3613
REVIEW
Development 138, 3613-3623 (2011) doi:10.1242/dev.058172
© 2011. Published by The Company of Biologists Ltd
The evolution of nervous system patterning: insights from
sea urchin development
Lynne M. Angerer1,*, Shunsuke Yaguchi2, Robert C. Angerer1 and Robert D. Burke3
Recent studies of the sea urchin embryo have elucidated the
mechanisms that localize and pattern its nervous system. These
studies have revealed the presence of two overlapping regions
of neurogenic potential at the beginning of embryogenesis,
each of which becomes progressively restricted by separate, yet
linked, signals, including Wnt and subsequently Nodal and
BMP. These signals act to specify and localize the embryonic
neural fields – the anterior neuroectoderm and the more
posterior ciliary band neuroectoderm – during development.
Here, we review these conserved nervous system patterning
signals and consider how the relationships between them
might have changed during deuterostome evolution.
Key words: Wnt, TGF, Nodal, BMP, Dorsal-ventral axis, Anteriorposterior axis, Lefty, Chordin, Six3, FoxQ2, Ciliary band, Apical
organ, Ectoderm patterning, Neural development, Neural
patterning
Introduction
Rapid progress has been made in the last five years in understanding
the mechanisms that govern ectoderm, and hence nervous system,
patterning in sea urchin embryos. These findings have revealed that
a fascinating cascade of Wnt-dependent pathways functions during
sea urchin development to regulate Nodal and bone morphogenetic
protein (BMP) signaling. Together, these restrict the neurogenic
capacity that is initially present throughout the entire embryo to
specific locations, generating first neuroectoderm at the anterior end
of the embryo [the anterior neuroectoderm (ANE), see Glossary, Box
1] and then, within and along a band of ciliated ectoderm, the ciliary
band ectoderm (the CBE, see Glossary, Box 1). These neuroectoderm
territories are established at different times during development by
molecularly distinct signaling mechanisms in sea urchin embryos and
neural patterning occurs in the absence of any complex tissue layer
movements or interactions, factors that facilitate a clearer
understanding of how embryonic nervous systems are shaped by
these signals. Importantly, because the sea urchin shares a common
ancestor with the chordates, analyses of the mechanisms that pattern
the sea urchin nervous system can reveal the shared and derived
features of chordate neural patterning, providing us with insights into
the evolution of deuterostome (see Glossary, Box 1) nervous systems.
The organization of the nervous system is a defining feature of
an animal body plan. Vertebrates and flies possess a centralized
nervous system (see Glossary, Box 1), whereas a more primitive
and diffuse nervous system (see Glossary, Box 1) is characteristic
1
National Institute for Dental and Craniofacial Research, National Institutes of
Health, Bethesda, MD 20892, USA. 2Shimoda Marine Research Center, University of
Tsukuba, Shimoda 415-0025, Japan. 3Biochemistry/Microbiology Department,
University of Victoria, Victoria V3P 5C2, Canada.
*Author for correspondence ([email protected])
Box 1. Glossary
Aboral ectoderm. Ectoderm on the dorsal side of the embryo and
a small region anterior to the anus on the ventral side that together
form a squamous epithelium.
Anterior neuroectoderm (ANE). Ectoderm that is derived from
the animal pole region of the egg, has the potential to produce
nerves, and is restricted by canonical Wnt signaling-dependent
processes to the anterior end of the embryo.
Ascidians. A class of urochordates, also known as sea squirts, that
are marine filter-feeders.
Bilaterians. Organisms with bilateral symmetry that arose from prebilaterians and include all the remaining animal phyla divided into
two major groups: the protostomes and deuterostomes.
Centralized nervous system. A system of nerves that is restricted
to specific regions of ectoderm, such as the dorsal nerve cords of
vertebrates and ventral nerve cords of arthropods, or the ANE and
CBE of sea urchin embryos.
Cephalochordates. One of the three chordate subphyla, along
with vertebrates and urochordates. A modern representative is
amphioxus, or lancelet.
Ciliary band ectoderm (CBE). Ectoderm that has the potential to
produce nerves and is restricted by TGF signaling to a narrow band
of cells (CB) between ventral and dorsal ectoderm.
Cnidarians. A sister group to the bilateria. Includes corals, sea
anemones, jellyfish and hydroids.
Ctenophores. Commonly known as comb jellies, a pre-bilaterian
phylum of marine animals that use groups of cilia (‘combs’) to swim.
Diffuse nervous system. A system of nerves that is distributed
throughout the ectoderm and can form a network of
interconnected neurons, such as is found in pre-bilaterians.
Deuterostome. Organisms in which the blastopore becomes the
anus and a second opening forms the mouth; coeloms develop as
hollow outgrowths of the gut. Deuterostomes mentioned here
include echinoderms (sea urchins), hemichordates (Saccoglossus
kowalevskii, Ptychodera flava), cephalochordates (amphioxus),
urochordates (ascidians) and vertebrates (fish, frog, chick, mouse).
Hemichordates. A phylum of marine deuterostomes, considered
to be a sister group of the echinoderms.
Pre-bilateria. Organisms with radial symmetry; extant members
include cnidarians, ctenophores and sponges.
Protostomes. Organisms in which the blastopore becomes the
mouth and coeloms develop from solid masses of mesodermal cells.
Protostomes mentioned here include ecdysozoans (flies) and
lophotrochozoans (polychaete annelids and snails).
Urbilaterian. Refers to the hypothetical last common ancestor of
bilaterians.
of pre-bilaterians (see Glossary, Box 1), including cnidarians (see
Glossary, Box 1) (Marlow et al., 2009) and ctenophores (see
Glossary, Box 1). Transforming growth factor  (TGF) signaling
centralizes the nervous system along the dorsal side in vertebrates
and along the ventral side in flies, suggesting that the common
bilaterian (see Glossary, Box 1) ancestor had a centralized system
that was sculpted by the same patterning mechanism (Arendt et al.,
2008; Denes et al., 2007; Mizutani and Bier, 2008).
DEVELOPMENT
Summary
Development 138 (17)
M
m
D
F
E
m
m
a
R
m
L
a
Key
Animal plate
Ciliary band
Ventral ectoderm
Blastoporal endodermadjacent/dorsal ectoderm
Serotonergic
neurons
Fig. 1. The organization of the sea urchin embryo nervous
system. (A-C)Sea urchin larvae showing the position of neurons
(magenta) and serotonergic neurons (green). (A)Ventral/posterior view.
Bilateral symmetry is indicated by the dashed line; the left (L) and right
(R) sides of the larva are marked. Cell nuclei are labeled in blue.
(B)Lateral view indicating the embryonic AP and DV axes (see Box 2 for
explanation). (C)Ventral view, indicating the position of the mouth (m).
Several examples of cell bodies are indicated (arrows). Scale bar: 20m.
(D-F)Schematic representations of the larvae shown in A-C,
highlighting regions of the ectoderm and the positions of the anterior
neuroectoderm containing the animal plate (red), the ciliary band
neuroectoderm (green), mouth (m) and anus (a).
The fact that nervous systems are localized to either the dorsal
(as in vertebrates) or ventral (as in flies) side of embryos, with
the position of the mouth defining the ventral side, sparked an
interesting and strongly debated hypothesis that the dorsalventral (DV) axis inverted during evolution (De Robertis and
Sasai, 1996; Gerhart, 2000), which led investigators to examine
evolutionarily more basal organisms, such as ascidians and the
hemichordates (see Glossary, Box 1). Attention was focused on
the Saccoglossus kowalevskii hemichordate embryo, which was
considered to represent a good basal model for chordate nervous
system patterning because its anterior-posterior (AP) gene
expression patterns are the same as those observed in chordate
embryos (Lowe et al., 2003). However, the nervous system of
Saccoglossus embryos is, in part, diffuse rather than centralized,
and nerve development was found to be insensitive to changes
in BMP signaling, in contrast to other organisms (Lowe et al.,
2006). Although part of the nervous system of the Saccoglossus
embryo is distributed throughout the epidermis, other
components are localized along both the dorsal and the ventral
midlines (Bullock and Horridge, 1965; Knight-Jones, 1952;
Nomaksteinsky et al., 2009). These localization patterns are also
found in the nervous system of another enteropneust
hemichordate adult, Ptychodera flava, as judged by the
expression patterns of several neural marker genes
(Nomaksteinsky et al., 2009). However, very little is understood
about the molecular mechanisms that pattern the nervous
systems of hemichordate embryos.
By contrast, a great deal of progress has been made in recent
years in elucidating the mechanisms that pattern the neural and
non-neural ectoderm territories in embryos of the sea urchin, which
represents another basal deuterostome. Although a sister phylum
of the hemichordates, the echinoderms have routinely been
excluded from studies of the origins of the chordate nervous
system, largely because their adult body plans lack apparent AP
polarity and are thought to be highly derived. The nervous system
of echinoderms has also been said to be diffuse throughout
development. However, sea urchin embryos clearly have AP as
well as DV polarity, with nerves organized into highly restricted
patterns within the ectoderm, as is the case for centralized nervous
systems (Fig. 1). Importantly, the creation of these localized
neuroectoderm domains is now understood at a mechanistic level,
owing to recent functional studies.
Here, we review the signals and mechanisms that act to specify
and localize the anterior and ciliary band neural ectoderm fields
(the ANE and the CBE) during sea urchin embryo development.
We also compare these neural patterning mechanisms with those
used by vertebrates, highlighting the shared features and signaling
pathways utilized. Finally, based on these comparisons, we discuss
the evolution of these neural patterning mechanisms and propose
that, although the same mechanisms might have been used
throughout evolution, the regulatory relationships between these
signaling mechanisms have changed.
