Twinning and embryonic left-right asymmetry

Mirror, mirror? An evaluation of identical twin mirroring in tooth crown morphology

It has been estimated that 25% of monozygotic ("identical") twin pairs exhibit reverse asymmetry (RA) or "mirroring" of minor anatomical features as a result of delayed zygote division. Here, we examine whether identical twin mirroring accounts for patterns of dental asymmetry in a sample of monozygotic and dizygotic ("fraternal") twins. We focus on crown morphology to approach the following question: is there an association between dental RA frequency and twin type suggestive of the presence of mirror image twins in our sample? Data were collected from 208 deciduous and 196 permanent dentitions of participants of the University of Adelaide Twin Study using Arizona State University Dental Anthropology System standards. RA frequencies were compared across morphological complexes (deciduous, permanent), twin types (monozygotic, dizygotic), and traits. Fisher's exact tests were performed to formally evaluate the association between twin type and dental RA. Across the entire dataset, RA rates failed to exceed 8% for any twin type. In monozygotic twins, deciduous mirroring totaled 5.3% of observed cases, while permanent mirroring totaled 7.8% of observed cases. We found no statistically significant association between RA and twin type for any morphological character (p-value range: 0.07-1.00). Our results suggest the timing of monozygotic twin division does not explain the structure of asymmetry for our morphology dataset and that published estimates of identical twin mirroring rates may be inflated or contingent upon phenotype. Instead, rates reported for this sample more closely align with the proposed etiology of this condition.

Neural induction drives body axis formation during embryogenesis, but a neural induction-like process drives tumorigenesis in postnatal animals

Characterization of cancer cells and neural stem cells indicates that tumorigenicity and pluripotency are coupled cell properties determined by neural stemness, and tumorigenesis represents a process of progressive loss of original cell identity and gain of neural stemness. This reminds of a most fundamental process required for the development of the nervous system and body axis during embryogenesis, i.e., embryonic neural induction. Neural induction is that, in response to extracellular signals that are secreted by the Spemann-Mangold organizer in amphibians or the node in mammals and inhibit epidermal fate in ectoderm, the ectodermal cells lose their epidermal fate and assume the neural default fate and consequently, turn into neuroectodermal cells. They further differentiate into the nervous system and also some non-neural cells via interaction with adjacent tissues. Failure in neural induction leads to failure of embryogenesis, and ectopic neural induction due to ectopic organizer or node activity or activation of embryonic neural genes causes a formation of secondary body axis or a conjoined twin. During tumorigenesis, cells progressively lose their original cell identity and gain of neural stemness, and consequently, gain of tumorigenicity and pluripotency, due to various intra-/extracellular insults in cells of a postnatal animal. Tumorigenic cells can be induced to differentiation into normal cells and integrate into normal embryonic development within an embryo. However, they form tumors and cannot integrate into animal tissues/organs in a postnatal animal because of lack of embryonic inducing signals. Combination of studies of developmental and cancer biology indicates that neural induction drives embryogenesis in gastrulating embryos but a similar process drives tumorigenesis in a postnatal animal. Tumorigenicity is by nature the manifestation of aberrant occurrence of pluripotent state in a postnatal animal. Pluripotency and tumorigenicity are both but different manifestations of neural stemness in pre- and postnatal stages of animal life, respectively. Based on these findings, I discuss about some confusion in cancer research, propose to distinguish the causality and associations and discriminate causal and supporting factors involved in tumorigenesis, and suggest revisiting the focus of cancer research.

Multi-scale Chimerism: An experimental window on the algorithms of anatomical control

Despite the immense progress in genetics and cell biology, major knowledge gaps remain with respect to prediction and control of the global morphologies that will result from the cooperation of cells with known genomes. The understanding of cooperativity, competition, and synergy across diverse biological scales has been obscured by a focus on standard model systems that exhibit invariant species-specific anatomies. Morphogenesis of chimeric biological material is an especially instructive window on the control of biological growth and form because it emphasizes the need for prediction without reliance on familiar, standard outcomes. Here, we review an important and fascinating body of data from experiments utilizing DNA transfer, cell transplantation, organ grafting, and parabiosis. We suggest that these are all instances (at different levels of organization) of one general phenomenon: chimerism. Multi-scale chimeras are a powerful conceptual and experimental tool with which to probe the mapping between properties of components and large-scale anatomy: the laws of morphogenesis. The existing data and future advances in this field will impact not only the understanding of cooperation and the evolution of body forms, but also the design of strategies for system-level outcomes in regenerative medicine and swarm robotics.

