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Focus Article
Left–right axis determination
Marta Ibañes1 and Juan Carlos Izpisúa Belmonte2,3∗
Vertebrates display left-right (L-R) asymmetric organ positioning and morphologies, which are established during embryonic development. These asymmetries
are conserved among individuals and species. How, when and where do embryos
first break the symmetry? Why is it broken in a consistent direction? How is the
asymmetry transmitted to and coordinated within the whole embryo? Which of
these elements are conserved between different organisms? These questions have
been the focus of intense research during the last decade, and much has been
learned. Nonetheless, our understanding of how tissue and organ L-R differences
are established during embryogenesis is scarce. A systems biology approach may
enable us to better understand the dynamics of gene networks, epigenetics, cilia,
fluids, and charged molecules as well as other processes involved in the generation
of the vertebrate L-R axis.  2009 John Wiley & Sons, Inc. WIREs Syst Biol Med 2009 1 210–219
INTRODUCTION
T
he external vertebrate body plan exhibits bilateral symmetry. However, internally, both the
morphology and position of organs are left–right
(L–R) asymmetric (Figure 1). These asymmetries are
conserved among species and consistently preserved
among individuals within a species. Thus, humans
have their heart on the left side, as do mice, chickens,
zebrafish, and frogs, to name a few examples. Accurate L–R asymmetric positioning and morphogenesis
of organs is essential for proper body functioning, and
alterations of the asymmetric body plan in humans
result in severe medical conditions.1 Laterality defects
at the genetic and morphological level can involve
complete inversion of the asymmetry, randomization
of the asymmetry (within individuals or within the
different organs of a single individual), or bilaterality,
and can be classified in different types accordingly.2
These phenotypes emphasize three main aspects of
L–R axis determination: the consistent orientation of
the asymmetry, the symmetry-breaking event, and the
transmission and coordination of asymmetric information between different parts of the body at different
times.
∗ Correspondence
to: [email protected]; [email protected]
1
Department of Estructura i Constituents de la Matèria, University
of Barcelona, Barcelona, Spain
2
Gene Expression Laboratory, Salk Institute for Biological Studies,
La Jolla, CA, USA
3 Centre
of Regenerative Medicine in Barcelona, Barcelona, Spain
DOI: 10.1002/wsbm.031
210
The L–R axis is determined during embryonic
development, after the two other primary body
axes [anteroposterior (A-P) and dorsoventral (D-V)]
have been established. In the last decade, pioneering
studies showed that L–R axis determination is under
genetic control.3 Subsequently, genetic cascades and
epigenetic factors controlling L–R axis determination
have been unveiled (see Refs. 4,5,6–12 for reviews).
Some molecular elements directing L–R asymmetry
have been found to be conserved among species
(e.g., the expression of transcription growth-β factor
Nodal (Figure 1) and the paired-like homeodomain
transcription factor Pitx2 on the left lateral plate
mesoderm (LPM)), while others remain specific to
each species, with it remaining still unknown whether
L–R axis determination follows an evolutionary
conserved design with characteristic species-specific
details, or it involves a key divergent step.13–16
Research in L–R axis determination has been devoted
to finding asymmetric cues, such as asymmetric
gene expression patterns (e.g., Nodal, Pitx2, Shh to
name a few), asymmetric activities and molecular
concentrations (e.g., H+ /K+ -ATPase activity, Notch
activity, intracellular and extracellular calcium), and
asymmetric transports (e.g., nodal fluid flow, ionic
transport through gap junctions). Altogether, these
decades of research have shown that L–R axis
determination involves molecular, cellular, and tissue
dynamics with complex biological, chemical, and
physical processes.
Systems biology aims at deciphering a rationale
of the system dynamics and commonly involves its
quantification. This kind of approach is starting in
 2009 Jo h n Wiley & So n s, In c.
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Nodal expression
in the LPM
WIREs Systems Biology and Medicine
Left–right axis determination
(a)
(b)
Right
Left
(c)
Right
Left–Right
phenotype
Situs solitus
Lungs
Heart
Left
(d)
Right
Heterotaxia
Left
Isomerism
Right
Left
Situs inversus
Spleen
Liver
Stomach
Right
Left
Right
Left
Right
Left
Right
Left
FIGURE 1 | Genetic control of left–right phenotypes. (Top) Schematic drawing of four different patterns of Nodal expression in the lateral plate
mesoderm (LPM). (Bottom) Example of phenotypes arising for these expression patterns. Drawings are a modification from Capdevila et al.5
(a) Wildtype expression and phenotype. (b) An example of heterotaxia is shown where the organs are morphologically or positionally inverted.