The primary (AP) and secondary (DV) axes of the
sea urchin embryo
Because the patterning of ectoderm into neural and non-neural fields
in the sea urchin embryo is intimately connected to the initial
mechanisms that control cell fate specification along the body axes,
we begin with a brief review of these axes and their developmental
properties. An explanation for the axial designations used in this
review and a comparison of these designations with other commonly
used nomenclatures for these axes are provided in Box 2.
The ectoderm, endoderm and mesoderm are arrayed along
the AP axis
The developmental capacities of the animal and vegetal halves of
the fertilized sea urchin egg differ, implying that the animal-vegetal
(AV) axis is established during oogenesis; isolated animal halves
become permanent blastulae in which many neurons develop,
whereas the vegetal halves often form complete embryos
(Hörstadius, 1973; Wikramanayake and Klein, 1997; Yaguchi et al.,
2006). The egg AV axis corresponds to the primary axis of the
embryo, which extends from the animal pole of the embryo to the
blastopore (see Box 2), which is the prospective anus of the larva.
The prospective ectoderm, endoderm and mesoderm are arrayed
along this axis (see Box 2). The mouth forms as a secondary
opening which, along with bilateral symmetry (Fig. 1A), is a
hallmark feature of deuterostomes.
Posterior signals are required for mesoderm and
endoderm specification
In the last decade, a paradigm has emerged that early canonical
Wnt signaling establishes AP polarity in bilaterians and marks the
site of gastrulation (Petersen and Reddien, 2009). In sea urchin
embryos, canonical Wnt signaling also plays a role in specifying
mesoderm and endoderm fates in blastomeres derived from the
vegetal regions of the egg and, based on this, the vegetal pole of
the sea urchin embryo can be considered analogous to the posterior
of vertebrate embryos (see Box 2).
DEVELOPMENT
3614 REVIEW
Development 138 (17)
The secondary (DV) axis is patterned by Nodal and BMP2/4
The zygotic expression of Nodal, a member of the TGF
superfamily, is both necessary and sufficient to specify ventral
structures and to initiate cell fate specification along the secondary
(DV) axis of sea urchin embryos (Duboc et al., 2004). During late
cleavage stages, nodal transcription is restricted to the presumptive
ventral ectoderm (see Box 2), where it upregulates its own
expression and that of four key genes – bmp2/4, chordin, lefty and
gsc – all of which play major roles in cell fate specification and
patterning along the secondary axis (Angerer et al., 2001; Angerer
et al., 2000; Bradham et al., 2009; Duboc et al., 2004; Lapraz et al.,
2009; Saudemont et al., 2010) (Fig. 2B). The mechanism that
controls the spatially restricted expression of nodal is not yet
completely understood, but might be linked to a maternal redox
gradient along the secondary axis (Coffman et al., 2004). In the
unfertilized egg, mitochondria are more abundant on the future
ventral side, and when they are artificially transplanted to the
prospective dorsal side the secondary axis often is reversed
(Coffman et al., 2009). In addition, the redox-sensitive kinase p38
is required for secondary axis polarity (Bradham and McClay,
2006) and cis-regulatory studies have identified several cis
elements within the nodal promoter that may bind transcription
factors, the orthologs of which are known to be redox sensitive
(Nam et al., 2007; Range et al., 2007). However, redox-sensitive
activation of a transcription factor required for nodal transcription
in the sea urchin embryo has not yet been demonstrated. Two other
ubiquitous factors required to initiate nodal expression are Univin,
a member of the growth differentiation factor (GDF) subfamily of
TGFs, and the transcription factor SoxB1 (Range et al., 2007). As
in other organisms, strong autoregulation of nodal is thought to
amplify a small initial asymmetry to lock down its localized
expression, which is then maintained by Lefty, a direct Nodal target
and Nodal signaling antagonist (Duboc et al., 2008; Meno et al.,
1996; Thisse and Thisse, 1999). Consequently, Nodal function is
Box 2. A note on axis designations
A Embryo
B Larva
Primary axis
A
A
Ectoderm
Secondary
axis
s
V
V
Nodal
Nodal
BMP2/4
BMP2/4
D
D
Endoderm
Mesoderm
cWnt
P
P
Classically, the vertebrate egg axis has been referred to as the animalvegetal (AV) axis; the embryo develops primarily from the animal half,
whereas the vegetal half contains most of the yolk. However, the
primary axis of an early vertebrate (e.g. Xenopus) embryo does not
align with the egg AV axis and is instead referred to as the anteriorposterior (AP) axis, with the head marking the anterior and the
blastopore marking the posterior. By contrast, in sea urchins the
blastopore forms in the region corresponding to the egg vegetal pole,
leading most investigators to use AV for both the egg and embryonic
primary axes. Because the early molecular mechanisms that pattern
sea urchin embryos along this axis (A; canonical Wnt, blue triangle)
are similar to those operating along the vertebrate AP axis, in this
review we refer to the sea urchin embryonic axis as primary or AP.
The most commonly used designation for the orthogonal, secondary
axis of the sea urchin embryo has been oral-aboral. However, again
because the mechanisms used for patterning this axis (A; Nodal,
yellow arrow; BMP2/4, blue arrow) are similar to those that pattern
the DV axes of other deuterostomes, we refer to this axis as dorsalventral (DV), with the dorsal and ventral surfaces corresponding
approximately to the aboral and oral tissue territories. As the embryo
develops to the pluteus larva stage, the gut bends to form the mouth
and skeletogenesis supports expansion of the dorsal ectoderm,
creating the longest dimension of the larva, which has been referred
to as the AP axis. However, this axis does not align with the earlier
signaling axes (B; green and pink arrows) that establish the body plan
and the organization of the embryonic nervous system. Therefore,
here we use only the early AP and DV axial designations to compare
the mechanisms that control the positioning of the nervous systems
in sea urchin and vertebrate embryos.
confined to within several cell diameters of its initial expression
site. In the sea urchin embryo this is in ectoderm on the ventral side
(Yaguchi et al., 2007), restricted there presumably because Lefty
diffuses further than Nodal and effectively interferes with Nodal
autoregulation (Bolouri and Davidson, 2009; Duboc et al., 2008)
(Fig. 2B).
BMP2/4, another member of the TGF superfamily, is
functionally polarized along the secondary axis of sea urchins, as
in all other bilaterians (Lapraz et al., 2009). It is transcribed only
on the ventral side of embryos under the control of Nodal, as is the
TGF antagonist Chordin (Angerer et al., 2000; Bradham et al.,
2009; Duboc and Lepage, 2006; Duboc et al., 2004). However,
BMP2/4 functions to specify aboral ectoderm (see Glossary, Box
1) on the opposite side of the embryo (Angerer et al., 2000;
Bradham et al., 2009; Duboc et al., 2004; Lapraz et al., 2009).
BMP2/4 has been shown to be part of a relay mechanism from
Nodal to the dorsal side, as cell-autonomous activation of the
Nodal signaling pathway in only one blastomere of an 8-cell
embryo results in the transcription of bmp2/4, which is sufficient
DEVELOPMENT
In sea urchins, canonical Wnt signaling is first detectable as a
wave of nuclear -catenin that begins at the 16-cell stage in the
most posterior cells, which are called the micromeres, and passes
during the next two cleavage stages through concentric tiers of
progressively less posterior cells that will form mesoderm,
endoderm and a small part of the ectoderm (Logan et al., 1999;
Wikramanayake et al., 1998) (Fig. 2A). Micromeres are sufficient
to induce endomesoderm because, when transplanted to the anterior
end, they induce these tissues in presumptive ectoderm (Hörstadius,
1973; Minokawa and Amemiya, 1999; Ransick and Davidson,
1995). Four known micromere signals, all of which depend on
canonical Wnt signaling, are known to exist. These include Delta,
an activator of the Notch signaling pathway (Sherwood and
McClay, 2001; Sweet et al., 2002), ActivinB (Sethi et al., 2009),
Wnt8 (Minokawa et al., 2005; Smith et al., 2008; Smith et al.,
2007; Wikramanayake et al., 2004) and an undefined fourth signal
that downregulates the transcription factor SoxB1, which is a
negative regulator of canonical Wnt signaling in mesoderm tissues
(Oliveri et al., 2003; Sethi et al., 2009). The most upstream
component of the canonical Wnt signaling pathway to be identified
thus far is Dishevelled (Dsh), which is highly concentrated in
vegetal egg cortex (Byrum et al., 2009; Leonard and Ettensohn,
2007; Weitzel et al., 2004). Elucidation of the importance of
canonical Wnt signaling in endomesoderm development and
gastrulation led to large-scale screens to identify and construct the
gene regulatory networks controlling these processes (Davidson,
2009; Davidson et al., 2002; Oliveri et al., 2008).
REVIEW 3615
3616 REVIEW
Development 138 (17)
B
A Nodal
A
Lefty
BMP2/4
Lefty
BMP2/4
Chordin
Chordin
Wnt
P
P
C
D
V
A
D
Canonical Wnt-dependent processes position the ANE at
the animal pole
A
Six3
Nodal
FoxQ2
BMP2/4
X
X
cWnt
cWnt
P
P
V
Key
D
V
signature of the sea urchin ANE is similar to that of vertebrate
anterior neuroectoderm. After ANE restriction, most of the
remaining ectoderm retains a regulatory state that can support the
development of posterior neuroectoderm. However, this
neuroectoderm is subsequently restricted to a narrow strip of cells
(the ciliary band or CB) as the CBE. The expression of Hnf6, the
earliest known CBE marker gene, indicates that the CB is specified
by the end of the mesenchyme blastula stage at the border between
ventral and dorsal ectoderm tissues, which are diverted to an
epidermal fate by Nodal and BMP2/4, respectively, as described
above.
