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From pluripotency to differentiation:
laying foundations for the body pattern
in the mouse embryo
Magdalena Zernicka-Goetz1 and Anna-Katerina Hadjantonakis2
rstb.royalsocietypublishing.org
Introduction
Cite this article: Zernicka-Goetz M,
Hadjantonakis A-K. 2014 From pluripotency to
differentiation: laying foundations for the body
pattern in the mouse embryo. Phil.
Trans. R. Soc. B 369: 20130535.
http://dx.doi.org/10.1098/rstb.2013.0535
One contribution of 14 to a Theme Issue
‘From pluripotency to differentiation: laying
the foundations for the body pattern in the
mouse embryo’.
Subject Areas:
cellular biology, developmental biology
Keywords:
mouse embryo, pluripotency, differentiation
Authors for correspondence:
Magdalena Zernicka-Goetz
e-mail: [email protected]
Anna-Katerina Hadjantonakis
e-mail: [email protected]
1
Department of Physiology, Development and Neuroscience, Gurdon Institute, University of Cambridge,
Downing St., Cambridge CB2 3DY, UK
2
Developmental Biology Program, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center,
New York, NY 10065, USA
A central question in contemporary stem and developmental biology and
modern medicine is how developmental potential becomes progressively
restricted as development proceeds. How totipotency—namely the ability to
give rise to both embryonic and extraembryonic tissues and, in an ideal situation to the entire organism—is lost. How do cells choose between alternative
fates, and how do they stereotypically organize themselves into developing tissues? Today, one can strip differentiated cells of their identity by inducing
pluripotency, but it is still largely unknown how pluripotency emerges in its
native context within the early embryo. Indeed, the only true pluripotent cell
population—the epiblast—exists in vivo within the early mammalian embryo.
Regenerative medicine holds enormous promise for patients with degenerative diseases and tissue lesions. However, the foundations for safe and effective
treatments depend on a deep understanding of the mechanisms of lineage commitment and specification, and the optimal environment for maintaining the
phenotypic stability of pluripotent stem cells, and the specific lineages that
can be directed to differentiate from them. These foundations will, in large
part, be laid through experimental investigation of factors affecting pluripotency and lineage specification during development, in embryos and in stem
cell culture paradigms.
Recently, there has been a reinvigorated interest in studies of the earliest
stages of mammalian development. Studies on early mammalian embryos,
exemplified by the mouse, have seen a new beginning. We have begun to lay
a molecular and cell biological framework of the critical events surrounding
early development of the mouse embryo. In part, this reflects renewed interest
in cellular differentiation and reprogramming with a focus on pluripotent cell
states, and the potential for directed cellular differentiation. Experimental
and conceptual progress has been galvanized by the availability of new experimental approaches for following the contributions of individual cells to
development of the mouse embryo, coupled to a molecular understanding of
the very first cell fate decisions, and the cellular events that precede them.
This rebirth of interest in the earliest stages of mouse development has given
new insights into our understanding of how cells evolve specific fates and
how they are assigned to particular lineages, of the changes in cellular properties that facilitate fate decisions, and of signalling pathways that control these
events. It is now timely to link these early events to later developmental processes that immediately precede and include gastrulation, the process during
which the three founder tissues of the body are established.
This collection of review articles and hypotheses presents a contemporary
overview of our current understanding of the earliest stages of mouse development, and the biology of embryo-derived pluripotent stem cells. The focus is
upon the role of signalling pathways, transcriptional and epigenetic mechanisms, and the activity of genetic networks that contribute to cell lineage
commitment and tissue morphogenesis.
It is within the very earliest embryonic stages that fate decisions send cells in
the direction of either pluripotency or differentiation. In the first set of articles,
the authors address the earliest assignments of cells to particular lineages, the
& 2014 The Author(s) Published by the Royal Society. All rights reserved.
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Phil. Trans. R. Soc. B 369: 20130535
the pre- and postimplantation periods of embryonic development, and are beginning to reveal how cells establish their
fates at this stage. Zernicka-Goetz and co-workers take us
from the early preimplantation cell fate decisions through
implantation in the first steps towards understanding the
mechanism of how a simple ball of blastocyst cells transforms
into the more complex structure of the egg cylinder [6]. They
present a new hypothesis that a formation of the epiblast
rosette at the time of implantation is the primordium that,
with the help of extraembryonic tissues, provides the essential
foundation for the future body in mammalian embryos.
Papanayoutou & Collignon [7], whose work led to discovery of the role of Activin/Nodal signalling in the early mouse
embryo, discuss the role of this signalling pathway at pre- and
early postimplantation stages of development, and how it
controls the expression of target genes in ESCs. They review
the TGFb-related ligands that determine the activity of
Activin/Nodal signalling, and the Smad2/3-dependent
mechanisms underlying developmental progression. These
recent studies indicate a role for Nodal signalling earlier
than previously expected.
The body plan of the developing embryo is established,
and the cardinal axes (AP, dorsal–ventral and left–right) are
elaborated at early postimplantation stages of development.
Establishment of the AP axis is governed by complex series
of interactions between the various adjacent tissues of the
early embryo. A key role is played by a specialized population
of migratory epithelial cells, referred to as the anterior visceral endoderm (AVE), which are derived from the primitive
endoderm of the blastocyst. Stower & Srinivas [8] discuss
our current understanding of the formation and function of
the AVE.