(c) Left isomerism is shown, arising for bilateral expression of Nodal. (d) In the absence of Nodal expression, complete reversal of situs can be found.
Data in Hamada et al.4 has been used as source to link gene expression patterns with phenotypes (Bottom Panels). (Reprinted with permission from
Ref 3. Copyright 2000 Elsevier.
the context of L–R axis determination. Thus, we
find examples where multidisciplinary approaches,
which involve theoretical and computational analyses
and engineered systems jointly with challenging
experiments utilizing molecular biology, have been
used recently to tackle different questions of L–R
axis determination.17 Herein, we review some aspects
of L–R axis determination in which this kind of
multidisciplinary approaches have been essential to
foster our knowledge and emphasize open issues on
this broad topic that lie within the field of systems
biology.
DIRECTIONAL SYMMETRY BREAKING
The robust specification of the left side with respect
to the two remaining body axes poses a challenge on
how L–R axis is determined. When first defining left
and right body sides, two different fates must appear
and cells must decide whether to be a left-sided cell or
a right-sided cell. Cells must decide this in the proper
place and direction relative to the two other body axes:
if the A-P axis or the D-V axis is inverted, the L–R axis
is inverted as well. This kind of asymmetry is termed
fixed or directional asymmetry.18 To account for the
directionality of L–R axis establishment, Brown and
Wolpert proposed a theoretical model based on a
chiral molecule.19 This molecule, the ‘F-molecule’,
contains the information of all three axes, exhibiting
an asymmetric L–R shape that results from A-P and
D-V axes. According to the scenario envisaged by
Vo lu me 1, September/Octo ber 2009
Brown and Wolpert, this chiral molecule indicates the
left and right directions and thus settles down the bias
for directional L–R asymmetry at the molecular level.
Then, transport mechanisms coupled to the molecular
asymmetry defined by the chiral molecule can spread
the asymmetry first to the whole cell and to the tissue
level afterwards.
Recently, there has been much evidence that
highlights cilia, which involve a chiral structure
of motor proteins, as a potential F-molecule-like
candidate that can drive L–R axis determination.20
An initial correlation between cilia and L–R axis
specification came from a congenital disease in
humans termed Kartagener’s syndrome.21 The human
population with this syndrome has immotile cilia
in the sperm and trachea and has a high incidence
of L–R reversed organs. A similar condition can
be found in mutant mice inversus viscerum (iv),22
eliciting a thorough study of ciliary dynamics and
L–R asymmetry in these mammals (see Ref. 23 for
a review). The first asymmetric event observed in
mouse development occurs around embryonic days
7.5–8 and corresponds to a leftward fluid flow in a
cavity termed the node. The node is located along
the midline at the anterior end of the primitive streak
and has a few hundred monocilited cells. The cilia in
these cells, with a 9 + 0 architecture, rotate clockwise
when viewed from the ventral side and create a flow
of extraembryonic fluid crossing the node from right
to left.24 The absence of fluid flow when cilia are
lacking or are immotile suggested that the motion
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V
P
A
D
L
id
Flu w
Flo
Nodal
cell
(a)
R
P
V
A
D L
id
Flu w
l
Fo
(b)
FIGURE 2 | Nodal ciliary motion and leftward fluid flow. Cells (grey
circles) are monociliated. Left–right asymmetry is broken at the level of
each single cell and is transferred to a field of cells through the fluid
flow (green arrow). (a) Motion in wildtype mouse embryos. The motion
is clockwise (red arrow) when viewed ventrally and tilted to the
posterior (dashed line). As a guide to the eye, the simplified trajectory of
the tip of cilia is depicted by a dotted line. Cilia perform a two-phase
nonplanar beating motion with a power stroke (blue) and a recovery
stroke (orange). The shape and bending of cilia along its motion (blue
and orange lines) are depicted in four cells. In two additional cells, the
extent of the two phases of motion is schematically drawn in the cone
defined by the motion of the tip of cilia. A leftward fluid flow arises
(green arrow). (b) Motion in inv mutants. A minority of cilia perform an
inverted beating motion, tilted to the anterior (red dashed line), which
elicits swirls and slowing down of the leftward flow (green arrow).
of cilia drives directional flow.24–27 In addition, the
flow itself was shown to direct L–R asymmetry,
since reversing it in wildtype embryos elicits L–R
inversion, while introducing a leftward fast fluid flow
in iv mutants restores proper L–R asymmetry.28,29
Therefore, these data provided a scenario in which
cilia induce a leftward fluid flow that directs the
asymmetry (Figure 2(a)).