D
Animal plate
Pre-ciliary band ectoderm
(ciliary band in B)
Endoderm/mesoderm
Ventral ectoderm
Dorsal ectoderm
Blastoporal endodermadjacent ectoderm
Fig. 2. Signals that specify cell fate along the sea urchin
developmental axes. (A)Beginning at the 16-cell stage, a wave of
canonical Wnt signaling (blue arrow) passes through successive tiers to
specify mesoderm and endoderm during subsequent cleavages.
Anterior neuroectoderm (ANE) fate is eliminated from the remainder of
the ectoderm, much of which has a pre-ciliary band neuroectoderm
(CBE) fate. (B)TGF signaling specifies ectodermal fates along the DV
axis, beginning with Nodal, which specifies ventral ectoderm. Nodal is
necessary for the subsequent expression of BMP2/4, which specifies
dorsal ectoderm and ectoderm adjacent to the blastopore endoderm.
Nodal upregulates its own expression and induces the expression of
Lefty (a Nodal antagonist) and Chordin (a BMP antagonist), which
together protect the ciliary band (green strip) from the epidermispromoting influences of Nodal and BMP2/4. (C)Wnt-dependent
processes determine the anterior position of the ANE (red) through an
unknown intermediate process ‘X’. Mutual antagonism between
canonical Wnt-dependent and Six3-dependent processes is thought to
determine the border of the ANE. (D)Canonical Wnt signals support
the elimination of FoxQ2 from the ectoderm except at the anterior end
of the embryo, probably through the same intermediate process ‘X’ as
in C. This allows Nodal signaling to increase through autoregulation to
levels sufficient to initiate DV axis patterning. cWnt, canonical Wnt.
to rescue patterning along the entire secondary axis (Lapraz et al.,
2009). BMP2/4 signaling is blocked on the ventral side, where it is
inhibited by Chordin (Lapraz et al., 2009; Saudemont et al., 2010),
and moves to the dorsal side, possibly aided by Chordin, as has
been shown in other embryos (Shimmi et al., 2005).
Positioning the ANE and the CBE
Canonical Wnt signals not only specify mesoderm and endoderm
tissues along the AP axis and promote gastrulation at the posterior
end of the embryo, but they are also required to restrict the domain
of ANE to the opposite end of the embryo. The regulatory protein
When canonical Wnt signaling in sea urchin embryos is blocked,
either by removing the posterior half of the cleavage-stage embryo
or by inhibiting -catenin nuclearization, a remarkable phenotype
is produced: the embryos consist almost entirely of ANE that
contains many serotonergic neurons, a neural cell type that in
normal three-day embryos develops only in the ANE (Yaguchi et
al., 2006). This observation suggested that Wnt signaling represses
ANE specification in posterior ectoderm and led to genome-wide
screens for candidate ANE regulatory genes that are repressed,
rather than activated, by canonical Wnt signaling in this ectoderm
(Wei et al., 2006; Wei et al., 2009). This resulted in the discovery
that the transcription factor Six3 is necessary to drive ANE fate
and is sufficient to greatly expand this domain throughout the
embryo (Wei et al., 2009). The transcription of six3 and other early
ANE factors is initially activated broadly in the cleavage-stage
embryo, presumably by maternally encoded factors, and is
subsequently repressed by canonical Wnt-dependent mechanisms
(Wei et al., 2009; Yaguchi et al., 2008) at the early blastula stage
in all regions except the ANE (Fig. 2C). Six3 is required for the
formation of all ANE nerves and also for the expression of a large
number of downstream early ANE regulatory genes, many of
which have orthologs that are expressed in the vertebrate forebrain
(Wei et al., 2009). Thus, much of the regulatory apparatus of the
sea urchin embryo ANE corresponds to that of the anterior
ectoderm of higher vertebrates, further supporting the view that
the sea urchin primary axis is functionally analogous to the AP
axes of other embryos. Furthermore, like the vertebrate forebrain,
the ANE expresses several Six3-dependent orthologs of known
canonical Wnt antagonists, and Six3 misexpression can suppress
Wnt ligand transcription (Wei et al., 2009). Thus, canonical Wnt
signaling and Six3 affect ANE development in reciprocal ways:
misexpression of stabilized -catenin or loss of Six3 eliminates
nerves, whereas misexpression of Six3 or loss of canonical Wnt
signaling greatly expands the ANE and increases the number of
nerves (Wei et al., 2009; Yaguchi et al., 2006). These studies
suggest that the balance between Six3-dependent and canonical
Wnt-dependent processes regulates the size of the ANE domain
(Fig. 2C).
Nodal and BMP2/4 signaling position the CBE
The CB is specified as a 4- to 5-cell-wide strip of cells that is
carved out, in part, by converting regions of the dorsal and ventral
ectoderm to an epidermal fate (Duboc et al., 2008; Duboc et al.,
2004; Lapraz et al., 2009; Saudemont et al., 2010). The earliest
molecular evidence of CB formation is the expression of the
transcription factor Hnf6 (or onecut) within the presumptive CB
during mesenchyme blastula stages (Otim et al., 2004; Poustka et
al., 2004), several hours after Nodal and BMP2/4 signaling have
DEVELOPMENT
A
Development 138 (17)
Coordination of ANE and CBE positioning mechanisms
The canonical Wnt signaling mechanism that positions the ANE is
distinct from the TGF-based mechanism that positions the CBE,
but these mechanisms are connected in a very interesting way.
When canonical Wnt signaling is blocked, secondary axis polarity
is also lost. As shown previously (Yaguchi et al., 2008), the loss of
this polarity results from a disruption in Nodal autoregulatory
amplification, a key process operating at the top of the secondary
axis regulatory system. Surprisingly, this blockade requires FoxQ2,
a transcription factor involved in ANE development (Fig. 2D),
which is initially expressed throughout the anterior half of the
cleavage-stage embryo and is then progressively restricted to the
ANE. In embryos lacking canonical Wnt signaling, FoxQ2
continues to be expressed ectopically throughout the expanded
ANE. If it is also removed from these embryos, nodal expression
and secondary patterning are restored, allowing the formation of a
CB (Yaguchi et al., 2008). This double-repression mechanism of
canonical Wnt signaling downregulating FoxQ2, which suppresses
nodal, ensures that, in the absence of any embryonic signaling, the
ANE fate is downregulated in posterior ectoderm precursors before
they are subject to secondary axis patterning. In the normal
embryo, FoxQ2 retains its ability to suppress Nodal autoregulation
in the ANE, thereby helping to maintain its neuroectodermal fate.
Nervous system development within the ANE and
CBE territories
The specification of the localized neuroectoderm territories, the
ANE and CBE, is completed in the blastula stages (18-27 hours).
Neuronal precursors are present during late blastula and gastrula
stages and appear to differentiate and become functional in early
larval stages (55-96 hours). As in flies and vertebrates, the
regulation of neuronal differentiation within the sea urchin ANE
and CBE territories involves Delta-Notch-mediated lateral
inhibition (Wei et al., 2011). Not all of the cell types within the
ANE and CBE have been defined. The ANE contains serotonergic
and non-serotonergic neurons that express Synaptotagmin B and
some cells that produce long, immotile cilia (Hörstadius, 1939),
which might have sensory function, whereas the CBE is
predominantly composed of specialized ciliated cells and functions
as a swimming and feeding organ (Strathmann, 1975).
Nerves within the ANE
The ANE territory is currently thought to consist of two parts: a
central disk called the animal plate and a surrounding torus of cells.
The first nerves to form are the serotonergic neurons, which appear
after gastrulation is complete (Bisgrove and Burke, 1986; Yaguchi
et al., 2000) (Fig. 3A, hatched green ovals). Serotonergic neurons
are restricted to the dorsal margin of the animal plate as a
consequence of Nodal-mediated suppression of their development
on the ventral side (Yaguchi et al., 2007) (Fig. 2B, Fig. 3B). Later,
additional non-serotonergic nerves expressing Synaptotagmin B
(Burke et al., 2006) appear around the animal plate and form axons
and dendrites.
The expression of ANE marker genes during blastula stages
indicates that the anterior ectoderm is already molecularly patterned
during gastrulation. At the mesenchyme blastula/early gastrula stage,
six3 transcripts accumulate primarily at the edge of the animal plate
where serotonergic and other non-serotonergic neurons will develop,
whereas foxQ2 is expressed in the central animal plate region. Six3
is necessary for development of the thickened columnar epithelial
structure of the ANE and for the development of all neurons that
develop there (Wei et al., 2009). FoxQ2 is also necessary for
serotonergic neuron development, as well as for the expression of the
transcription factor Nk2.1 (Yaguchi et al., 2008), which in turn
controls the expression of a novel, ankryn repeat-containing protein
(AnkAT) that is required for the formation of apical tuft cilia (Dunn
et al., 2007; Yaguchi et al., 2010b). At the late mesenchyme blastula
stage, cells expressing delta appear in the animal plate (Lapraz et al.,
2009; Rottinger et al., 2006; Saudemont et al., 2010; Walton et al.,
2006) and neuron number is controlled by lateral inhibition (Wei et
al., 2011). All animal plate cells express Hnf6 (Poustka et al., 2004;
Yaguchi et al., 2006) and neuronal 3-tubulin (Duboc et al., 2004;
Casano et al., 1996). Thus, the ANE is specified by Six3-dependent
regulatory factors and behaves like a neural epithelium, with many
cells having neural potential.