Far from solved: a perspective on what we know about early mechanisms of left-right asymmetry

Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.

Consistent left-right asymmetry cannot be established by late organizers in Xenopus unless the late organizer is a conjoined twin

How embryos consistently orient asymmetries of the left-right (LR) axis is an intriguing question, as no macroscopic environmental cues reliably distinguish left from right. Especially unclear are the events coordinating LR patterning with the establishment of the dorsoventral (DV) axes and midline determination in early embryos. In frog embryos, consistent physiological and molecular asymmetries manifest by the second cell cleavage; however, models based on extracellular fluid flow at the node predict correct de novo asymmetry orientation during neurulation. We addressed these issues in Xenopus embryos by manipulating the timing and location of dorsal organizer induction: the primary dorsal organizer was ablated by UV irradiation, and a new organizer was induced at various locations, either early, by mechanical rotation, or late, by injection of lithium chloride (at 32 cells) or of the transcription factor XSiamois (which functions after mid-blastula transition). These embryos were then analyzed for the position of three asymmetric organs. Whereas organizers rescued before cleavage properly oriented the LR axis 90% of the time, organizers induced in any position at any time after the 32-cell stage exhibited randomized laterality. Late organizers were unable to correctly orient the LR axis even when placed back in their endogenous location. Strikingly, conjoined twins produced by late induction of ectopic organizers did have normal asymmetry. These data reveal that although correct LR orientation must occur no later than early cleavage stages in singleton embryos, a novel instructive influence from an early organizer can impose normal asymmetry upon late organizers in the same cell field.

Epigenesis of behavioural lateralization in humans and other animals

Despite several decades of research, the epigenesis of behavioural and brain lateralization is still elusive, although its knowledge is important in understanding developmental plasticity, function and evolution of lateralization, and its relationship with developmental disorders. Over the last decades, it has become clear that behavioural lateralization is not restricted to humans, but a fundamental principle in the organization of behaviour in vertebrates. This has opened the possibility of extending descriptive studies on human lateralization with descriptive and experimental studies on other vertebrate species. In this review, we therefore explore the evidence for the role of genes and environment on behavioural lateralization in humans and other animals. First, we discuss the predominant genetic models for human handedness, and conclude that their explanatory power alone is not sufficient, leaving, together with ambiguous results from adoption studies and selection experiments in animals, ample opportunity for a role of environmental factors. Next, we discuss the potential influence of such factors, including perinatal asymmetrical perception induced by asymmetrical head position or parental care, and social modulation, both in humans and other vertebrates, presenting some evidence from our own work on the domestic chick. We conclude that both perinatal asymmetrical perception and later social modulation are likely candidates in influencing the degree or strength of lateralization in both humans and other vertebrates. However, in most cases unequivocal evidence for this is lacking and we will point out further avenues for research.

Is left-right asymmetry a form of planar cell polarity?

Consistent left-right (LR) patterning is a clinically important embryonic process. However, key questions remain about the origin of asymmetry and its amplification across cell fields. Planar cell polarity (PCP) solves a similar morphogenetic problem, and although core PCP proteins have yet to be implicated in embryonic LR asymmetry, studies of mutations affecting planar polarity, together with exciting new data in cell and developmental biology, provide a new perspective on LR patterning. Here we propose testable models for the hypothesis that LR asymmetry propagates as a type of PCP that imposes coherent orientation onto cell fields, and that the cue that orients this polarization is a chiral intracellular structure.

What's left in asymmetry?

