Laterality defects in conjoined twins

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.

An atypical basement membrane forms a midline barrier during left-right asymmetric gut development in the chicken embryo

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1 + midline barrier to separate the two sides of the embryo, without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals are poorly understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

Monosymmetros Cephalothoracopagus Tetrabrachius and Tetrapus Piglets with Syndromic Evolution

Conjoined twins are rare congenital malformations that have been reported in mammals. Two different cases are presented in this study. Case No. 1 features monocephalic, thoracopagus-conjoined twin piglets with anencephaly and palatoschisis of the Pietrain breed, and case No. 2 features monocephalic, thoracopagus conjoined twin piglets with palatoschisis and bifid root tongue of a mixed breed. These cases were examined using post-mortem and computed tomography (CT) examinations. In both cases, the conjoined symmetrical twins had a single head, one neck, and fused thoracic cavities, while the abdominal cavities were separated. Similarly, in both cases, they had four forelimbs and four hindlimbs and duplicated foramen magnum. During CT examination, in case No. 1, severe abnormalities were observed in the skull and vertebral column. In the left twin, occult dysraphism was seen from the C2 vertebra until the end of the vertebral column, and in the right twin, from the C3 vertebra until the end of the state vertebral level. In case No. 2, the oral cavity contained a tongue with a bifid root connected with one hyoid bone, and the soft palate presented a small cleft. During CT examination, the parietal bone and the occipital bones were partially duplicated. This case also presented occult dysraphism, but only in the cervical vertebrae, C1-C6 for the left twin and C1-C5 for the right twin. In both cases, abnormalities of the internal organs were revealed during necropsy. Conjoined twins with multiple congenital anomalies presented here enhance our understanding of the various clinical forms of conjoined cases in veterinary medicine.

A Case Report of Dextrocardia with Situs Inversus: A Rare Condition and Its Clinical Importance

Situs inversus totalis (SIT) is a rare medical condition characterized by a complete mirror-image reversal of the normal positioning of the internal organs. Aristotle initially described situs inversus in animals, while Fabricius first characterized it in humans. Although the specific cause is uncertain, there is evidence of autosomal recessive and X-linked inheritance. Before seeking treatment for an unrelated ailment, many people with SIT are unaware of their distinct anatomy, as in our case. The presented case is a 30-year-old female patient from Central Ethiopia, presented to Hakim Gizaw Teaching Hospital outpatient department of medicine with the complaint of right-sided anterior chest pain for five days. Clinically, the apical beat was heard in the right 5th intercostal space. After undergoing an electrocardiogram (ECG), echocardiogram, chest X-ray, and abdominal ultrasound, she was diagnosed with situs inversus totalis. The clinical implications of SIT encompass challenges in diagnosis and procedures, potential congenital cardiac abnormalities, considerations for organ transplantation, and clinical decision-making, particularly in emergency scenarios. Key Clinical Messages. This case highlights the clinical implications of dextrocardia with situs inversus totalis, emphasizing the importance of awareness for accurate diagnosis, procedural challenges, and informed clinical decision-making. Understanding this rare condition is crucial for providing appropriate medical care and to navigate potential complications in affected individuals.

Collective intelligence: A unifying concept for integrating biology across scales and substrates

A defining feature of biology is the use of a multiscale architecture, ranging from molecular networks to cells, tissues, organs, whole bodies, and swarms. Crucially however, biology is not only nested structurally, but also functionally: each level is able to solve problems in distinct problem spaces, such as physiological, morphological, and behavioral state space. Percolating adaptive functionality from one level of competent subunits to a higher functional level of organization requires collective dynamics: multiple components must work together to achieve specific outcomes. Here we overview a number of biological examples at different scales which highlight the ability of cellular material to make decisions that implement cooperation toward specific homeodynamic endpoints, and implement collective intelligence by solving problems at the cell, tissue, and whole-organism levels. We explore the hypothesis that collective intelligence is not only the province of groups of animals, and that an important symmetry exists between the behavioral science of swarms and the competencies of cells and other biological systems at different scales. We then briefly outline the implications of this approach, and the possible impact of tools from the field of diverse intelligence for regenerative medicine and synthetic bioengineering.