DEVELOPMENT
begun, as indicated by phospho-Smad2/3 and phospho-Smad1/5
accumulation, respectively (Bergeron et al., 2010; Lapraz et al.,
2009; Yaguchi et al., 2007). When Nodal (and consequently
BMP2/4) signals are eliminated, the expression domain of Hnf6
and those of many other genes that are normally restricted to the
CB expand into the presumptive ventral and dorsal ectoderm
(Bradham et al., 2009; Duboc et al., 2004; Yaguchi et al., 2010a;
Yaguchi et al., 2006; Saudemont et al., 2010), suggesting that the
regulatory state in the absence of TGF signaling supports a CBlike fate, as originally proposed by Lepage and colleagues (Duboc
et al., 2004). Similarly, when only BMP2/4 signaling is eliminated,
the expression of CBE markers expands into the dorsal ectoderm,
but not into the ventral ectoderm where Nodal signaling remains
active (Lapraz et al., 2009; Yaguchi et al., 2010a; Saudemont et al.,
2010). The altered expression of CBE markers in response to these
perturbations is not observed in the most posterior ectoderm
adjacent to the blastopore (Saudemont et al., 2010; Yaguchi et al.,
2010a). Because this region of ectoderm derives from posterior
blastomeres, in which canonical Wnt signaling is active during
cleavage stages, it is likely to be regulated, at least in part, by
distinct signals.
The Nodal and BMP signaling antagonists Lefty and Chordin are
thought to prevent conversion of the CBE to epidermal fates. In
Lefty morphants, Nodal expression expands, ventralizing the
embryo and eliminating the CBE except in the ANE (Duboc et al.,
2008; Saudemont et al., 2010; Yaguchi et al., 2010a). Conversely,
in Lefty-misexpressing embryos, the CBE is radialized, as is also
observed in Nodal morphants. In Chordin morphants, the
secondary axis patterning mechanism is also compromised because
elevated, ectopic BMP2/4 signaling on the ventral side interferes
with Nodal expression, causing an expanded CBE phenotype
(Lapraz et al., 2009; Saudemont et al., 2010). By contrast, in
Chordin-misexpressing embryos, as in embryos lacking BMP
signaling (Lapraz et al., 2009; Yaguchi et al., 2010a), the CBE
expands to the dorsal side of the embryo. Thus, Nodal and BMP
inhibit CBE development, whereas Lefty and Chordin support it by
inhibiting the inhibitors of CBE development. An interesting, but
yet unanswered, question is how these diffusible signals and
antagonists precisely control the position and the width of the CB
in the normal embryo.
In summary, the ANE is established first at the animal pole
during cleavage stages by a balance of opposing canonical Wntdependent and Six3-dependent processes, whereas the CB is
established later at the border between the ventral and dorsal
ectoderm by factors that antagonize Nodal and BMP signaling. In
the sea urchin embryo these territories are created at separate times
and places by distinct mechanisms – properties that have facilitated
dissection of the mechanisms driving the development of each.
REVIEW 3617
3618 REVIEW
A Ventral/posterior view
Development 138 (17)
position of the CBE, which has neurogenic potential, but not the
number of nerves that develop within it. In addition, TGF
signaling appears to regulate the structure and connectivity of
CB neurons (Yaguchi et al., 2010a). These signaling mechanisms
appear to be the same as those used by chordate embryos to
shape their neuroectoderm territories.
B Lateral view
m
m
a
Key
Animal plate
Ciliary band
Serotonergic neurons
Ventral ectoderm
Blastoporal endodermadjacent/dorsal ectoderm
Ciliary band neurons
Fig. 3. The structure of neuroectodermal territories.
Ventral/posterior (A) and lateral (B) views of a three-day old sea urchin
larva. At this stage, the ANE is composed of the animal plate (red) and
a surrounding torus of cells. Within the animal plate are cells with long,
immotile cilia that constitute the apical tuft (black lines in B).
Serotonergic neurons (hatched green ovals) develop at the dorsal edge
of the animal plate. Neural precursors in the region of the ciliary band
are indicated by pink ovals. The positions of the mouth (m) and anus (a)
are indicated.
Nerves within the CB
The CBE also contains scattered delta-expressing cells and cells that
express orthologs of genes that encode neural markers in other
embryos (Saudemont et al., 2010). Several groups of cells first
appear during gastrulation near the CB, beneath the ventral ectoderm
and the lateral sections of the CB, inside the dorsal ectoderm (Fig.
3A). Subsequently, ~40-50 regularly spaced neural cell bodies
connected by processes form a highly structured chain along the
ventral side of the CB (Fig. 1C, Fig. 3B). Axons of these neurons are
either bundled in the central tract or they project towards and beneath
the dorsal ectoderm (Nakajima et al., 2004) (Fig. 1, Fig. 3B). How
these CB nerves connect to each other or to nerves in the ANE is
unknown. In addition, how the early CB-associated neurons respond
to TGF signals is unclear, but in the absence of these signals the
scattered nerves that develop within the expanded CBE do not form
interconnecting bundled axonal tracts (Yaguchi et al., 2010a). In
addition, Nodal and BMP signals might play a later role in regulating
neural identity, as has been found in insects and vertebrates (Mizutani
et al., 2006; Rusten et al., 2002).
The distribution of nerves within the CBE is controlled by the
positioning of the CBE, as shown by Nodal and BMP2/4
perturbations that alter the position, size and shape of the CBE
(Bradham et al., 2009; Lapraz et al., 2009; Saudemont et al.,
2010; Yaguchi et al., 2010a). The observation that ectopic
neurons produced under a variety of experimental conditions
[e.g. in Nodal morphants or following inhibition of BMP
signaling (Saudemont et al., 2010; Yaguchi et al., 2010a)] remain
largely confined to the CBE strongly suggests that the CBE
provides an environment conducive to neural differentiation.
When the Nodal or BMP2/4 signaling pathways are blocked by
interfering with components that function cell-autonomously
(e.g. their receptors and Smads), the distribution, but not the
number, of nerves changes in proportion to the size of the CBE.
This implies that TGF signaling primarily controls the size and
Deuterostome neural system patterning: shared
mechanisms
The pre-signaling neuroectodermal regulatory states of early
embryonic cells and the signaling mechanisms that specify neural
versus non-neural fates in the sea urchin are remarkably similar to
those observed in other deuterostome embryos. The highly ordered
localized arrangement of nerves in sea urchin embryos strongly
reinforces the recent findings that hemichordate nervous systems
are more centralized (Nomaksteinsky et al., 2009) than previously
thought (Lowe et al., 2003). Centralization mechanisms also exist
in the protostome (see Glossary, Box 1) polychaete annelid
Platynereis dumerilii (Denes et al., 2007), supporting their
existence in the urbilaterian ancestor (see Glossary, Box 1). Below,
we highlight the mechanisms of neural patterning that are shared
among deuterostomes.
Patterning of the nervous system along the AP axis of
deuterostomes employs an ancient Wnt-based set of
signals
Embryonic cells become anterior neuroectoderm unless
instructed otherwise
In the absence of all known zygotic signaling (i.e. in the absence
of canonical Wnt), virtually the entire sea urchin embryo assumes
an ANE fate (Yaguchi et al., 2006). Similarly, in Xenopus embryos,
when early signals that promote non-neural ectodermal fates are
DEVELOPMENT
a
A four-step model for nervous system
development in sea urchin embryos
Based on the studies discussed above, a four-step model for
patterning the nervous system of sea urchin embryos is presented
in Fig. 4 and is summarized below.
First, zygotic gene expression establishes a pre-ANE gene
regulatory state throughout the embryonic ectoderm. Initially, the
early ectoderm displays an ANE bias established by the early broad
zygotic transcription of six3 and other genes in the ANE gene
regulatory network. Second, canonical Wnt-mediated signaling from
posterior blastomeres downregulates the expression of six3, foxQ2
and other members of the ANE regulatory network, progressively
restricting the expression of these genes to the definitive ANE
domain. An additional consequence is that FoxQ2 is removed from
all but the anterior ectoderm, allowing nodal autoregulation to occur.
Third, Nodal upregulation activates the secondary axial patterning
mechanism and, together with BMP, suppresses neurogenesis in the
ventral and dorsal ectoderm, thus respecifying these tissues as
epidermis. A strip of cells between these regions retains the early
neuroectoderm specification of the CBE, presumably through the
activities of Lefty and Chordin, which suppress Nodal and BMP2/4
function in this region. Fourth, nerves develop in the ANE and CBE,
regions that are protected from canonical Wnt and TGF signals.
The two themes of this model are that the embryo starts with two
regulatory layers of broad overlapping neurogenic potential (the
ANE and the CBE), each of which is sequentially downregulated
by successive and distinct signals. Protection from signaling
appears to be the principal mechanism that results in restricted
regions that differentiate as neuroepithelia.
Development 138 (17)
REVIEW 3619
A Egg
B Cleavage
C Embryo
D Larva
Nodal
BMP2/4
cWnt
A
V
D
P
0 hour
(fertilized egg)
Key
8 hour
(16-cell)
24 hour
(blastula)
48 hour
(prism)
Animal plate
Pre-ciliary band ectoderm
(ciliary band in C and D)
Endoderm/mesoderm
Ventral ectoderm
Dorsal ectoderm
Endoderm-adjacent ectoderm
Serotonergic neuron precursor in C
Differentiated serotonergic neuron in D
Ciliary band neuron precursor in C
Differentiated ciliary band neuron in D
Fig. 4. A four-step model for specification and organization of the sea urchin embryo nervous system. (A)In the first step, an anterior
neuroectoderm regulatory state (red) is present throughout the egg and much of the embryo during early cleavage stages. (B)In the second step,
which occurs during very early blastula stages, this state is eliminated by canonical Wnt (cWnt)-dependent signals from all but the anterior
neuroectoderm, revealing a ciliary band-like neuroectoderm (green) that contains scattered neural precursors (light pink circles). (C)In the third step,
which occurs during the mesenchyme blastula/early gastrula stages, Nodal and BMP2/4 signals convert ventral and dorsal ectoderm to non-neural
ectoderm except in the anterior neuroectoderm (red) and ciliary band (green), which are protected from these signals. (D)During the fourth and
final step, by which point the embryo has transitioned into a larva, CBE and ANE neural progenitors differentiate. The timeline indicates hours postfertilization and embryonic stages.