But still there was an open issue: could the
clockwise motion of cilia per se elicit a leftward
fluid flow? A whole set of techniques have been put
forward to solve this hydrodynamic problem.30–38
The small spatial and velocity scales involved in the
node sets the fluid dynamics in a regime where our
212
intuition fails.30 In this regime (called low Reynolds
number), viscosity dominates over inertia. A first
novel theoretical and computational approach based
on steady fluid dynamics at low Reynolds numbers
predicted a key element: the motion of cilia must
be around an axis tilted to the posterior to elicit a
leftward flow.30 Subsequently, high-speed recording
of the projection of the motion of the tip of cilia
on the surface of the node provided evidence that
the projected trajectory has an elliptic shape as could
be expected from a motion around an axis tilted
to the posterior.33,34 Indeed, by assuming such a
motion, the angle of tilting could be inferred and
a posterior tilting angle of 30◦ –45◦ was found.34
Scanning electron microscopy images of cilia in
wildtype mice and iv/iv mutants confirmed a posterior
tilting of cilia, while an engineered mechanical
system supported experimentally that posteriorly
tilted clockwise rotation can induce a leftward flow.33
Taking into account the two-phase beating motion
9 + 2 cilia and flagella perform, a theoretical and
computational study that characterized, for the first
time, the spatiotemporal dynamics of cilia and of
beads embedded in the fluid flow proposed that the
motion of cilia corresponds indeed to an asymmetric
two-phase nonplanar beating which ensures the
asymmetric flow.32 According to the nonplanar
beating motion proposal, cilia move faster towards the
left when moving far from the cell surface (effective or
power stroke), and bend and move more slowly when
passing rightwards close to the cell surface (recovery
stroke).32 The propulsion of the flow is favored
during the power stroke, setting the asymmetry of
the flow.32,39–41 Physical arguments on the force
required to bend cilia point out that both internal
(e.g., cilia motors) and external (e.g., viscous resistance
created by the cell surface) factors are necessary to
dictate the directionality of the power and recovery
strokes.32 The two-phase beating motion of cilia was
supported by high-speed temporal imaging of nodal
ciliary motion.32,34 In addition, this kind of motion
clarified the origin of the features exhibited by the flow
in inversion of embryonic turning (inv) mutants.32 In
this mutant, the flow is leftward, albeit slower and
more swirly.26 The presence of a minority of cilia
(20%) with reversed beating motion (i.e., rightward
power stroke and leftward recovery stroke) in a node
containing mainly immobile and wildtype cilia elicits
a slower meandering motion of beads embedded in
the flow, which overall move from right to left, as
observed through video microscopy and confirmed
computationally32,34 (Figure 2(b)).
Altogether, these results suggest that, in mice,
symmetry is broken first at the molecular and at the
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Left–right axis determination
single-cell levels through the chiral structure of motor
proteins involved in cilia motion (clockwise leftward
nonplanar beating) coupled to A-P and D-V broken
symmetries (orientation of cilia motion along the A-P
axis and their protrusion from the ventral side). In this
context, it is interesting to mention that theoretical and
numerical studies on cilia motion have found that the
structure of cilia does not prevent counterclockwise
motion to arise;31,37 albeit clockwise direction is the
main beating mode when feedback between the motor
activity and the displacement of microtubule doublets
of cilia is taken into account.37 Moreover, the cilia and
fluid flow mechanism exemplifies how the information
single cells have regarding where their left and right
sides are can become an asymmetry that leads to
defining a body axis (i.e., an asymmetry over a field of
cells that establishes the left and right sides across the
midline, Figure 2). In this case, the nodal fluid flow is
in charge of expanding the asymmetry along the node.
A CONSERVED EVENT?
Importantly, the cilia and fluid flow mechanism is
able to break bilateral symmetry in a consistent way,
without requiring any L–R preexisting asymmetry.