Nested Selves: Self-Organization and Shared Markov Blankets in Prenatal Development in Humans

The immune system is a central component of organismic function in humans. This paper addresses self-organization of biological systems in relation to-and nested within-other biological systems in pregnancy. Pregnancy constitutes a fundamental state for human embodiment and a key step in the evolution and conservation of our species. While not all humans can be pregnant, our initial state of emerging and growing within another person's body is universal. Hence, the pregnant state does not concern some individuals but all individuals. Indeed, the hierarchical relationship in pregnancy reflects an even earlier autopoietic process in the embryo by which the number of individuals in a single blastoderm is dynamically determined by cell- interactions. The relationship and the interactions between the two self-organizing systems during pregnancy may play a pivotal role in understanding the nature of biological self-organization per se in humans. Specifically, we consider the role of the immune system in biological self-organization in addition to neural/brain systems that furnish us with a sense of self. We examine the complex case of pregnancy, whereby two immune systems need to negotiate the exchange of resources and information in order to maintain viable self-regulation of nested systems. We conclude with a proposal for the mechanisms-that scaffold the complex relationship between two self-organising systems in pregnancy-through the lens of the Active Inference, with a focus on shared Markov blankets.

Phenotypically Discordant Anomalies in Conjoined Twins: Quirks of Nature Governed by Molecular Pathways?

A multitude of additional anomalies can be observed in virtually all types of symmetrical conjoined twins. These concomitant defects can be divided into different dysmorphological patterns. Some of these patterns reveal their etiological origin through their topographical location. The so-called shared anomalies are traceable to embryological adjustments and directly linked to the conjoined-twinning mechanism itself, inherently located within the boundaries of the coalescence area. In contrast, discordant patterns are anomalies present in only one of the twin members, intrinsically distant from the area of union. These dysmorphological entities are much more difficult to place in a developmental perspective, as it is presumed that conjoined twins share identical intra-uterine environments and intra-embryonic molecular and genetic footprints. However, their existence testifies that certain developmental fields and their respective developmental pathways take different routes in members of conjoined twins. This observation remains a poorly understood phenomenon. This article describes 69 cases of external discordant patterns within different types of otherwise symmetrical mono-umbilical conjoined twins and places them in a developmental perspective and a molecular framework. Gaining insights into the phenotypes and underlying (biochemical) mechanisms could potentially pave the way and generate novel etiological visions in the formation of conjoined twins itself.

Characterizing the coalescence area of conjoined twins to elucidate congenital disorders in singletons

Shared anomalies, always located close to the area of coalescence and observable in virtually every type of conjoined twinning, are currently seen as separate anomalies caused by mostly unknown and seemingly unrelated pathways rather than being connected to the twinning mechanism itself. Therefore, most (case) reports about conjoined twins are mere descriptions of (external) dysmorphologies lacking reflections on the possible origin of their concomitant anomalies. As we will demonstrate in this article, shared anomalies are influenced, and in some cases solely and sequentially explained, by interaction aplasia and neo‐axial orientation; two embryological mechanisms to which each set of conjoined twins is subjected and are responsible for their ultimate phenotypical fate. In this review, we consider how the ventral, lateral and caudal conjunction types and their intermediates determine the phenotypic presentation of the twins, including patterns of shared malformations and anomalies, which in themselves can be indistinguishable from those encountered in singleton cases. Hence, it can be hypothesized that certain anomalies in singletons originate in a fashion similar to that in conjoined twins.

A parapagus dicephalus tripus tribrachius conjoined twin with a unique morphological pattern: a case report

Background

Conjoined twinning is a rare congenital malformation with an incidence of about 1.5 per 100,000 births. Because no consensus has been reached regarding the dysmorphology, thorough descriptions of conjoined twins as part of teratological collections can be useful to increase knowledge of this congenital malformation. In this case report, we describe a parapagus dicephalus twin from the collection of the Department of Anatomy of the University Medical Center Utrecht in the Netherlands. External anatomical characteristics were assessed through a detailed macroscopic examination and internal characteristics by means of whole-body computed tomography and magnetic resonance imaging (3 Tesla).

Case presentation

Macroscopic examination showed a Caucasian male parapagus dicephalus tripus tribrachius conjoined twin a type of conjoined twinning in which there are two heads side by side, one rump, and three upper and three lower limbs. In addition, anencephaly was observed in the left twin. Radiological imaging showed a normal central nervous system in the right twin and absence of the calvaria, cerebrum, diencephalon, and mesencephalon in the left twin. There was clear duplication of the vertebral column, rib cage, respiratory system, and gastrointestinal system at least up to and including the first part of the duodenum. The heart consisted of a monoatrium with two separate ventricles. There was a fused liver with a single gallbladder, a single spleen, three kidneys, two bladders, and duplication of the penis. The third upper and lower extremities were articulating with a fused glenoid and acetabulum, respectively. The third foot showed both polydactyly and syndactyly of the toes.