Wnt signaling defines the posterior pole and restricts the
ANE to the opposite pole
As discussed recently by others (Niehrs, 2010; Petersen and
Reddien, 2009), canonical Wnt signaling is an ancient function that
operates at the posterior end of nearly all bilaterian embryos.
Similarly, in sea urchin embryos a gradient of nuclear -catenin
emanates from posterior blastomeres during cleavage stages and is
necessary to specify mesoderm and endoderm (Logan et al., 1999;
Wikramanayake et al., 1998). At the opposite end of vertebrate and
sea urchin embryos, the tissues that develop are of anterior
character. Whereas suppression of canonical Wnt signaling
anteriorizes all three germ layers in vertebrates, this process is
known thus far to affect only the ectoderm in sea urchin embryos,
as blocking canonical Wnt completely abrogates endomesoderm
development. Thus, since the ectoderm at the anterior end of sea
urchin embryos has many properties of anterior neuroectoderm,
and as canonical Wnt signaling functions at the opposite end in
these embryos, the primary axis of the sea urchin resembles the AP
axis of other embryos.
The canonical Wnt-dependent remodeling of a broad
neuroectoderm territory is an ancient function. In sea urchin
embryos, it restricts FoxQ2 and the apical tuft, which depends on
FoxQ2 (Yaguchi et al., 2010b), to the anterior end of the embryo.
The same canonical Wnt-mediated restriction has recently been
described in Saccoglossus (Darras et al., 2011). FoxQ2 is also
expressed at the anterior end of the embryo of the cephalochordate
(see Glossary, Box 1) amphioxus (Branchiostoma floridae) (Yu et
al., 2002), raising the possibility that a similar mechanism also
operates in these organisms. Amazingly, this process also occurs in
the hydrozoan cnidarian Clytia hemisphaerica: FoxQ2 is expressed
where the apical tuft forms, at the pole opposite to canonical Wnt
signaling (Momose et al., 2008), which is where gastrulation
occurs (Wikramanayake et al., 2003). Further, as in sea urchin
embryos, the size of the FoxQ2 expression domain in Clytia is
regulated by canonical Wnt signaling (Momose et al., 2008). This
indicates that canonical Wnt-restricted anterior expression of
FoxQ2 predates the cnidarian-bilaterian split.
The canonical Wnt-dependent restriction of expression of other
genes, such as six3, to the ANE of sea urchin embryos or to the
anterior neural plate and forebrain of vertebrate embryos is
conserved (Braun et al., 2003; Lagutin et al., 2003; Wei et al.,
2009). Furthermore, an AP gradient of nuclear -catenin has been
demonstrated in the presumptive Xenopus neural plate ectoderm
during gastrulation, around the time when the AP and DV axes
begin to separate (Kiecker and Niehrs, 2001). Thus, anterior
neuroectoderm fate requires low levels of canonical Wnt signaling
in both sea urchin and vertebrate embryos.
Roles of canonical Wnt and TGF signaling in positioning
anterior neuroectoderm
In the sea urchin embryo, canonical Wnt-mediated suppression
of ANE fate occurs in non-ANE ectoderm during cleavage
stages by a mechanism that has not yet been defined. It requires
neither Nodal nor BMP2/4, nor their antagonists (Wei et al.,
2009). When canonical Wnt signaling is blocked, the entire
embryo assumes ANE fate. By contrast, in vertebrate embryos,
canonical Wnt signaling results in localized production of BMP
DEVELOPMENT
eliminated, the entire embryo expresses Sox2 (Reversade and De
Robertis, 2005; Reversade et al., 2005), a transcription factor that
supports the neural precursor state in mouse embryos (Bylund et
al., 2003; Graham et al., 2003). Other anterior neural markers are
expressed in the early medial epiblast cells of chick embryos
(reviewed by Wilson and Houart, 2004), in the epiblast of mouse
embryos (reviewed by Levine and Brivanlou, 2007) and in mouse
embryonic stem cells in the absence of signals that promote nonneural ectodermal fates (Tropepe et al., 2001; Vallier et al., 2004).
Thus, it is likely that in vertebrates, as in sea urchin embryos, ANE
fate is not induced but rather is driven by early maternal regulatory
factors.
signaling antagonists, which protect the neuroectoderm
regulatory state in the dorsal ectoderm. Thus, when canonical
Wnt signaling is blocked in vertebrate embryos, they completely
lack neuroectoderm and are strongly ventralized (Heasman et al.,
2000). In addition, canonical Wnt signal activity posteriorizes
the neuroectoderm except where it is protected by Wnt
antagonists (reviewed by Yamaguchi, 2001; Houart et al., 2002).
These two processes, canonical Wnt-dependent creation of a
neural plate by antagonizing BMP signaling and posteriorization
by high levels of canonical Wnt activity in posterior regions of
the neural plate, occur during gastrulation. If both BMP signaling
and canonical Wnt signaling are blocked, then the underlying
neuroectoderm regulatory state can be detected and it is anterior
in character, as posteriorizing signals are lacking (see Reversade
et al., 2005). Thus, in both vertebrate and sea urchin embryos,
there is a basal, probably maternally driven, ANE regulatory
state that is restricted to anterior ectoderm regions by canonical
Wnt signaling. Where the differences lie is in the regulatory
relationships between canonical Wnt and TGF signaling
activities. In sea urchin embryos, the production of Nodal and
BMP2/4 (and their respective antagonists) depends on canonical
Wnt signaling, whereas in the vertebrate embryo this is only the
case for the BMP antagonists. Furthermore, in sea urchin
embryos, the creation of the more posterior CBE via TGF
signaling depends on the prior restriction of the ANE to the
anterior end of the embryo, whereas in vertebrate embryos these
are independent events that may occur at approximately the same
time. Thus, the development of non-neural and neural ectoderm
territories in sea urchin and vertebrate embryos make use of the
same signaling pathways, but these are deployed differently as a
result of the different regulatory linkages among them.
Some regulatory properties of the sea urchin embryo ANE
are conserved
The ANE regulatory properties of the sea urchin embryo are
similar to those described for mouse embryos. In both mouse and
sea urchins, Six3 functions near the top of the ANE regulatory
networks (Lagutin et al., 2003; Lavado et al., 2008; Wei et al.,
2009). Furthermore, many of the same transcription factors,
including, Rx, Achaete-Scute, Zic2, Ebf3, Fez, Nkx2.1, SoxC
and the Notch ligand Delta (Wei et al., 2009) are expressed in
the ANE of both mouse and sea urchin embryos. Mutual
antagonism between Six3 and canonical Wnt signals operates in
both systems. The sea urchin ANE is likely to be protected from
canonical Wnt signals by Six3-dependent Wnt antagonists, such
as Dkk1, just as the zebrafish telencephalon is protected by a
secreted frizzled-related protein, a Wnt antagonist called Tlc
(Houart et al., 2002).
Many of the transcription factors expressed in the sea urchin
embryo ANE (Kenny et al., 2003; Wei et al., 2009) are also
expressed in the ectodermal layer of the cnidarian Nematostella
vectensis [e.g. six3, hbn, rx, otx, soxb1, soxb2 (Marlow et al.,
2009)]. Some of these are expressed in patterns similar to those
observed in the sea urchin embryo ANE, consistent with the
intriguing possibility that the cnidarian ectoderm is similar to the
sea urchin ANE. Both regions resist respecification by canonical
Wnt signaling and contain many neurons. An additional
provocative hypothesis suggests that most of the body of Hydra,
another cnidarian, gave rise to the heads of deuterostomes and
protostomes (Meinhardt, 2002). This shared and ancient state of
early ectoderm might thus be part of the default state of early, presignaling ectoderm in vertebrate embryos as well.
Development 138 (17)
Nervous system positioning along the DV axes of
deuterostomes relies on TGF signals
Much of the ectoderm adopts a CBE fate unless instructed
otherwise by Nodal and BMP2/4
Nodal and BMP2/4 carve out the CB by diverting ectoderm on the
ventral and dorsal sides of the embryo to a non-neural epidermal fate.
This process shares some properties with neuroectoderm patterning
mechanisms along the DV axes of other bilaterian embryos. It
operates along an axis orthogonal to the canonical Wnt system,
which specifies cell fates along the AP axis. Although nodal, lefty,
bmp2/4 and chordin are all transcribed on the ventral side of the sea
urchin embryo, BMP2/4 signaling occurs only on the side opposite
to that of nodal and chordin expression, as is seen in vertebrate and
arthropod embryos. As discussed above, most of the widespread
latent bias toward neuroectoderm and neural structures in vertebrate
embryos is suppressed by early BMP signaling and maintained
where antagonistic activities prevent this signaling. In addition,
fibroblast growth factor (FGF) signaling and inhibition of canonical
Wnt signaling promote neural development by reducing the half-life
of the BMP effector protein Smad (Fuentealba et al., 2007), and, in
Xenopus embryos, this is required in addition to secreted antagonists
for neural development (Pera et al., 2003). Although it is clear that
downregulation of BMP2/4 signals is important for neuroectoderm
development in the sea urchin embryo, whether FGF signaling also
plays a role in this process is not yet known.
