is identical and includes widespread apoptosis and the activation of the oral-specifying Wnt signaling pathway. By 8 hr post amputation, Wnt signaling became restricted to oral regeneration. Wnt pathway genes were also upregulated in puncture wounds, and these wounds induced the formation of ectopic oral structures if pre-existing organizers were simultaneously amputated. Our work suggests that oral patterning is activated as part of a generic injury response in Hydra, and that alternative injury outcomes are dependent on signals from the surrounding tissue. Furthermore, Wnt signaling is likely part of a conserved wound response predating the split of cnidarians and bilaterians.