Conclusion

This case report describes a unique case of a male dicephalus parapagus tripus tribrachus conjoined twin discordant for anencephaly. Radiological imaging proved to be an adequate noninvasive method to provide insights into the internal (dys)morphology of this specific specimen, improving its scientific and educational value. This approach could be generally applied to other teratological specimens, thereby strengthening arguments regarding pathogenetic hypotheses, which may lead to new or improved insights into both normal and abnormal embryonic development.

Two is a Crowd: Two is a Crowd: On the Enigmatic Etiopathogenesis of Conjoined Twinning

In this article, we provide a comprehensive overview of multiple facets in the puzzling genesis of symmetrical conjoined twins. The etiopathogenesis of conjoined twins remains matter for ongoing debate and is currently cited-in virtually every paper on conjoined twins-as partial fission or secondary fusion. Both theories could potentially be extrapolated from embryological adjustments exclusively seen in conjoined twins. Adoption of these, seemingly factual, theoretical proposals has (unconsciously) resulted in crystallized patterns of verbal and graphic representations concerning the enigmatic genesis of conjoined twins. Critical evaluation on their plausibility and solidity remains however largely absent. As it appears, both the fission and fusion theories cannot be applied to the full range of conjunction possibilities and thus remain matter for persistent inconclusiveness. We propose that initial duplication of axially located morphogenetic potent primordia could be the initiating factor in the genesis of ventrally, laterally, and caudally conjoined twins. The mutual position of two primordia results in neo-axial orientation and/or interaction aplasia. Both these embryological adjustments result in conjunction patterns that may seemingly appear as being caused by fission or fusion. However, as we will substantiate, neither fission nor fusion are the cause of most conjoined twinning types; rather what is interpreted as fission or fusion is actually the result of the twinning process itself. Furthermore, we will discuss the currently held views on the origin of conjoined twins and its commonly assumed etiological correlation with monozygotic twinning. Finally, considerations are presented which indicate that the dorsal conjunction group is etiologically and pathogenetically different from other symmetric conjoined twins. This leads us to propose that dorsally united twins could actually be caused by secondary fusion of two initially separate monozygotic twins. An additional reason for the ongoing etiopathogenetic debate on the genesis of conjoined twins is because different types of conjoined twins are classically placed in one overarching receptacle, which has hindered the quest for answers. Clin. Anat. 32:722-741, 2019. © 2019 Wiley Periodicals, Inc.

From cytoskeletal dynamics to organ asymmetry: a nonlinear, regulative pathway underlies left-right patterning

Consistent left-right (LR) asymmetry is a fundamental aspect of the bodyplan across phyla, and errors of laterality form an important class of human birth defects. Its molecular underpinning was first discovered as a sequential pathway of left- and right-sided gene expression that controlled positioning of the heart and visceral organs. Recent data have revised this picture in two important ways. First, the physical origin of chirality has been identified; cytoskeletal dynamics underlie the asymmetry of single-cell behaviour and patterning of the LR axis. Second, the pathway is not linear: early disruptions that alter the normal sidedness of upstream asymmetric genes do not necessarily induce defects in the laterality of the downstream genes or in organ situs Thus, the LR pathway is a unique example of two fascinating aspects of biology: the interplay of physics and genetics in establishing large-scale anatomy, and regulative (shape-homeostatic) pathways that correct molecular and anatomical errors over time. Here, we review aspects of asymmetry from its intracellular, cytoplasmic origins to the recently uncovered ability of the LR control circuitry to achieve correct gene expression and morphology despite reversals of key 'determinant' genes. We provide novel functional data, in Xenopus laevis, on conserved elements of the cytoskeleton that drive asymmetry, and comparatively analyse it together with previously published results in the field. Our new observations and meta-analysis demonstrate that despite aberrant expression of upstream regulatory genes, embryos can progressively normalize transcriptional cascades and anatomical outcomes. LR patterning can thus serve as a paradigm of how subcellular physics and gene expression cooperate to achieve developmental robustness of a body axis.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Xenopus, an ideal model organism to study laterality in conjoined twins

Conjoined twins occur at low frequency in all vertebrates including humans. Many twins fused at the chest or abdomen display a very peculiar laterality defect: while the left twin is normal with respect to asymmetric organ morphogenesis and placement (situs solitus), the organ situs is randomized in right twins. Although this phenomenon has fascinated already some of the founders of experimental embryology in the 19th and early 20th century, such as Dareste, Fol, Warynsky and Spemann, its embryological basis has remained enigmatic. Here we summarize historical experiments and interpretations as well as current models, argue that the frog Xenopus is the only vertebrate model organism to tackle the issue, and outline suitable experiments to address the question of twin laterality in the context of cilia-based symmetry breakage.