Although antagonism of BMP signaling is considered to be the
primary mechanism that prevents ectoderm from differentiating as
epidermis in higher deuterostomes, Nodal signaling also overrides
an early neuroectoderm regulatory state. In Xenopus, continued
suppression of effectors downstream of both Nodal and BMP
signaling is required for neural induction (Chang and Harland, 2007),
and, in zebrafish and mice, neural fates have been shown to emerge
in presumptive endomesoderm cells in the absence Nodal signaling
(Camus et al., 2006; Feldman et al., 2000; Schier and Talbot, 2001).
DV axis patterning mechanisms are connected to different
upstream polarizing mechanisms
Although the antagonism of TGF signaling is employed in
positioning the nervous system in virtually all cases that have been
examined, this process is connected to distinctly different initial
polarizing mechanisms, and the timing and manner in which these
mechanisms are utilized are highly variable. For example, DV axial
specification in Xenopus is provided by the sperm entry point and
by cytoplasmic rotation (Moon and Kimelman, 1998). In flies, it
depends on a gradient of Dorsal activity controlled by follicle cell
activities (Steward and Govind, 1993), whereas in sea urchins
redox asymmetry is thought to be involved (Coffman et al., 2009;
Coffman et al., 2004; Steward and Govind, 1993). The sea urchin
embryo provides an especially clear example of that connection
because both the establishment of the nervous system and the
specification of cell fates along the secondary axis can be directly
traced to a single event: the activation of Nodal in the presumptive
ventral ectoderm, possibly dependent on the mitochondrial gradient
in the egg. The fact that Nodal controls the expression of BMP2/4,
and that Nodal and BMP2/4 then play repressive roles in
positioning the nervous system, make Nodal a key regulator of this
process in the sea urchin embryo.
Patterns of Nodal expression during evolution
The nodal gene appeared sometime after the cnidarian-bilaterian
split in the common ancestor of protostomes and deuterostomes, as
it is expressed in echinoderms and in snails (Grande and Patel,
DEVELOPMENT
3620 REVIEW
2009). In snails, it functions to specify right-sidedness, assuming
the mouth is ventral. This, along with the fact that BMP2/4
signaling in sea urchins and BMP2/4 expression in hemichordates
is dorsal (Lowe et al., 2006), suggest that the positioning of major
parts of the nervous system in basal deuterostomes should be
ventral. This is not the case in sea urchin embryos, largely because
Nodal specifies that region as non-neural ventral ectoderm. During
deuterostome evolution the position of Nodal signaling and its
relative importance in nervous system positioning have changed.
In vertebrate embryos, the primary function of Nodal is in
specifying mesoderm and endoderm rather than ectoderm, whereas
in the cephalochordate amphioxus (Onai et al., 2010; Yu et al.,
2007) it supports dorsal/anterior fates in both mesoderm and
ectoderm (Onai et al., 2010). Although the wide variations in the
patterns of Nodal expression in the ectoderm among deuterostomes
make it impossible to clearly identify ancestral versus derived
traits, studies on Nodal function in sea urchin embryos demonstrate
that it plays a crucial role in determining non-neural fates and, as
a consequence, the organization of the nervous system.
Conclusions
In sea urchin embryos, the positioning of the ANE and the CBE
within a broad neuroectoderm territory is controlled by successive
primary and secondary axis patterning mechanisms. The regulatory
links between these signaling mechanisms allow their functions to
be separated in time – Nodal expression does not become
established until the ANE is restricted to the anterior end of the
embryo by canonical Wnt-dependent processes, and BMP2/4
expression does not begin without Nodal. By contrast, the
regulatory scheme in vertebrate embryos compresses these events
temporally and spatially, in part because BMP expression initially
occurs broadly and independently of canonical Wnt and Nodal. As
a result, the positioning of the nervous system dorsally in higher
deuterostomes or ventrally in protostomes depends heavily on the
localized production of activities that suppress BMP signaling.
Although the signals and their antagonists are the same, the
regulatory connections between the Wnt and BMP pathways are
different in the embryos of sea urchins and vertebrates, yielding
distinct neural versus non-neural patterns. The neural patterning
mechanisms in the common ancestor of echinoderms and other
deuterostomes cannot be determined, but a clearer understanding
of the evolution of extant deuterostomes can be obtained by
detailed comparisons of diverse embryos. Importantly, it is
becoming evident that to understand their role in neural patterning
we must not simply determine the expression patterns of Wnt and
TGF signaling pathway components, but also test their regulatory
relationships, as has been done in the sea urchin embryo.
Acknowledgements
We thank Adi Sethi, Diane Adams and Ryan Range for discussions and
especially Zheng Wei for experimental and bioinformatic contributions.
Support for this work was provided by the Division of Intramural Research in
the National Institute for Dental and Craniofacial Research, National Institutes
of Health (L.M.A. and R.C.A.), Special Coordination Funds for Promoting
Science and Technology of the Ministry of Education, Culture, Sports, Science
and Technology of the Japanese Government (MEXT), by a Grant-in Aid for
Young Scientists (Start-up) (S.Y.) and NSERC (R.D.B.). Deposited in PMC for
release after 12 months.
Competing interests statement
The authors declare no competing financial interests.
References
Angerer, L. M., Oleksyn, D. W., Logan, C. Y., McClay, D. R., Dale, L. and
Angerer, R. C. (2000). A BMP pathway regulates cell fate allocation along the
sea urchin animal-vegetal embryonic axis. Development 127, 1105-1114.
REVIEW 3621
Angerer, L. M., Oleksyn, D. W., Levine, A. M., Li, X., Klein, W. H. and
Angerer, R. C. (2001). Sea urchin goosecoid function links fate specification
along the animal-vegetal and oral-aboral embryonic axes. Development 128,
4393-4404.
Arendt, D., Denes, A. S., Jekely, G. and Tessmar-Raible, K. (2008). The
evolution of nervous system centralization. Philos. Trans. R. Soc. Lond. B Biol. Sci.
363, 1523-1528.
Bergeron, K. F., Xu, X. and Brandhorst, B. P. (2010). Oral-aboral patterning and
gastrulation of sea urchin embryos depend on sulfated glycosaminoglycans.
Mech. Dev. 128, 71-89.
Bisgrove, B. W. and Burke, R. D. (1986). Development of serotongic neurons in
embryos of the sea urchin, Strongylocentrotus purpuratus. Dev. Growth Differ.
28, 569-574.
Bolouri, H. and Davidson, E. H. (2009). The gene regulatory network basis of
the ‘community effect’, and analysis of a sea urchin embryo example. Dev. Biol.
340, 170-178.
Bradham, C. A. and McClay, D. R. (2006). p38 MAPK is essential for secondary
axis specification and patterning in sea urchin embryos. Development 133, 2132.
Bradham, C. A., Oikonomou, C., Kuhn, A., Core, A. B., Modell, J. W., McClay,
D. R. and Poustka, A. J. (2009). Chordin is required for neural but not axial
development in sea urchin embryos. Dev. Biol. 328, 221-233.
Braun, M. M., Etheridge, A., Bernard, A., Robertson, C. P. and Roelink, H.
(2003). Wnt signaling is required at distinct stages of development for the
induction of the posterior forebrain. Development 130, 5579-5587.
Bullock, T. and Horridge, G. (1965). Hemichordata. Structure and Function of the
Nervous System of Invertebrates, pp. 1567-1577. San Francisco: W. H. Freeman.
Burke, R. D., Osborne, L., Wang, D., Murabe, N., Yaguchi, S. and Nakajima,
Y. (2006). Neuron-specific expression of a synaptotagmin gene in the sea urchin
Strongylocentrotus purpuratus. J. Comp. Neurol. 496, 244-251.
Bylund, M., Andersson, E., Novitch, B. G. and Muhr, J. (2003). Vertebrate
neurogenesis is counteracted by Sox1-3 activity. Nat. Neurosci. 6, 1162-1168.
Byrum, C. A., Xu, R., Bince, J. M., McClay, D. R. and Wikramanayake, A. H.
(2009). Blocking Dishevelled signaling in the noncanonical Wnt pathway in sea
urchins disrupts endoderm formation and spiculogenesis, but not secondary
mesoderm formation. Dev. Dyn. 238, 1649-1665.
Camus, A., Perea-Gomez, A., Moreau, A. and Collignon, J. (2006). Absence of
Nodal signaling promotes precocious neural differentiation in the mouse
embryo. Dev. Biol. 295, 743-755.
Casano, C., Ragusa, M., Cutrera, M., Costa, S. and Gianguzza, F. (1996).
Spatial expression of alpha and beta tubulin genes in the late embryogenesis of
the sea urchin Paracentrotus lividus. Int. J. Dev. Biol. 40, 1033-1041.
Chang, C. and Harland, R. M. (2007). Neural induction requires continued
suppression of both Smad1 and Smad2 signals during gastrulation.
Development 134, 3861-3872.
Coffman, J. A., McCarthy, J. J., Dickey-Sims, C. and Robertson, A. J. (2004).
Oral-aboral axis specification in the sea urchin embryo. II. Mitochondrial
distribution and redox state contribute to establishing polarity in
Strongylocentrotus purpuratus. Dev. Biol. 273, 160-171.
Coffman, J. A., Coluccio, A., Planchart, A. and Robertson, A. J. (2009). Oralaboral axis specification in the sea urchin embryo III. Role of mitochondrial redox
signaling via H2O2. Dev. Biol. 330, 123-130.
Darras, S., Gerhart, J., Terasaki, M., Kirschner, M. and Lowe, C. J. (2011).
{beta}-Catenin specifies the endomesoderm and defines the posterior organizer
of the hemichordate Saccoglossus kowalevskii. Development 138, 959-970.
Davidson, E. H. (2009). Network design principles from the sea urchin embryo.
Curr. Opin. Genet. Dev. 19, 535-540.