Left-right patterning is a fascinating problem of morphogenesis, linking evolutionary and cellular signaling mechanisms across many levels of organization. In the past 15 years, enormous progress has been made in elucidating the molecular details of this process in embryos of several model species. While many outside the field seem to believe that the fundamental aspects of this pathway are now solved, workers on asymmetry are faced with considerable uncertainties over the details of specific mechanisms, a lack of conceptual unity of mechanisms across phyla, and important questions that are not being pursued in any of the popular model systems. Here, we suggest that data from clinical syndromes, cryptic asymmetries, and bilateral gynandromorphs, while not figuring prominently in the mainstream work on LR asymmetry, point to crucial and fundamental gaps of knowledge about asymmetry. We identify 12 big questions that provide exciting opportunities for fundamental new advances in this field.

Perspectives and open problems in the early phases of left-right patterning

Embryonic left-right (LR) patterning is a fascinating aspect of embryogenesis. The field currently faces important questions about the origin of LR asymmetry, the mechanisms by which consistent asymmetry is imposed on the scale of the whole embryo, and the degree of conservation of early phases of LR patterning among model systems. Recent progress on planar cell polarity and cellular asymmetry in a variety of tissues and species provides a new perspective on the early phases of LR patterning. Despite the huge diversity in body-plans over which consistent LR asymmetry is imposed, and the apparent divergence in molecular pathways that underlie laterality, the data reveal conservation of physiological modules among phyla and a basic scheme of cellular chirality amplified by a planar cell polarity-like pathway over large cell fields.

KCNQ1 and KCNE1 K+ channel components are involved in early left-right patterning in Xenopus laevis embryos

Several ion transporters have been implicated in left-right (LR) patterning. Here, we characterize a new component of the early bioelectrical circuit: the potassium channel KCNQ1 and its accessory subunit KCNE1. Having cloned the native Xenopus versions of both genes, we show that both are asymmetrically localized as maternal proteins during the first few cleavages of frog embryo development in a process dependent on microtubule and actin organization. Molecular loss-of-function using dominant negative constructs demonstrates that both gene products are required for normal LR asymmetry. We propose a model whereby these channels provide an exit path for K(+) ions brought in by the H(+),K(+)-ATPase. This physiological module thus allows the obligate but electroneutral H(+),K(+)-ATPase to generate an asymmetric voltage gradient on the left and right sides. Our data reveal a new, bioelectrical component of the mechanisms patterning a large-scale axis in vertebrate embryogenesis.

Left-right patterning from the inside out: widespread evidence for intracellular control

The field of left-right (LR) patterning--the study of molecular mechanisms that yield directed morphological asymmetries in otherwise symmetrical organisms--is in disarray. On one hand is the undeniably elegant hypothesis that rotary beating of inclined cilia is the primary symmetry-breaking step: they create an asymmetric extracellular flow across the embryonic midline. On the other hand lurk many early symmetry-breaking steps that, even in some vertebrates, precede the onset of ciliary flow. We highlight an intracellular model of LR patterning where gene expression is initiated by physiological asymmetries that arise from subcellular asymmetries (e.g. motor-protein function along oriented cytoskeletal tracks). A survey of symmetry breaking in eukaryotes ranging from protists to vertebrates suggests that intracellular cytoskeletal elements are ancient and primary LR cues. Evolutionarily, quirky effectors like ciliary motion were likely added later in vertebrates. In some species (like mice), developmentally earlier cues may have been abandoned entirely. Late-developing asymmetries pose a challenge to the intracellular model, but early mid-plane determination in many groups increases its plausibility. Multiple experimental tests are possible.

Fgf8 haploinsufficiency results in distinct craniofacial defects in adult zebrafish

Significant progress has been made toward understanding the role of fgf8 in directing early embryonic patterning of the pharyngeal skeleton. Considerably less is known about the role this growth factor plays in the coordinated development, growth, and remodeling of the craniofacial skeleton beyond embryonic stages. To better understand the contributions of fgf8 in the formation of adult craniofacial architecture, we analyzed the skeletal anatomy of adult ace(ti282a)/fgf8 heterozygous zebrafish. Our results revealed distinct skeletal defects including facial asymmetries, aberrant craniofacial geometry, irregular patterns of cranial suturing, and ectopic bone formation. These defects are similar in presentation to several human craniofacial disorders (e.g., craniosynostosis, hemifacial microsomia), and may be related to increased levels of bone metabolism observed in ace(ti282a)/fgf8 heterozygotes. Moreover, skeletal defects observed in ace(ti282a)/fgf8 heterozygotes are consistent with expression patterns of fgf8 in the mature craniofacial skeleton. These data reveal previously unrecognized roles for fgf8 during skeletogenesis, and provide a basis for future investigations into the mechanisms that regulate craniofacial development beyond the embryo.