Leftward Flow Determines Laterality in Conjoined Twins

Conjoined twins fused at the thorax display an enigmatic left-right defect: although left twins are normal, laterality is disturbed in one-half of right twins [1-3]. Molecularly, this randomization corresponds to a lack of asymmetric Nodal cascade induction in right twins [4]. We studied leftward flow [5, 6] at the left-right organizer (LRO) [7, 8] in thoracopagus twins in Xenopus, which displayed a duplicated, fused, and ciliated LRO. Cilia were motile and produced a leftward flow from the right LRO margin of the right to the left margin of the left twin. Motility was required for correct laterality in left twins, as knockdown of dynein motor dnah9 prevented Nodal cascade induction. Nodal was rescued by parallel knockdown of the inhibitor dand5 [9, 10] on the left side of the left twin. Lack of Nodal induction in the right twin, despite the presence of flow, was due to insufficient suppression of dand5. Knockdown of dand5 at the center of the fused LRO resulted in asymmetric Nodal cascade induction in the right twin as well. Manipulation of leftward flow and dand5 in a targeted and sided manner induced the Nodal cascade in a predictable manner, in the left twin, the right one, both, or neither. Laterality in conjoined twins thus was determined by cilia-driven leftward fluid flow like in single embryos, which solves a century-old riddle, as the phenomenon was already studied by some of the founders of experimental embryology, including Dareste [11], Fol and Warynsky [12], and Spemann and Falkenberg [13] (reviewed in [14]).

A chick embryo with a yet unclassified type of cephalothoracopagus malformation and a hypothesis for explaining its genesis

Cephalothoracopagus embryos are conjoined twins, who share parts of their heads, necks and bodies. Our study aims at presenting a detailed morphological analysis of a cephalothoracopagus chick embryo of developmental stage 31. Because none of the existing theories can explain the genesis of the phenotype of this embryo, we also suggest a hypothesis, which explains it. Beside the cephalothoracopagus embryo, we investigated five control embryos. With the aid of the high-resolution episcopic microscopy (HREM) technique, we created digital volume data and three-dimensional (3D) computer models of the organs and arteries of the embryos. We used the 3D models for topological analysis and for measuring the diameters of the great intrathoracic arteries. The malformed embryo showed two body backs, each containing a notochord, spinal cord and dorsal aorta. The body backs continued into separated lower bodies. The embryo had a single, four-chambered heart, single respiratory tract and single upper alimentary tract. The topology of the pharyngeal arch arteries was normal, and the diameters of these arteries were similar to that of the control embryos. We classified the embryo we investigated as a yet unknown malformation and suggest a hypothesis explaining its genesis.

Causal Hypothesis for Some Congenital Anomalies

Congenital anomalies are a major cause of fetal and neonatal death and of childhood morbidity. Chromosomal and other genetic abnormalities, environmental teratogens and some nutritional deficiencies account for some congenital anomalies but the majority are of unknown etiology. The hypothesis is here proposed that a significant proportion of congenital anomalies and cerebral palsy of unknown etiology are attributable to a monozygotic multiple conception with monochorionic placentation and that these anomalies, even in singletons, may be explained by early, unrecognized or unrecorded loss of one conceptus in a monochorionic monozygotic conception. The pathological mechanism is hemodynamic instability with episodes of acute feto–fetal transfusion that produce ischemic organ impairment in either or both twins. The resultant clinical abnormality will depend on range of severity (fetal death, infant death, congenital anomaly, normal infant), site or combination of sites (which organ[s] present[s] with the congenital anomaly) and timing (early, middle or late in gestation as shown by variation in brain pathology that is observed).

Absence of the spleen(s) in conjoined twins: a diagnostic clue of laterality defects? Radiological study of historical specimens

BACKGROUND

Laterality defects are quite common in thoracoileopagus and parapagus dicephalus but rare in other types of conjoined twins.

OBJECTIVE

To present the presumed laterality defects in cephalothoracoileopagus and prosopothoracoileopagus conjoined twins, based on the unilateral or bilateral absence or duplication of the spleen.

