Researchers Reconstruct Early Stages of Embryo Development

Researchers Reconstruct Early Stages of Embryo Development

Tuesday, 04 November 2014

Researchers at the University of Cambridge have managed to reconstruct the early stage of mammalian development using embryonic stem cells, showing that a critical mass of cells – not too few, but not too many – is needed for the cells to being self-organising into the correct structure for an embryo to form.

All organisms develop from embryos: a cell divides generating many cells. In the early stages of this process, all cells look alike and tend to aggregate into a featureless structure, more often than not a ball. Then, the cells begin to 'specialise' into different types of cell and space out asymmetrically, forming an axis which begins to provide a structure for the embryo to develop along.

In animal embryos this stage is followed by a process known as gastrulation: a choreographed movement of the cells that, using the initial axis as a reference, positions the head and the tail, the front and the back. During the process, the cells begin to form three distinct layers: the endoderm, mesoderm and ectoderm, determining which tissues or organs the cells will then develop into.

Professor Alfonso Martinez-Arias from the Department of Genetics at the University of Cambridge, who led the research, says:

"Gastrulation was described by biologist Professor Lewis Wolpert as being 'truly the most important event in your life' because it creates the blueprint of an organism. Axis formation and gastrulation are the two central processes that initiate the development of an organism and are inextricably associated with the embryo. We have managed to recreate this for the first time in the lab."

Professor Martinez-Arias and colleagues, supported by the European Research Council and the Wellcome Trust, have reconstructed these early stages of development using mouse embryonic stem cells. Embryonic stem cells, discovered in the Department of Genetics in the 1980s (for which Sir Martin Evans was awarded the Nobel Prize in Physiology or Medicine 2007), have become an important tool for developmental biology, understanding disease, and in regenerative medicine due to the ability to give rise to all cell types in culture. Over the last few years, they have been used to 'grow' organs including the eye and the cerebral cortex; surprisingly, these structures develop without an axis.

In research published today in the journal Development, the researchers report a way to coax cells to reorganize in the manner that they do in an embryo, creating an axis and undergoing movements and organisations that mimic the process of gastrulation. Over the years researchers have been making aggregates of embryonic stem cells to obtain certain cell types, for example red blood cells. However, these aggregates lack structure and the different cell types emerge in a disorganised fashion. This is the first time that researchers have been able to elicit axis formation, spatial organisation and gastrulation-like movements from aggregates of embryonic stem cells.

The researchers show that if the number of cells aggregated initially is similar to that of a mouse embryo, the cells generate a single axis and this serves as a template for a sequence of events that mimics those of the early embryo. By manipulating the signals that the cells see at a particular time, the researchers were able to influence what type of cell they become and how they are organised. In one of the experiments, for example, activation of a particular signal at the correct time elicits the appearance of the mesoderm, endoderm and ectoderm – the precursors of all cell types – with a spatial organization similar to that of an embryo.

Using this experimental system, the researchers were able to generate the early stages of a spinal cord, which they showed forms as part of the process of gastrulation. This finding complements previous research from the University of Edinburgh and the National Institute for Medical Research which showed that embryonic stem cells can be coaxed into this spinal cord cells; however, the Cambridge researchers showed that in the embryo-like aggregates, the structural organization is more robust and allows for the polarised growth of the tissue.

Professor Martinez-Arias adds:

"It is early days but this system promises insights into the early stages of development and what determines the specification of the different cell types. This will allow more robust protocols for differentiation with cues that mimic those that the cells are subject to in embryos.”

"Most significantly, the system will provide a means to test, experimentally, how a homogeneous group of cells organizes itself in space, a central process in the development of any organism, and the ability to recreate in culture the niches that adult stem cells create during embryogenesis and which have remained elusive experimentally."