Davidson, E. H., Rast, J. P., Oliveri, P., Ransick, A., Calestani, C., Yuh, C. H.,
Minokawa, T., Amore, G., Hinman, V., Arenas-Mena, C. et al. (2002). A
provisional regulatory gene network for specification of endomesoderm in the
sea urchin embryo. Dev. Biol. 246, 162-190.
De Robertis, E. M. and Sasai, Y. (1996). A common plan for dorsoventral
patterning in Bilateria. Nature 380, 37-40.
Denes, A. S., Jekely, G., Steinmetz, P. R., Raible, F., Snyman, H.,
Prud’homme, B., Ferrier, D. E., Balavoine, G. and Arendt, D. (2007).
Molecular architecture of annelid nerve cord supports common origin of nervous
system centralization in bilateria. Cell 129, 277-288.
Duboc, V. and Lepage, T. (2006). A conserved role for the nodal signaling
pathway in the establishment of dorso-ventral and left-right axes in
deuterostomes. J. Exp. Zool. B Mol. Dev. Evol. 310, 41-53.
Duboc, V., Rottinger, E., Besnardeau, L. and Lepage, T. (2004). Nodal and
BMP2/4 signaling organizes the oral-aboral axis of the sea urchin embryo. Dev.
Cell 6, 397-410.
Duboc, V., Lapraz, F., Besnardeau, L. and Lepage, T. (2008). Lefty acts as an
essential modulator of Nodal activity during sea urchin oral-aboral axis
formation. Dev. Biol. 320, 49-59.
Dunn, E. F., Moy, V. N., Angerer, L. M., Angerer, R. C., Morris, R. L. and
Peterson, K. J. (2007). Molecular paleoecology: using gene regulatory analysis
to address the origins of complex life cycles in the late Precambrian. Evol. Dev. 9,
10-24.
DEVELOPMENT
Development 138 (17)
Feldman, B., Dougan, S. T., Schier, A. F. and Talbot, W. S. (2000). Nodal-related
signals establish mesendodermal fate and trunk neural identity in zebrafish. Curr.
Biol. 10, 531-534.
Fuentealba, L. C., Eivers, E., Ikeda, A., Hurtado, C., Kuroda, H., Pera, E. M.
and De Robertis, E. M. (2007). Integrating patterning signals: Wnt/GSK3
regulates the duration of the BMP/Smad1 signal. Cell 131, 980-993.
Gerhart, J. (2000). Inversion of the chordate body axis: are there alternatives?
Proc. Natl. Acad. Sci. USA 97, 4445-4448.
Graham, V., Khudyakov, J., Ellis, P. and Pevny, L. (2003). SOX2 functions to
maintain neural progenitor identity. Neuron 39, 749-765.
Grande, C. and Patel, N. H. (2009). Nodal signalling is involved in left-right
asymmetry in snails. Nature 457, 1007-1011.
Heasman, J., Kofron, M. and Wylie, C. (2000). Beta-catenin signaling activity
dissected in the early Xenopus embryo: a novel antisense approach. Dev. Biol.
222, 124-134.
Hörstadius, S. (1939). The mechanics of sea urchin development, studied by
operative methods. Biol. Rev. Camb. Philos. Soc. 14, 48.
Hörstadius, S. (1973). Experimental Embryology of Echinoderms. Oxford:
Clarendon Press.
Houart, C., Caneparo, L., Heisenberg, C., Barth, K., Take-Uchi, M. and
Wilson, S. (2002). Establishment of the telencephalon during gastrulation by
local antagonism of Wnt signaling. Neuron 35, 255-265.
Kenny, A. P., Oleksyn, D. W., Newman, L. A., Angerer, R. C. and Angerer, L.
M. (2003). Tight regulation of SpSoxB factors is required for patterning and
morphogenesis in sea urchin embryos. Dev. Biol. 261, 412-425.
Kiecker, C. and Niehrs, C. (2001). A morphogen gradient of Wnt/beta-catenin
signalling regulates anteroposterior neural patterning in Xenopus. Development
128, 4189-4201.
Knight-Jones, E. (1952). On the nervous system of Saccoglossus cambrensis
(Enteropneusta). Philos. Trans. R. Soc. Lond. B Biol. Sci. 236, 315-354.
Lagutin, O. V., Zhu, C. C., Kobayashi, D., Topczewski, J., Shimamura, K.,
Puelles, L., Russell, H. R., McKinnon, P. J., Solnica-Krezel, L. and Oliver, G.
(2003). Six3 repression of Wnt signaling in the anterior neuroectoderm is
essential for vertebrate forebrain development. Genes Dev. 17, 368-379.
Lapraz, F., Besnardeau, L. and Lepage, T. (2009). Patterning of the dorsalventral axis in echinoderms: insights into the evolution of the BMP-chordin
signaling network. PLoS Biol. 7, e1000248.
Lavado, A., Lagutin, O. V. and Oliver, G. (2008). Six3 inactivation causes
progressive caudalization and aberrant patterning of the mammalian
diencephalon. Development 135, 441-450.
Leonard, J. D. and Ettensohn, C. A. (2007). Analysis of dishevelled localization
and function in the early sea urchin embryo. Dev. Biol. 306, 50-65.
Levine, A. J. and Brivanlou, A. H. (2007). Proposal of a model of mammalian
neural induction. Dev. Biol. 308, 247-256.
Logan, C. Y., Miller, J. R., Ferkowicz, M. J. and McClay, D. R. (1999). Nuclear
beta-catenin is required to specify vegetal cell fates in the sea urchin embryo.
Development 126, 345-357.
Lowe, C. J., Wu, M., Salic, A., Evans, L., Lander, E., Stange-Thomann, N.,
Gruber, C. E., Gerhart, J. and Kirschner, M. (2003). Anteroposterior
patterning in hemichordates and the origins of the chordate nervous system.
Cell 113, 853-865.
Lowe, C. J., Terasaki, M., Wu, M., Freeman, R. M., Jr, Runft, L., Kwan, K.,
Haigo, S., Aronowicz, J., Lander, E., Gruber, C. et al. (2006). Dorsoventral
patterning in hemichordates: insights into early chordate evolution. PLoS Biol. 4,
e291.
Marlow, H. Q., Srivastava, M., Matus, D. Q., Rokhsar, D. and Martindale, M.
Q. (2009). Anatomy and development of the nervous system of Nematostella
vectensis, an anthozoan cnidarian. Dev. Neurobiol. 69, 235-254.
Meinhardt, H. (2002). The radial-symmetric hydra and the evolution of the
bilateral body plan: an old body became a young brain. BioEssays 24, 185-191.
Meno, C., Saijoh, Y., Fujii, H., Ikeda, M., Yokoyama, T., Yokoyama, M.,
Toyoda, Y. and Hamada, H. (1996). Left-right asymmetric expression of the
TGF beta-family member lefty in mouse embryos. Nature 381, 151-155.
Minokawa, T. and Amemiya, S. (1999). Timing of the potential of micromeredescendants in echinoid embryos to induce endoderm differentiation of
mesomere-descendants. Dev. Growth Differ. 41, 535-547.
Minokawa, T., Wikramanayake, A. H. and Davidson, E. H. (2005). cisRegulatory inputs of the wnt8 gene in the sea urchin endomesoderm network.
Dev. Biol. 288, 545-558.
Mizutani, C. M. and Bier, E. (2008). EvoD/Vo: the origins of BMP signaling in the
neuroectoderm. Nat. Rev. Genet. 9, 663-677.
Mizutani, C. M., Meyer, N., Roelink, H. and Bier, E. (2006). Thresholddependent BMP-mediated repression: a model for a conserved mechanism that
patterns the neuroectoderm. PLoS Biol. 4, e313.
Momose, T., Derelle, R. and Houliston, E. (2008). A maternally localised Wnt
ligand required for axial patterning in the cnidarian Clytia hemisphaerica.
Development 135, 2105-2113.
Moon, R. T. and Kimelman, D. (1998). From cortical rotation to organizer gene
expression: toward a molecular explanation of axis specification in Xenopus.
BioEssays 20, 536-545.
Development 138 (17)
Nakajima, Y., Kaneko, H., Murray, G. and Burke, R. D. (2004). Divergent
patterns of neural development in larval echinoids and asteroids. Evol. Dev. 6,
95-104.
Nam, J., Su, Y. H., Lee, P. Y., Robertson, A. J., Coffman, J. A. and Davidson, E.
H. (2007). Cis-regulatory control of the nodal gene, initiator of the sea urchin
oral ectoderm gene network. Dev. Biol. 306, 860-869.
Niehrs, C. (2010). On growth and form: a Cartesian coordinate system of Wnt and
BMP signaling specifies bilaterian body axes. Development 137, 845-857.
Nomaksteinsky, M., Rottinger, E., Dufour, H. D., Chettouh, Z., Lowe, C. J.,
Martindale, M. Q. and Brunet, J. F. (2009). Centralization of the
deuterostome nervous system predates chordates. Curr. Biol. 19, 1264-1269.
Oliveri, P., Davidson, E. H. and McClay, D. R. (2003). Activation of pmar1
controls specification of micromeres in the sea urchin embryo. Dev. Biol. 258,
32-43.
Oliveri, P., Tu, Q. and Davidson, E. H. (2008). Global regulatory logic for
specification of an embryonic cell lineage. Proc. Natl. Acad. Sci. USA 105, 59555962.
Onai, T., Yu, J. K., Blitz, I. L., Cho, K. W. and Holland, L. Z. (2010). Opposing
Nodal/Vg1 and BMP signals mediate axial patterning in embryos of the basal
chordate amphioxus. Dev. Biol. 344, 377-389.