MATERIALS AND METHODS

Three human anatomical specimens of craniothoracoileopagus (CTIP) twins and one of prosopothoracoileopagus (PTIP) twins were investigated. The specimens were part of the Museum Vrolik collection of the Department of Anatomy and Embryology of the Academic Medical Centre, University of Amsterdam, The Netherlands. The specimens were taken out of their jars and scanned with multidetector CT and volumetric T2-weighted MRI at 1.5 T.

RESULTS

The internal anatomy of the specimens was largely in accordance with previous reports. However, there was no recognisable spleen in the right twin in one CTIP specimen, in the left twin in one other CTIP specimen, and in both twins in the third CTIP specimen and in the PTIP specimen.

CONCLUSION

Asplenia and polysplenia are considered reliable indicators of right and left isomerism, respectively. However, three of our four specimens had laterality patterns that did not correspond with those previously reported. Since no other parameters of laterality defects could be verified in these specimens, we concluded that asplenia was unlikely to be caused by laterality defects.

Conjoined twins: a worldwide collaborative epidemiological study of the International Clearinghouse for Birth Defects Surveillance and Research

Conjoined twins (CT) are a very rare developmental accident of uncertain etiology. Prevalence has been previously estimated to be 1 in 50,000 to 1 in 100,000 births. The process by which monozygotic twins do not fully separate but form CT is not well understood. The purpose of the present study was to analyze diverse epidemiological aspects of CT, including the different variables listed in the Introduction Section of this issue of the Journal. The study was made possible using the International Clearinghouse for Birth Defects Surveillance and Research (ICBDSR) structure. This multicenter worldwide research includes the largest sample of CT ever studied. A total of 383 carefully reviewed sets of CT obtained from 26,138,837 births reported by 21 Clearinghouse Surveillance Programs (SP) were included in the analysis. Total prevalence was 1.47 per 100,000 births (95% CI: 1.32-1.62). Salient findings including an evident variation in prevalence among SPs: a marked variation in the type of pregnancy outcome, a similarity in the proportion of CT types among programs: a significant female predominance in CT: particularly of the thoracopagus type and a significant male predominance in parapagus and parasitic types: significant differences in prevalence by ethnicity and an apparent increasing prevalence trend in South American countries. No genetic, environmental or demographic significant associated factors were identified. Further work in epidemiology and molecular research is necessary to understand the etiology and pathogenesis involved in the development of this fascinating phenomenon of nature.

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.

Malformations in a chornobyl-impacted region

OBJECTIVE

One of the populations most exposed to chronic low-dose radiation from Chornobyl (Chernobyl in Russian) lives in Polissia, the region representing the northern half of Rivne Province (Oblast) in Ukraine. Here the patterns and population rates of malformations are reported and possible etiologic factors and regional contrasts are explored.

PATIENTS AND METHODS

Malformations, as defined by international standards, noted among all 96 438 births in Rivne between 2000 and 2006, were analyzed statistically. Contrasts of rates in Polissia compared with the rest of Rivne also were investigated.

RESULTS

The overall rate of neural tube defects in Rivne is among the highest in Europe (22.2 per 10,000 live births). The rates of conjoined twins and teratomas also seem to be elevated. In Polissia, the overall rates of neural tube defects are even higher (27.0 vs 18.3, respectively; odds ratio: 1.46 [95% confidence interval: 1.13-1.93]), and the rates of microcephaly and microphthalmia may also be elevated.

CONCLUSIONS

The malformation patterns observed suggest early disruptions of blastogenesis, manifesting as alterations of body axes, twinning, duplications, laterality, and midline formation. The results are sufficiently compelling to justify continuing and expanding this investigation of malformations in chronic low-dose radiation-impacted regions of Ukraine.

Gnatho-thoracopagus conjoined twins with near di-symmetry: a case report of an unusual form of conjoined twinning

The term gnatho-thoracopagus was used previously to describe a remote case of conjoined twins with union involving the mandible. Few reported cases of this type of union exist in the literature. A case of ventrally conjoined twins, with union involving the mandible and near di-symmetry, is presented here and compared with similar previously reported cases. The presented case represents an example of prosopo-thoracopagus, an intermediate between cephalopagus and thoracopagus conjoined twins, although this case does have unique variations.