Moreover, support to this mechanism in zebrafish
and Xenopus laevis embryos has been provided as
well.42–45 In addition, it could potentially play a role
in rabbits and medaka embryos where a leftward
flow and posteriorly tilted clockwise ciliary motion
have been observed, but no evidence for their relation
to L–R axis determination has been provided yet.34
Thus, it may be the conserved symmetry-breaking
event developmental biologists have been searching
for. Interestingly, the actual quantitative details of cilia
and fluid flow motion differ among species (Table 1),
which can be evaluated through mathematical and
computational modeling.32 For instance, ciliated
organs with more sparsely distributed or stiffer cilia
are expected to exhibit slower leftward fluid flows.32
Significantly, those elements relevant for settling down
a leftward fluid flow are conserved: low Reynolds
numbers and the directional features of cilia motion
and position (Table 1). However, inv mutants, which
have a leftward fluid flow but reverse L–R asymmetry,
exemplifies the notion that the L–R axis is not just
defined by the direction of flow and that additional
information such as the flow speed may play a role as
well. Accordingly, further features may be conserved,
such as the Peclet number that takes into account
the speed of the flow and evaluates which is the
dominating transport mechanism.30
Several data suggest additional complexities.
The leftward fluid flow is the first known asymmetry
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in mice. In contrast, earlier L–R asymmetries exist in
fish and amphibians before the action of cilia and the
leftward fluid flow. In zebrafish embryos the earliest
element known to control L–R axis determination is
the action of the H+ /K+ ATPase.43 Interestingly, this
activity does not alter cilia or fluid flow dynamics,
which emerge at later stages.43 Therefore, L–R
axis determination seems to use parallel processes
to transfer L–R asymmetric information, at least
in fish. In Xenopus, asymmetries in microfilamentdependent organization have been suggested during
the first cell cycle,46 while H+ /K+ -ATPase (H-V)
mRNA is asymmetrically localized along the three
spatial axes during the first cleavage stages.47–49 These
asymmetries are observed at stages much earlier than
the emergence of the leftward fluid flow.45 It remains
to be elucidated whether these early asymmetries
in Xenopus embryos alter cilia and the nodal flow
dynamics. Moreover, in rabbits, the fibroblast growth
factor fgf8 has been shown to control L–R asymmetry
at earlier stages than the onset of asymmetric fluid
flow.50 Finally, while nodal cilia have been reported in
chick embryos,51 it remains to be elucidated whether
they are motile, induce a leftward fluid flow, and
control L–R asymmetry.
The controversy of whether cilia and fluid flow
are the first symmetry-breaking events highlights
further possibilities.6,7,11,52–56 A model for a conserved
mechanism of L–R asymmetry (the ‘intracellular
model’) has been proposed.53,57 According to this
model, L–R asymmetry arises at very early stages
inside cells driven by an F-like molecule, which
is proposed to be a cytoskeletal element.53 The
F-molecule elicits a directional transport of key
molecules (maternal mRNAs and ion-transporter
proteins) within a cell, which subsequently can
revert on the whole body through ion fluxes.
Notably, despite the lack of identification of the Fmolecule, motor proteins, ion fluxes, and gap junction
communication have been shown to be relevant for
L–R axis determination in a wide variety of species,
ranging from ciliates and plants, to mammals.53
TRANSFER OF THE ASYMMETRY
A conserved feature among vertebrate embryos is
the left-sided expression of the gene Nodal in the
LPM, which is key for the proper establishment
of asymmetric morphologies such as the looping of
the heart and gut (Figure 1).5,58,59 Nodal asymmetric
expression in the LPM requires Nodal expression
around the node at earlier stages.60–64 Actually, in
all vertebrate embryos examined so far, the cascade
of L–R asymmetric information goes at some point
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TABLE 1 Cilia and Flow Dynamics in Different Model Organisms
WT Mouse
iv Mutant
inv/inv
Mutant
Rabbit
Zebrafish
Medaka
Xenopus
∼5
∼9
∼3
∼5
∼3
Cilia
length (µm)
∼5
Cilia rotation
speed (Hz)
11 ± 3
0
∼100 (50%)
7±2
∼26
43 ± 3
∼20, 25
Cilia rotation
direction
CW
—
CW CCW
(5%)
CW
CCW (Dv)
CW
CW
Cilia tilting
P (40o )
—
A (20%)
P(40o )
P (45o )
P (40o )
P (60%)
Cilia protrusion
V
V
V
V
D
V
V
Ciliated organ
Node
Node
Node
NP
KV
KV
GP
and dimensions
100 µm
100 µm
100 µm
150 µm
600 µm
150 µm
240 µm
Main direction
of flow
L
None
L
L
L
L,CCW
L
Speed fluid flow
(µm/s)
4–19
0
1
—
7
4
References
34
26
Less than
wildtype
26 and 34
34
42 , 43 and 44
34
45
Slow and fast phases of cilia motion has been studied only for wildtype (WT) and inv mutant mouse embryos. Direction of rotation from ventral view, if not
stated otherwise (Dv). NP, KV, GP stands for notochordal plate, Kupffer’s vesicle, gastrocoel roof plate, respectively. CW and CCW indicate clockwise and
counterclockwise motion. L, R, A, P, D,V indicate left, right, anterior, posterior, dorsal, and ventral, respectively. Percentages refer to the fraction of cilia within
a node.
through the node, located along the midline. During
the last decade, the molecular cascades that process
and transfer the L–R asymmetric information from
the node to the organs have been a focus of
thorough study. For recent reviews on the molecular
cascades controlling L–R axis determination, see Refs.