Otim, O., Amore, G., Minokawa, T., McClay, D. R. and Davidson, E. H. (2004).
SpHnf6, a transcription factor that executes multiple functions in sea urchin
embryogenesis. Dev. Biol. 273, 226-243.
Pera, E. M., Ikeda, A., Eivers, E. and De Robertis, E. M. (2003). Integration of
IGF, FGF, and anti-BMP signals via Smad1 phosphorylation in neural induction.
Genes Dev. 17, 3023-3028.
Petersen, C. P. and Reddien, P. W. (2009). Wnt signaling and the polarity of the
primary body axis. Cell 139, 1056-1068.
Poustka, A. J., Kuhn, A., Radosavljevic, V., Wellenreuther, R., Lehrach, H.
and Panopoulou, G. (2004). On the origin of the chordate central nervous
system: expression of onecut in the sea urchin embryo. Evol. Dev. 6, 227-236.
Range, R., Lapraz, F., Quirin, M., Marro, S., Besnardeau, L. and Lepage, T.
(2007). Cis-regulatory analysis of nodal and maternal control of dorsal-ventral
axis formation by Univin, a TGF-{beta} related to Vg1. Development 134, 36493664.
Ransick, A. and Davidson, E. H. (1995). Micromeres are required for normal
vegetal plate specification in sea urchin embryos. Development 121, 3215-3222.
Reversade, B. and De Robertis, E. M. (2005). Regulation of ADMP and
BMP2/4/7 at opposite embryonic poles generates a self-regulating
morphogenetic field. Cell 123, 1147-1160.
Reversade, B., Kuroda, H., Lee, H., Mays, A. and De Robertis, E. M. (2005).
Depletion of Bmp2, Bmp4, Bmp7 and Spemann organizer signals induces
massive brain formation in Xenopus embryos. Development 132, 3381-3392.
Rottinger, E., Croce, J., Lhomond, G., Besnardeau, L., Gache, C. and Lepage,
T. (2006). Nemo-like kinase (NLK) acts downstream of Notch/Delta signalling to
downregulate TCF during mesoderm induction in the sea urchin embryo.
Development 133, 4341-4353.
Rusten, T. E., Cantera, R., Kafatos, F. C. and Barrio, R. (2002). The role of TGF
beta signaling in the formation of the dorsal nervous system is conserved
between Drosophila and chordates. Development 129, 3575-3584.
Saudemont, A., Emmanuel, H., Mekpoh, F., Bessodes, N., Quirin, M., Lapraz,
F., Duboc, V., Rottinger, E., Range, R., Oisel, A. et al. (2010). Gene regulatory
network analysis in an echinoderm reveals ancestral regulatory circuits regulating
ectoderm patterning and morphogenesis downstream of Nodal and BMP2/4.
PLoS Genet. 6, e1001259.
Schier, A. F. and Talbot, W. S. (2001). Nodal signaling and the zebrafish
organizer. Int. J. Dev. Biol. 45, 289-297.
Sethi, A. J., Angerer, R. C. and Angerer, L. M. (2009). Gene regulatory network
interactions in sea urchin endomesoderm induction. PLoS Biol. 7, e1000029.
Sherwood, D. R. and McClay, D. R. (2001). LvNotch signaling plays a dual role in
regulating the position of the ectoderm-endoderm boundary in the sea urchin
embryo. Development 128, 2221-2232.
Shimmi, O., Umulis, D., Othmer, H. and O’Connor, M. B. (2005). Facilitated
transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the
Drosophila blastoderm embryo. Cell 120, 873-886.
Smith, J., Theodoris, C. and Davidson, E. H. (2007). A gene regulatory network
subcircuit drives a dynamic pattern of gene expression. Science 318, 794-797.
Smith, J., Kraemer, E., Liu, H., Theodoris, C. and Davidson, E. (2008). A
spatially dynamic cohort of regulatory genes in the endomesodermal gene
network of the sea urchin embryo. Dev. Biol. 313, 863-875.
Steward, R. and Govind, S. (1993). Dorsal-ventral polarity in the Drosophila
embryo. Curr. Opin. Genet. Dev. 3, 556-561.
Strathmann, R. (1975). Larval feeding in echinoderms. Am. Zool. 15, 717-730.
Sweet, H. C., Gehring, M. and Ettensohn, C. A. (2002). LvDelta is a mesoderminducing signal in the sea urchin embryo and can endow blastomeres with
organizer-like properties. Development 129, 1945-1955.
Thisse, C. and Thisse, B. (1999). Antivin, a novel and divergent member of the
TGFbeta superfamily, negatively regulates mesoderm induction. Development
126, 229-240.
DEVELOPMENT
3622 REVIEW
Tropepe, V., Hitoshi, S., Sirard, C., Mak, T. W., Rossant, J. and van der Kooy,
D. (2001). Direct neural fate specification from embryonic stem cells: a primitive
mammalian neural stem cell stage acquired through a default mechanism.
Neuron 30, 65-78.
Vallier, L., Reynolds, D. and Pedersen, R. A. (2004). Nodal inhibits
differentiation of human embryonic stem cells along the neuroectodermal
default pathway. Dev. Biol. 275, 403-421.
Walton, K. D., Croce, J. C., Glenn, T. D., Wu, S. Y. and McClay, D. R. (2006).
Genomics and expression profiles of the Hedgehog and Notch signaling
pathways in sea urchin development. Dev. Biol. 300, 153-164.
Wei, Z., Angerer, R. C. and Angerer, L. M. (2006). A database of mRNA
expression patterns for the sea urchin embryo. Dev. Biol. 300, 476-484.
Wei, Z., Yaguchi, J., Yaguchi, S., Angerer, R. C. and Angerer, L. M. (2009). The
sea urchin animal pole domain is a Six3-dependent neurogenic patterning
center. Development 136, 1179-1189.
Wei, Z., Angerer, R. C. and Angerer, L. M. (2011). Direct development of
neurons within foregut endoderm of sea urchin embryos. Proc. Natl. Acad. Sci.
USA 108, 9143-9147.
Weitzel, H. E., Illies, M. R., Byrum, C. A., Xu, R., Wikramanayake, A. H. and
Ettensohn, C. A. (2004). Differential stability of {beta}-catenin along the animalvegetal axis of the sea urchin embryo mediated by dishevelled. Development
131, 2947-2956.
Wikramanayake, A. H. and Klein, W. H. (1997). Multiple signaling events
specify ectoderm and pattern the oral-aboral axis in the sea urchin embryo.
Development 124, 13-20.
Wikramanayake, A. H., Huang, L. and Klein, W. H. (1998). beta-Catenin is
essential for patterning the maternally specified animal-vegetal axis in the sea
urchin embryo. Proc. Natl. Acad. Sci. USA 95, 9343-9348.
Wikramanayake, A. H., Hong, M., Lee, P. N., Pang, K., Byrum, C. A., Bince, J.
M., Xu, R. and Martindale, M. Q. (2003). An ancient role for nuclear betacatenin in the evolution of axial polarity and germ layer segregation. Nature
426, 446-450.
Wikramanayake, A. H., Peterson, R., Chen, J., Huang, L., Bince, J. M.,
McClay, D. R. and Klein, W. H. (2004). Nuclear beta-catenin-dependent Wnt8
REVIEW 3623
signaling in vegetal cells of the early sea urchin embryo regulates gastrulation
and differentiation of endoderm and mesodermal cell lineages. Genesis 39, 194205.
Wilson, S. W. and Houart, C. (2004). Early steps in the development of the
forebrain. Dev. Cell 6, 167-181.
Yaguchi, S., Kanoh, K., Amemiya, S. and Katow, H. (2000). Initial analysis of
immunochemical cell surface properties, location and formation of the
serotonergic apical ganglion in sea urchin embryos. Dev. Growth Differ. 42, 479488.
Yaguchi, S., Yaguchi, J. and Burke, R. D. (2006). Specification of ectoderm
restricts the size of the animal plate and patterns neurogenesis in sea urchin
embryos. Development 133, 2337-2346.
Yaguchi, S., Yaguchi, J. and Burke, R. D. (2007). Sp-Smad2/3 mediates
patterning of neurogenic ectoderm by nodal in the sea urchin embryo. Dev. Biol.
302, 494-503.
Yaguchi, S., Yaguchi, J., Angerer, R. C. and Angerer, L. M. (2008). A WntFoxQ2-nodal pathway links primary and secondary axis specification in sea
urchin embryos. Dev. Cell 14, 97-107.
Yaguchi, S., Yaguchi, J., Angerer, R. C., Angerer, L. M. and Burke, R. D.
(2010a). TGF signaling positions the ciliary band and patterns neurons in the
sea urchin embryo. Dev. Biol. 347, 71-81.
Yaguchi, S., Yaguchi, J., Wei, Z., Shiba, K., Angerer, L. M. and Inaba, K.
(2010b). ankAT-1 is a novel gene mediating the apical tuft formation in the sea
urchin embryo. Dev. Biol. 348, 67-75.
Yamaguchi, T. P. (2001). Heads or tails: Wnts and anterior-posterior patterning.
Curr. Biol. 11, R713-R724.
Yu, J. K., Holland, L. Z. and Holland, N. D. (2002). An amphioxus nodal gene
(AmphiNodal) with early symmetrical expression in the organizer and mesoderm
and later asymmetrical expression associated with left-right axis formation. Evol.
Dev. 4, 418-425.
Yu, J. K., Satou, Y., Holland, N. D., Shin, I. T., Kohara, Y., Satoh, N., BronnerFraser, M. and Holland, L. Z. (2007). Axial patterning in cephalochordates and
the evolution of the organizer. Nature 445, 613-617.
DEVELOPMENT
Development 138 (17)