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quarta-feira, maio 26, 2010

Study Sheds Light Into the Nature of Embryonic Stem Cells

ScienceDaily (May 25, 2010) — New insight into what stem cells are and how they behave could help scientists to grow cells that form different tissues.

A study at the University of Edinburgh has shown that embryonic stem cells consist of cells that switch back and forth between precursors of different cell types. This may be linked to their potential to become any cell type in the body.

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The findings could help scientists catch embryonic stem cells at exactly the right point when they are primed to differentiate into cells that form specific tissues.

The study indicates that embryonic stem cells are not a single cell type as previously thought, but comprise a mixture of different cell types from the early embryo that can transform themselves from one type to another.

Scientists previously thought that embryonic stem cells were only able to become the embryonic precursors for adult cells, a property known as pluripotency.

The researchers have now found that they can also turn into cells associated with the placenta. These cells -- known as the primitive endoderm -- form the yolk sac that helps provide nutrients to the early embryo.

Read more here/Leia mais aqui: Science Daily


Functional Heterogeneity of Embryonic Stem Cells Revealed through Translational Amplification of an Early Endodermal Transcript

Maurice A. Canham1¤, Alexei A. Sharov2, Minoru S. H. Ko2,Joshua M. Brickman1*

1 Institute for Stem Cell Research, Medical Research Council – Centre for Regenerative Medicine, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom, 2 Laboratory of Genetics, National Institute on Aging, National Institutes of Health Biomedical Research Center, Baltimore, Maryland, United States of America


ES cells are defined as self-renewing, pluripotent cell lines derived from early embryos. Cultures of ES cells are also characterized by the expression of certain markers thought to represent the pluripotent state. However, despite the widespread expression of key markers such as Oct4 and the appearance of a characteristic undifferentiated morphology, functional ES cells may represent only a small fraction of the cultures grown under self-renewing conditions. Thus phenotypically “undifferentiated” cells may consist of a heterogeneous population of functionally distinct cell types. Here we use a transgenic allele designed to detect low level transcription in the primitive endoderm lineage as a tool to identify an immediate early endoderm-like ES cell state. This reporter employs a tandem array of internal ribosomal entry sites to drive translation of an enhanced Yellow Fluorescent Protein (Venus) from the transcript that normally encodes for the early endodermal marker Hex. Expression of this Venus transgene reports on single cells with low Hex transcript levels and reveals the existence of distinct populations of Oct4 positive undifferentiated ES cells. One of these cells types, characterized by both the expression of the Venus transgene and the ES cells marker SSEA-1 (V+S+), appears to represent an early step in primitive endoderm specification. We show that the fraction of cells present within this state is influenced by factors that both promote and suppress primitive endoderm differentiation, but conditions that support ES cell self-renewal prevent their progression into differentiation and support an equilibrium between this state and at least one other that resembles the Nanog positive inner cell mass of the mammalian blastocysts. Interestingly, while these subpopulations are equivalently and clonally interconvertible under self-renewing conditions, when induced to differentiate both in vivo and in vitro they exhibit different behaviours. Most strikingly when introduced back into morulae or blastocysts, the V+S+ population is not effective at contributing to the epiblast and can contribute to the extra-embryonic visceral and parietal endoderm, while the V−S+ population generates high contribution chimeras. Taken together our data support a model in which ES cell culture has trapped a set of interconvertible cell states reminiscent of the early stages in blastocyst differentiation that may exist only transiently in the early embryo.

Author Summary

Embryonic stem (ES) cells are karyotypically normal, embryo-derived cell lines that are pluripotent, i.e. capable of generating all the cell types of the future organism, but not the extra-embryonic lineages. What gives ES cells this unique capacity? Here, we use a fluorescent reporter cell line that employs translational amplification to visualize single ES cells expressing low levels of lineage-specific genes. With this reporter we split ES cell cultures into two fractions that both express certain stem cell markers but only one of which expresses low levels of an endodermal marker gene. Following purification, single cells from either fraction are equally competent to re-establish a heterogeneous culture. However, when challenged to differentiate immediately after purification, each exhibits strong lineage bias, with the endoderm marker-expressing fraction unexpectedly able to contribute to the extra-embryonic endoderm in chimeric embryos. These data suggest that ES cells expand under steady-state conditions as a heterogeneous mix of lineage-biased—but not lineage-committed—cell types. We propose that these observed uncommitted substates exist temporarily in vivo, but are perpetuated in vitro under the selectively self-renewing conditions of ES cell culture. Our findings suggest that pluripotency is determined by the capacity of a mixed population of lineage-biased intermediates to commit to different cell fates in specific contexts.

Citation: Canham MA, Sharov AA, Ko MSH, Brickman JM (2010) Functional Heterogeneity of Embryonic Stem Cells Revealed through Translational Amplification of an Early Endodermal Transcript. PLoS Biol 8(5): e1000379. doi:10.1371/journal.pbio.1000379

Academic Editor: Hiroshi Hamada, Osaka University, Japan

Received: June 1, 2009; Accepted: April 15, 2010; Published: May 25, 2010

This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

Funding: This research was supported by the Medical Research Council (MRC) (G0701428), Scottish Funding Council and, in part, by the Intramural Research Program of the National Institutes of Health (NIH), National Institute on Aging (Z01AG AG000656, Z01AG000662). JMB is supported by an MRC Senior Non-Clinical Fellowship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Abbreviations: AVE, anterior visceral endoderm; β-gal, β-galactosidase; dpc, days post coitum; EBs, embryoid bodies; ECATs, ES cell specific transcripts; ES cell, embryonic stem cell; FDR, false discovery rate; GO, gene ontology; HexRS, Hex RedStar; HV, hex venus; ICM, inner cell mass; IRES, internal ribosomal entry site; PrEc, primitive ectoderm; PrEn, primitive endoderm; VE, visceral endoderm; Xen cells, extra-embryonic endoderm cells

* E-mail:

¤ Current address: HPV Research Group, Division of Pathology, School of Molecular and Clinical Medicine, University of Edinburgh, Edinburgh, United Kingdom