7,8,10,23,65,66. Herein we focus on specific cases for
which theoretical modeling has been used to evaluate
which dynamics is taking place.
The pattern of Nodal expression around the
node differs between species. In chick embryos,
Nodal is asymmetrically expressed around the node
at stage Hamburger Hamilton 6 and requires Notch
signaling.62,63 The formulation and simulation of a
network of the main genes and proteins involved,
jointly with its experimental validation and testing,
helped deciphering how and which asymmetries of
earlier stages were transferred to the asymmetric
perinodal expression of Nodal.63 Asymmetries in
the activity of the H+ /K+ -ATPase at stage 449
elicits a transient higher left-sided extracellular Ca2+
level.63 This epigenetic asymmetry is translated
into a genetic asymmetric pattern of expression
by modifying the binding affinity of receptor
Notch with its ligands, as suggested computationally
and corroborated experimentally. According to the
numerical analysis, such change in binding affinities
enables a switch-like response of left-sided Nodal
expression at the border between ligand domains if
Notch signaling is properly enhanced63 (Figure 3).
214
The periodic waves of expression, from the posterior
end to anterior regions of the embryo, of the
glycosyltransferase Lunatic fringe, a modulator of
the Notch pathway, enhances the Notch pathway
increasingly over time63 and mainly on the left side
where receptor-ligand binding affinities are higher
because of extracellular Ca2+ . Thus, after a period
of time, the Notch pathway on the left side reaches
high enough activity to induce Nodal expression.
Therefore, a time-dependent coupling with the A-P
triggers left-sided Nodal expression.63 Interestingly,
time-dependent specification of left and right cell
fates mediated by Notch has been recently proposed
to be acting as well during the establishment of
subnuclear asymmetry between habenular nuclei in
zebrafish.67
Mouse embryos exhibit bilateral expression of
Nodal around the node, which depends as well on
the Notch pathway and which is essential for leftsided Nodal expression in the LPM.61 Recently, by
means of mathematical modeling supported by novel
experimental data, asymmetric expression of Nodal
in the LPM of mouse embryos has been proposed
to be driven by a reaction–diffusion mechanism
that transiently amplifies a slight asymmetry, which
comes from the leftward fluid flow along the
node.69 The nodal flow is assumed to drive a
small initial asymmetry of Nodal expression. The
reaction-diffusion system then amplifies it through
Nodal self-enhancement and long-range inhibition
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Left–right axis determination
Signal
wildtype
Right
Left
L–R asymmetry impaired
Right
Left
Right
Left
Right
Left
Switch dynamics
Left cell fate
Right cell fate
Left cell
fate
Right cell fate
FIGURE 3 | Transfer of body left–right asymmetric information. We show schematically two types of mechanisms (switch dynamics) that transfer
a signal (in blue) into two cell types (the left cell type, in blue, and the right cell type, in green). The first column shows the switch dynamics. Cells are
initially equivalent (grey), but under the influence of a signal decide to become a left cell or a right cell. The spatial pattern of the signal (depicted in
rectangles in the top row) controls where each cell fate is chosen. Subsequent columns show the outcome for each dynamic under the signal pattern
indicated at the top row. (middle row) This switch dynamics amplifies the signal or converts it into another type of asymmetry. Therefore, the cell
pattern correlates with the pattern of the signal. An example of this mechanism is the conversion of asymmetric extracellular Ca2+ into asymmetric
Nodal expression around the node in chick embryos.63 (bottom row) This switch dynamics amplifies the signal and ensures asymmetry through
communication (red line) between the states of cells at each side across the midline. Accordingly, for symmetric patterns of the signal, a random
outcome is found (50% show inverted phenotype). An example of this mechanism is reaction–diffusion dynamics involving short-range activation and
long-range inhibition.68,69 In the figure, the dotted line stands for the midline.
mediated by Lefty, another protein of the transforming
growth-β family which is activated by Nodal and
has longer range effects, suggesting it may diffuse
faster. Note that the dynamics of local activation
and long-range inhibition can ensure asymmetry
(Figure 3), as initially proposed from theoretical
grounds,68,70 since both left and right sides compete
for expression by interacting through diffusible
molecules. Therefore, reaction–diffusion dynamics
can elicit Nodal expression either on the left or on
the right side of the LPM, as found experimentally
in manipulated embryos,69 and the proper bias is
thought to be settled down by the nodal flow.