Diffusible growth factors, including members of the FGF,
Wnt, bone morphogenetic protein and Hedgehog families,
emanating from localized areas travel through the extracellular space and reach their target cells to specify the cell fate,
and coordinate tissue formation. Matsuo & Kimura-Yoshida
[9] discuss the current understanding of mechanisms by
which these growth factors travel great distances to their
target cells to control signalling activity within the early
mouse embryo. They discuss recent studies which reveal
that heparan sulfate proteoglycans that are located on the surface of cells, and within the extracellular matrix, play crucial
roles in regulating the extracellular distribution of growth factors, and thereby are likely to provide an additional level of
regulation in growth factor-mediated signalling.
The pluripotent epiblast is transformed into the three
definitive germ layers (ectoderm, mesoderm and endoderm),
as well as the primordial germ cells (PGCs), at gastrulation.
Surani and co-workers discuss how unlike the majority of epiblast cells which undergo differentiation towards somatic cell
lineages, PGCs initiate a unique cellular programme [10].
They discuss the concept of enhancer function in relation to
the factors which govern a PGC identity, namely how these
PGC-specific factors work by suppression of somatic differentiation genes, and concomitantly regulate the re-expression of
pluripotency and germ-cell-specific genes.
Tam and colleagues, whose work was key in compiling the
fate map of the mouse gastrula, and in understanding critical
signalling events leading to development of endoderm, discuss
how the formation of definitive endoderm in embryoid bodies
follows a similar process to germ layer formation as from the
epiblast, requiring an initial de-epithelialization event and
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changes in cellular properties that facilitate fate decisions and the
participating signalling pathways. When and how the cells first
become different is of great importance in all model systems, and
this knowledge has been lacking in mammalian development as
mammalian embryos are difficult to study in this respect—they
can compensate for perturbations. Takaoka & Hamada [1],
experts in early mouse development and whose work allowed
establishment of symmetry breaking events leading to
development of the anterior–posterior (AP) axis and left–right
asymmetry, present a new hypothesis of how pre-patterning
might become established in the mammalian embryo leading
to the first cell fate specification—formation of the inner cell
mass and trophectoderm. This hypothesis suggests that epigenetic asymmetry in zygotes, such as histone methylation and
DNA methylation between maternal and paternal genomes,
might explain pre-patterning and how previously discovered
effects of the inheritance of animal and vegetal parts of the
zygote bias cell fate from the 4-cell stage onwards.
The pluripotent epiblast represents the founding cell
population of the entire embryo, whereas the extraembryonic
tissues, primitive endoderm and trophectoderm, both support its development. Boroviak & Nichols [2] discuss how
following the early cleavage divisions and formation of the
blastocyst, cells of the inner cell mass lose totipotency. They
describe how developing epiblast cells transiently attain the
state of naive pluripotency, and acquire competence to selfrenew in vitro as mouse embryonic stem cells (ESCs).
ESCs possess the capacity to indefinitely self-renew in culture. Morgani & Brickman [3] raise the question of whether
ESCs, which have been historically defined as pluripotent, can,
under certain conditions, acquire a state of totipotency. Can
they revert to an earlier stage of embryonic development from
which they are derived? These authors discuss cell state transitions between pluripotency and totipotency in culture, and
the signalling and transcriptional networks that control them.
Following on, Hermitte & Chazaud [4], whose work
revealed the paradigm shifting salt-and-pepper distribution of
lineage-restricted progenitor cells within the inner cell mass of
the blastocyst, bring us back to the embryo to provide an overview of recent findings that lead to different hypotheses
concerning the execution of the second cell fate decision—
formation of the pluripotent epiblast and the second
extraembryonic tissue, the primitive endoderm. It is now
clear that this decision is not set up by cell position, as thought
for so long, but rather by pre-specification of cells within the
inner cell mass that will sort to their final positions. Recent
studies have cast more light on the roles of fibroblast growth
factor (FGF) signalling, cell heterogeneity and cell polarity on
the pre-specification of cell fate and cell sorting at the time
when cells are faced with a choice to commit to a pluripotent
epiblast fate.
Emergent epiblast exhibits a state of naive pluripotency.
Kalkan & Smith [5] discuss how, for normal development
to proceed, the naive state must be changed for lineage specification to occur. They discuss how in vitro differentiation
of naive ESCs cultured under defined conditions provides a
system for interrogating this critical transition, and discuss
the incipient road map of events.
The peri-implantation stage of development has previously been hidden from view, a conceptual ‘black box’.
However, the recent development of culture methods and
ways of imaging cells in situ in embryos is bringing a thus
far missing view of cellular and molecular events that link
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Phil. Trans. R. Soc. B 369: 20130535
The endoderm is one of three definitive germ layers generated at gastrulation. Cells descended from the endoderm will
constitute the multipotent progenitors of the respiratory and
digestive tracts, the endocrine glands and auditory and urinary
systems. In the final review, Hadjantonakis and co-workers
discuss their unexpected insights that lead to revision of our
thinking on the cellular dynamics that drive the initial phases
of endoderm formation [13]. The authors show that segregation
between the embryonic and extraembryonic lineages may not
be as strict as previously believed, by suggesting that the endoderm is composed of cells derived from both epiblast-derived
definitive endoderm and so-called extraembryonic endoderm,
descended from the primitive endoderm.
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subsequent re-epithelialization [11]. This requires the activity of
both TGFb signalling at the formative phase of endoderm
differentiation and specifically of Nodal, which cannot be substituted for by Activin A, which is commonly used as its in vitro
surrogate.
Parfitt & Shen [12] extend the discussion of how development of the mammalian embryo is coordinated from
blastocyst to gastrulation stages. They review how regulatory
networks constructed for different stem cell types relate to
the corresponding networks in vivo within the embryo.
They also highlight several studies that have contributed to
our current understanding of the molecular regulation of
the blastocyst –gastrula transition.