The influence of twinning on cardiac development

The risk for a cardiac anomaly in a twin pregnancy is increased, particularly in monochorionic twins. This is relevant in terms of fetal diagnosis as well as for the management of the pregnancy; there are also implications for the neonatal period and possibly beyond. The risk for a cardiac abnormality depends on the type of monochorionic twin as determined by the timing of embryonic division. Prenatal identification of twin type and the relative risks for a cardiac anomaly are discussed along with theories for the aetiology of the different cardiac lesions.

Prevalence and pathogenesis of congenital anomalies in cerebral palsy

BACKGROUND

It has been hypothesised that cerebral palsy (CP) and other congenital anomalies are attributable to feto-fetal transfusion problems in a monochorionic multiple gestation. Thus more than one organ could be compromised leading to the coexistence of two or more anomalies in a fetus. Such anomalies in a singleton birth may be attributable to early demise of the co-conceptus as a vanishing twin.

AIM

To determine whether the coexistence of congenital anomalies and CP is greater than a chance finding by comparing the prevalence of congenital anomalies in children with CP with that in the general population of children.

METHODS

A population-based register of children with CP born in 1966-1991 in the counties of Merseyside and Cheshire, UK, comprised the index population. Coexisting congenital anomalies were recorded. For comparison the population prevalence of congenital anomalies was obtained from eight congenital malformation registers in the UK.

RESULTS

Children with CP were found to have highly significant increases in risk for microcephaly, isolated hydrocephaly, congenital anomalies of the eye, congenital cardiac anomalies, cleft lip and/or palate and congenital dislocation of the hips and talipes (p<0.001) and atresias of the oesophagus (p<0.001) and intestines (p<0.01). The relative risks ranged from 3.1 (95% CI 1.9 to 4.8; p<0.001) for congenital malformations of the cardiac septa to 116.09 (95% CI 84.0 to 162.3; p<0.001) for microcephaly.

CONCLUSIONS

Congenital anomalies in children with CP are found much more frequently than expected by chance. A common pathogenic mechanism may account for the coexistence of disparate congenital anomalies. A hypothesis is proposed for such a common pathogenic mechanism.

A Case of Conjoined Cephalopagus Twinning in an Ostrich (Struthio camelus)

Conjoined twinnings have been reported in most domestic animal species and in some avian species. Cases of conjoined twins have not been reported in the ostrich so far. A hybrid Blue neck x African black male ostrich conjoined twinning was born at the end of artificial egg incubation and died spontaneously 24 h after the hatching. It was frozen and sent to the Unit of Veterinary Pathology of the University of Messina for gross examination. The most important gross findings involved the external body and most of the internal organs. On the basis of the duplication, the conjoined twins were classified as a cephalopagus. Radiological features included: development of one head containing a single brain, two spinal cords, deviated vertebral columnae with fusion of the two first cervical vertebrae. In one twin, the synsacrum was absent as well as portions of the vertebral column. Grossly, both twins showed two upper and lower limbs each. The gastro-enteric apparatuses of the twins were not completely developed and fused at different levels. One liver and one heart localized in the centre of the conjoined twins were observed. The authors conclude that the possible causes of the malformation could be related to a genetic factor.

[Twins: a response to the question of genetic/environmental influence on development?]

This article presents recent data about human twinning and explains how twin studies can bring precious informations about craniofacial growth. These natural experiences of growth phenomenon can give clues about genetic/environment interactions during development.

Anteriorward shifting of asymmetric Xnr1 expression and contralateral communication in left-right specification in Xenopus

Transient asymmetric Nodal signaling in the left lateral plate mesoderm (L LPM) during tailbud/early somitogenesis stages is associated in all vertebrates examined with the development of stereotypical left-right (L-R) organ asymmetry. In Xenopus, asymmetric expression of Nodal-related 1 (Xnr1) begins in the posterior L LPM shortly after the initiation of bilateral perinotochordal expression in the posterior tailbud. The L LPM expression domain rapidly shifts forward to cover much of the flank of the embryo before being progressively downregulated, also in a posterior-to-anterior direction. The mechanisms underlying the initiation and propagation of Nodal/Xnr1 expression in the L LPM, and its transient nature, are not well understood. Removing the posterior tailbud domain prevents Xnr1 expression in the L LPM, consistent with the idea that normal embryos respond to a posteriorly derived asymmetrically acting positive inductive signal. The forward propagation of asymmetric Xnr1 expression occurs LPM-autonomously via planar tissue communication. The shifting is prevented by Nodal signaling inhibitors, implicating an underlying requirement for Xnr1-to-Xnr1 induction. It is also unclear how asymmetric Nodal signals are modulated during L-R patterning. Small LPM grafts overexpressing Xnr1 placed into the R LPM of tailbud embryos induced the expression of the normally L-sided genes Xnr1, Xlefty, and XPitx2, and inverted body situs, demonstrating the late-stage plasticity of the LPM. Orthogonal Xnr1 signaling from the LPM strongly induced Xlefty expression in the midline, consistent with recent findings in the mouse and demonstrating for the first time in another species conservation in the mechanism that induces and maintains the midline barrier. Our findings suggest that there is long-range contralateral communication between L and R LPM, involving Xlefty in the midline, over a substantial period of tailbud embryogenesis, and therefore lend further insight into how, and for how long, the midline maintains a L versus R status in the LPM.