Early, H+-V-ATPase-dependent proton flux is necessary for consistent left-right patterning of non-mammalian vertebrates

Biased left-right asymmetry is a fascinating and medically important phenomenon. We provide molecular genetic and physiological characterization of a novel, conserved, early, biophysical event that is crucial for correct asymmetry: H+ flux. A pharmacological screen implicated the H+-pump H+-V-ATPase in Xenopus asymmetry, where it acts upstream of early asymmetric markers. Immunohistochemistry revealed an actin-dependent asymmetry of H+-V-ATPase subunits during the first three cleavages. H+-flux across plasma membranes is also asymmetric at the four- and eight-cell stages, and this asymmetry requires H+-V-ATPase activity. Abolishing the asymmetry in H+ flux, using a dominant-negative subunit of the H+-V-ATPase or an ectopic H+ pump, randomized embryonic situs without causing any other defects. To understand the mechanism of action of H+-V-ATPase, we isolated its two physiological functions, cytoplasmic pH and membrane voltage (Vmem) regulation. Varying either pH or Vmem, independently of direct manipulation of H+-V-ATPase, caused disruptions of normal asymmetry, suggesting roles for both functions. V-ATPase inhibition also abolished the normal early localization of serotonin, functionally linking these two early asymmetry pathways. The involvement of H+-V-ATPase in asymmetry is conserved to chick and zebrafish. Inhibition of the H+-V-ATPase induces heterotaxia in both species; in chick, H+-V-ATPase activity is upstream of Shh; in fish, it is upstream of Kupffer's vesicle and Spaw expression. Our data implicate H+-V-ATPase activity in patterning the LR axis of vertebrates and reveal mechanisms upstream and downstream of its activity. We propose a pH- and Vmem-dependent model of the early physiology of LR patterning.

Functional magnetic resonance imaging in adult craniopagus for presurgical evaluation

✓ Cranially conjoined twins are rare and pose unique challenges in the preoperative evaluation of cerebral language function. The authors report on their experience in the functional magnetic resonance (fMR) imaging evaluation of adult craniopagus (temporoparietooccipital fusion) to evaluate hemispheric language dominance and the eloquent language areas in the preoperative planning stages. Conventional clinical imaging hardware originally designed for individuals was adapted and tailored for use in the twins. They were assigned a selection of language tasks while undergoing fMR imaging. Significant blood oxygen level—dependent activations were detected in the main language regions in each twin, that is, the inferior frontal gyrus (around the Broca area), the middle and superior temporal lobes (around the Wernicke area) together with the inferior parietal lobe, and the middle and superior frontal gyri. Overall, the right-handed twin was strongly left lateralized for language, whereas the left-handed twin showed more bilateral activation during language tasks. Noninvasive language mapping with the aid of fMR imaging has been demonstrated for the first time in total craniopagus.

Roles for fgf8 signaling in left-right patterning of the visceral organs and craniofacial skeleton

Laterality is fundamental to the vertebrate body plan. Here, we investigate the roles of fgf8 signaling in LR patterning of the zebrafish embryo. We find that fgf8 is required for proper asymmetric development of the brain, heart and gut. When fgf8 is absent, nodal signaling is randomized in the lateral plate mesoderm, leading to aberrant LR orientation of the brain and visceral organs. We also show that fgf8 is necessary for proper symmetric development of the pharyngeal skeleton. Attenuated fgf8 signaling results in consistently biased LR asymmetric development of the pharyngeal arches and craniofacial skeleton. Approximately 1/3 of zebrafish ace/fgf8 mutants are missing Kupffer's vesicle (KV), a ciliated structure similar to Hensen's node. We correlate fgf8 deficient laterality defects in the brain and viscera with the absence of KV, supporting a role for KV in proper LR patterning of these structures. Strikingly, we also correlate asymmetric craniofacial development in ace/fgf8 mutants with the presence of KV, suggesting roles for KV in lateralization of the pharyngeal skeleton when fgf8 is absent. These data provide new insights into vertebrate laterality and offer the zebrafish ace/fgf8 mutant as a novel molecular tool to investigate tissue-specific molecular laterality mechanisms.