An open issue is how the leftward fluid flow is
translated into asymmetric information. One proposal
is that the fluid flow transports a morphogen
which becomes asymmetrically distributed.26 The
plausibility of this transport mechanism was first
confirmed by means of a theoretical analysis of
Vo lu me 1, September/Octo ber 2009
the nodal hydrodynamics which estimated the Peclet
number and numerically tested that the direct
transport driven by the flow is more relevant than
diffusive transport as well as pointed out the relevance
of protein half-lives.30 Experimental validation using
exogenous proteins has been confirmed afterwards.34
In addition, vesicular nodal parcels have been found
to be transported by the fluid flow.71 The plausibility
of this transport has been theoretically confirmed as
well;35 albeit physical arguments on the force exerted
by the flow and the cilia challenge the proposal that
these vesicles are broken down through collisions with
the node or cilia35 as it has been proposed from the
motion observed in video microscopy recording.71
Another scenario for translation of the fluid flow
asymmetric information proposes that the flow is
mechanosensed by cilia. According to this model,
some cilia in the node sense the pressure and stresses
of the fluid flow and respond accordingly. Results
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on mutants kif3a and iv, which exhibit distinct patterns of expression of Nodal in the LPM (bilateral and
absence or random asymmetric expression, respectively) while none of them has a fluid flow (iv has
immotile cilia and kif3a cannot assemble cilia), were
some of the evidences fostering this model.72 The finding of two population of cilia within the mouse node,
one of motile cilia and one of immotile sensing cilia,
supported this model.54,73 However, a hydrodynamic
study has challenged it, since the pressure and stresses
of the flow are expected to be symmetrical on both
sides of the node.30 Notice that both scenarios take
into account the direction of the flow as well as other
features such as its speed. In order to clarify these
issues, it may be worth it to investigate further how
Nodal becomes consistently expressed in the right
LPM in inv embryos which show a slow leftward and
swirly fluid flow.
CONCLUSION
L–R axis determination involves many different
aspects ranging from complex nonlinear molecular
and cell dynamics, symmetry breaking and pattern
formation processes, and hydrodynamics, to name
a few. Systems biology approaches characterize and
quantify all the relevant different aspects to provide an
integrated understanding. Herein, we have reviewed
specific examples in which mathematical and computational modeling of these aspects has been essential
to evaluate and propose frameworks as well as to
raise predictions to understand how the embryo first
decides which side is left and which side is right and
how it transmits this information to different spatial
regions. We have seen as well that these approaches
can help pinpointing which elements are essential for
the conservation of a mechanism.
Many issues in vertebrate L–R axis determination are still unanswered which pose challenges to
systems biology. This is the case of unveiling the mechanisms driving L–R asymmetric morphogenesis. At
present, the coupling of early asymmetric molecular
events with L–R asymmetric cellular behaviors is starting to be deciphered (see Refs 74,75,76 as examples
where multidisciplinary approaches have been used to
unveil the forces acting during left-right asymmetric
morphogenesis). Relating molecular asymmetries to
asymmetric morphogenesis will provide a systemslevel understanding of L–R axis determination and
will need novel combined computational and experimental efforts. Another issue is the structure of the
flow of L–R axis information along the embryo.
Is there really a hierarchical structure of information flow (and hence, a first symmetry breaking that
controls all subsequent processes)? Is information processed in parallel, eliciting nonlinear cascades? Can
L–R axis establishment be decoupled into independent functional modules (e.g., a module to generate
random asymmetry and a module for consistent
bias (chiral molecule) as proposed in Brown and
Wolpert19 ? Novel multidisciplinary systems biology
approaches that can identify and evaluate the plausibility and features of different networks of L–R
information flows are necessary to address these
questions.
NOTES
We apologize to the authors of original work not cited due to manuscript length constraints. Work in
the laboratory of JCIB was supported by grants from MEC, the Marato and the Cellex Foundation. MI
acknowledges support from the Ramon y Cajal Program the NIH, and FIS2006-05019 (Ministry of Science and
Innovation, Spain).
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