The key to left-right asymmetry

Establishment of left-right asymmetry in vertebrates involves cilia as essential components in the breaking of symmetry, an asymmetric signaling cascade, and a midline barrier that helps to maintain asymmetry. A new study suggests that a reaction-diffusion mechanism also plays a key role.

Is the early left-right axis like a plant, a kidney, or a neuron? The integration of physiological signals in embryonic asymmetry

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well-characterized, the left-right (LR) axis has only relatively recently begun to be understood at the molecular level. The mechanisms that ensure invariant LR asymmetry of the heart, viscera, and brain involve fundamental aspects of cell biology, biophysics, and evolutionary biology, and are important not only for basic science but also for the biomedicine of a wide range of birth defects and human genetic syndromes. The LR axis links biomolecular chirality to embryonic development and ultimately to behavior and cognition, revealing feedback loops and conserved functional modules occurring as widely as plants and mammals. This review focuses on the unique and fascinating physiological aspects of LR patterning in a number of vertebrate and invertebrate species, discusses several profound mechanistic analogies between biological regulation in diverse systems (specifically proposing a nonciliary parallel between kidney cells and the LR axis based on subcellular regulation of ion transporter targeting), highlights the possible importance of early, highly-conserved intracellular events that are magnified to embryo-wide scales, and lays out the most important open questions about the function, evolutionary origin, and conservation of mechanisms underlying embryonic asymmetry.

Left-right asymmetry in the vertebrate embryo: from early information to higher-level integration

Although vertebrates seem to be essentially bilaterally symmetrical on the exterior, there are numerous interior left-right asymmetries in the disposition and placement of internal organs. These asymmetries are established during embryogenesis by complex epigenetic and genetic cascades. Recent studies in a range of model organisms have made important progress in understanding how this laterality information is generated and conveyed to large regions of the embryo. Both commonalities and divergences are emerging in the mechanisms that different vertebrates use in left-right axis specification. Recent evidence also provides intriguing links between the establishment of left-right asymmetries and the symmetrical elongation of the anterior-posterior axis.

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.

The craniopagus malformation: classification and implications for surgical separation

Craniopagus twins (CPT) are an uncommon, highly fascinating accident of nature. The clinical pathology of this complex entity is reviewed and placed in perspective. A logical classification aids understanding of the anomaly, and is essential to gauge outcome from separation attempts. 'Partial forms' lack significant shared dural venous sinuses (SDVS) and 'Total forms' with SDVS also exhibit more severe compressional brain distortion. Our classification consists of Partial Angular (PA), Partial Vertical (PV), Total Angular (TA) and Total Vertical (TV, formerly O'Connell Types I-III). Total vertical has a continuous cranium, and inter-twin axial facial rotation <40 degrees (Type I), 140-180 degrees (Type II) or intermediate (Type III). The term 'Angular' denotes an inter-twin longitudinal angle below 140 degrees , regardless of axial rotation. Our review categorized 64 well-delineated CPT, including 41 operative separation attempts in small children since initial success in 1952. Just over one-half were TV, almost one-third TA, and partial forms accounted for the remaining one-sixth. About 30% of CPT had shared or fused brain tissue, and a similar percentage of TA twins shared a posterior fossa. Partial forms had significantly higher birth weights, were separated at an earlier age (6 versus 11 months) and had lower mortality and better outcome compared with Total forms. A multi-staged surgical separation for Total CPT had a significantly better mortality than single-staged separation. Discussion emphasizes embryological, anatomical and clinical aspects of the malformation, with emphasis upon obstacles to a successful outcome.