Left and right in the amphibian world: which way to develop and where to turn?

The last decade has seen a dramatic increase in studies on the development, function and evolution of asymmetries in vertebrates, including amphibians. Here we discuss current knowledge of behavioral and anatomical asymmetries in amphibians. Behavioral laterality in the response of both adult and larval anurans to presumed predators and competitors is strong and may be related, respectively, to laterality in the telencephalon of adults and the Mauthner neurons of tadpoles. These behavior lateralities, however, do not seem to correlate with visceral asymmetries in the same animals. We briefly compare what is known about the evolution and development of asymmetry in the structure and function of amphibians with what is known about asymmetries in other chordate and non-chordate groups. Available data suggest that the majority of asymmetries in amphibians fall into two independent groups: (1) related to situs viscerum and (2) of a neurobehavioral nature. We find little evidence linking these two groups, which implies different developmental regulatory pathways and independent evolutionary histories for visceral and telencephalic lateralizations. Studies of animals other than standard model species are essential to test hypotheses about the evolution of laterality in amphibians and other chordates.

The embryonic origins of left-right asymmetry

The bilaterally symmetric body plan of vertebrates features several consistent asymmetries in the placement, structure, and function of organs such as the heart, intestine, and brain. Deviations from the normal pattern result in situs inversus, isomerisms, or heterotaxia (independent randomization), which have significant clinical implications. The invariance of the left-right (LR) asymmetry of normal morphology, neuronal function, and phenotype of several syndromes raises fascinating and fundamental questions in cell, developmental, evolutionary, and neurobiology. While a pathway of asymmetrically expressed signaling factors has been well-characterized in several model systems, very early steps in the establishment of LR asymmetry remain poorly understood. In particular, the origin of consistently oriented asymmetry is unknown. Recently, a candidate for the origins of asymmetry has been suggested: bulk transport of extracellular morphogens by rotating primary cilia during gastrulation. This model is appealing because it 'bootstraps' morphological asymmetry of the embryo from the intrinsic structural (molecular) chirality of motile cilia. However, conceptual and practical problems remain with this hypothesis. Indeed, the genetic data are also consistent with a different mechanism: cytoplasmic transport roles of motor proteins. This review outlines the progress and remaining questions in the field of left-right asymmetry, and focuses on an alternative model for 'Step 1' of asymmetry. More specifically, based on wide-ranging data on ion fluxes and motor protein function in several species, it is suggested that laterality is driven by pH/voltage gradients across the midline, which are established by chiral movement of motor proteins with respect to the cytoskeleton.

Motor protein control of ion flux is an early step in embryonic left-right asymmetry

The invariant left-right asymmetry of animal body plans raises fascinating questions in cell, developmental, evolutionary, and neuro-biology. While intermediate mechanisms (e.g., asymmetric gene expression) have been well-characterized, very early steps remain elusive. Recent studies suggested a candidate for the origins of asymmetry: rotary movement of extracellular morphogens by cilia during gastrulation. This model is intellectually satisfying, because it bootstraps asymmetry from the intrinsic biochemical chirality of cilia. However, conceptual and practical problems remain with this hypothesis, and the genetic data is consistent with a different mechanism. Based on wide-ranging data on ion fluxes and motor protein action in a number of species, a model is proposed whereby laterality is generated much earlier, by asymmetric transport of ions, which results in pH/voltage gradients across the midline. These asymmetries are in turn generated by a new candidate for "step 1": asymmetric localization of electrogenic proteins by cytoplasmic motors.

Cerebral mirror-imaging in a monozygotic twin

A healthy monozygotic twin of discordant handedness was scanned with functional magnetic resonance imaging during a language and a mental rotation task. Mirror-imaging was found for both cerebral functions.