Source: University of Cambridge

Contact: Craig Brierley

References:

Symmetry breaking, germ layer specification and axial organisation in aggregates of mouse ES cells

Susanne C. van den Brink, Peter Baillie-Johnson, Tina Balayo, Anna-Katerina Hadjantonakis, Sonja Nowotschin, David A. Turner, and Alfonso Martinez Arias

Development 2014 141:4231-4242; doi:10.1242/dev.113001

Wnt/β-catenin and FGF signalling direct the specification and maintenance of a neuromesodermal axial progenitor in ensembles of mouse ES cells

David A. Turner, Penelope C. Hayward, Peter Baillie-Johnson, Pau Rué, Rebecca Broome, Fernando Faunes, and Alfonso Martinez Arias

Development; 2014 141:4243-4253; doi:10.1242/dev.112979

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ZenMaster

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For more on stem cells and cloning, go to CellNEWS at

http://cellnews-blog.blogspot.com/

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Researchers Coax Human Embryonic Stem Cells to Organize Themselves Using Geometry

Using Geometry, Researchers Coax Human Embryonic Stem Cells to Organize Themselves

Tuesday, 01 July 2014

About seven days after conception, something remarkable occurs in the clump of cells that will eventually become a new human being. They start to specialize. They take on characteristics that begin to hint at their ultimate fate as part of the skin, brain, muscle or any of the roughly 200 cell types that exist in people, and they start to form distinct layers.

Forty-two hours after they began to differentiate,

embryonic cells are clearly segregating into

endoderm (red), mesoderm (blue) and ectoderm

(black). Researchers say the key to achieving this

patterning in culture is confining the colonies

geometrically. Credit: Brivanlou Lab, the

Rockefeller University. 

Although scientists have studied this process in animals, and have tried to coax human embryonic stem cells into taking shape by flooding them with chemical signals, until now the process has not been successfully replicated in the lab. But researchers led by Ali Brivanlou, Robert and Harriet Heilbrunn Professor and head of the Laboratory of Stem Cell Biology and Molecular Embryology at The Rockefeller University, have done it, and it turns out that the missing ingredient is geometrical, not chemical.

"Understanding what happens in this moment, when individual members of this mass of embryonic stem cells begin to specialize for the very first time and organize themselves into layers, will be a key to harnessing the promise of regenerative medicine," Brivanlou says.

"It brings us closer to the possibility of replacement organs grown in petri dishes and wounds that can be swiftly healed."

In the uterus, human embryonic stem cells receive chemical cues from the surrounding tissue that signal them to begin forming layers — a process called gastrulation. Cells in the centre begin to form ectoderm, the brain and skin of the embryo, while those migrating to the outside become mesoderm and endoderm, destined to become muscle and blood and many of the major organs, respectively.

Brivanlou and his colleagues, including postdocs Aryeh Warmflash and Benoit Sorre as well as Eric Siggia, Viola Ward Brinning and Elbert Calhoun Brinning Professor and head of the Laboratory of Theoretical Condensed Matter Physics, confined human embryonic stem cells originally derived at Rockefeller to tiny circular patterns on glass plates that had been chemically treated to form "micropatterns" that prevent the colonies from expanding outside a specific radius. When the researchers introduced chemical signals spurring the cells to begin gastrulation, they found the colonies that were geometrically confined in this way proceeded to form endoderm, mesoderm and ectoderm and began to organize themselves just as they would have under natural conditions. Cells that were not confined did not.

By monitoring specific molecular pathways the human cells use to communicate with one another to form patterns during gastrulation — something that was not previously possible because of the lack of a suitable laboratory model — the researchers also learned how specific inhibitory signals generated in response to the initial chemical cues function to prevent the cells within a colony from all following the same developmental path.

"At the fundamental level, what we have developed is a new model to explore how human embryonic stem cells first differentiate into separate populations with a very reproducible spatial order just as in an embryo," says Warmflash.

"We can now follow individual cells in real time in order to find out what makes them specialize, and we can begin to ask questions about the underlying genetics of this process."

The research also has direct implications for biologists working to create "pure" populations of specific cells, or engineered tissues consisting of multiple cell types, for use in medical treatments.

"These cells have a powerful intrinsic tendency to form patterns as they develop," Warmflash says.

"Varying the geometry of the colonies may turn out to be an important tool that can be used to guide stem cells to form specific cell types or tissues."

Source: Rockefeller University 

Contact: Zach Veilleux

Reference:

A method to recapitulate early embryonic spatial patterning in human embryonic stem cells

Aryeh Warmflash, Benoit Sorre, Fred Etoc, Eric D. Siggia and Ali H. Brivanlou

Nature Methods online: June 29, 2014, doi:10.1038/nmeth.3016

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ZenMaster

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For more on stem cells and cloning, go to CellNEWS at

http://cellnews-blog.blogspot.com/

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