Human laterality disorders

Heterotaxia is a group of congenital disorders characterized by a misplacement of one or more organs according to the left-right axis. Bilateral asymmetry of internal organs is conserved among all vertebrate species. Analyses in animal models such as mouse, chicken, frog and zebrafish allowed for a remarkable progress of knowledge on the embryonic and genetic mechanisms underlying internal left-right asymmetry. In this review we focus on the insights from these model organisms that are useful for a better understanding of the etiology and pathogenesis of human heterotaxia. The known causes of human heterotaxia are reviewed and situated within the conceptual framework that originates from vertebrate model organisms. Furthermore, we attempt to apply the rapidly increasing insights gained from both animal models and human genetics to clinical practice in order to contribute to a more accurate conceptual classification, genetic diagnosis and counseling.

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.

Familial recurrence of heart defects in subjects with congenitally corrected transposition of the great arteries

Familial recurrence of congenitally corrected transposition of the great arteries (CCTGA) is considered uncommon. Most of the previous familial studies involved a small number of patients and referred to all situs and looping anomalies including single ventricle, heterotaxia, and other cardiac defects different from CCTGA. We performed a large, consecutive clinical case series study in order to detect the recurrence of congenital heart defects in families of children with the classic form of CCTGA. From January 1997 through December 2004, 102 consecutive patients with CCTGA were evaluated in four institutions. There were 59 male (57.8%) and 43 female (42.2%). Mean age was 8.6 +/- 7.8 years. Eighty-eight patients (86.3%) had situs solitus of the atria, 14 (13.7%) situs inversus. The cardiac and extracardiac anomalies among relatives and the patterns of familial recurrence were investigated. Relatives with congenital heart defects were found in 16/102 families (15.7%). Transposition of the great arteries (TGA) was the most common recurrent defect (6/102 families). Consanguinity was identified in the parents of three probands. Six probands had an unaffected twin-sib. Recurrence risks for congenital heart defects were calculated at 5.2% (6/116) for siblings. In conclusion, CCTGA is not always sporadic in families. The pattern of inheritance, the presence of consanguinity among parents and the recurrence of situs inversus could suggest, in some families, an autosomal recessive mechanism with similarities with that occurring in some pedigrees with heterotaxia. The recurrence of TGA and CCTGA in the same family suggests a pathogenetic link between these two anatomically different malformations.

Echocardiographic assessment of conjoined twins

OBJECTIVE

To determine the accuracy of prenatal and postnatal echocardiography in delineating the degree of cardiac fusion, intracardiac anatomy (ICA), and ventricular function of 23 sets of conjoined twins with thoracic level fusion presenting to a single centre over a 20 year period.

METHODS

13 thoracopagus, 5 thoraco-omphalopagus, and 5 parapagus pairs presenting to the authors' institution between 1985 and 2004 inclusive were assessed. Echocardiographic data were analysed together with operative intervention and outcome. Twins were classified according to the degree of cardiac fusion: separate hearts and pericardium (group A, n = 5), separate hearts and common pericardium (group B, n = 7), fused atria and separate ventricles (group C, n = 2), and fused atria and ventricles (group D, n = 9).

RESULTS

The degree of cardiac fusion was correctly diagnosed in all but one set. ICA was correctly diagnosed in all cases, although the antenatal diagnosis was revised postnatally in three cases. Abnormal ICA was found in one twin only in two group A pairs, one group B pair, and both group C pairs. All group D twins had abnormal anatomy. Ventricular function was good in all twins scanned prenatally, and postnatally function correlated well with clinical condition. Thirteen sets of twins in groups A-C were surgically separated; 16 of 26 survived. None from groups C or D survived.

CONCLUSIONS

Prenatal and postnatal echocardiography accurately delineates cardiac fusion, ICA, and ventricular function in the majority of twins with thoracic level fusion. It is integral in assessing feasibility of separation. The outcome in twins with fused hearts remains dismal.

Left-right asymmetry in embryonic development: a comprehensive review

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well characterized, the left-right (LR) axis has only recently begun to be understood at the molecular level. The mechanisms which ensure invariant LR asymmetry of the heart, viscera, and brain represent a thread connecting biomolecular chirality to human cognition, along the way involving fundamental aspects of cell biology, biophysics, and evolutionary biology. An understanding of LR asymmetry is important not only for basic science, but also for the biomedicine of a wide range of birth defects and human genetic syndromes. This review summarizes the current knowledge regarding LR patterning in a number of vertebrate and invertebrate species, discusses several poorly understood but important phenomena, and highlights some important open questions about the evolutionary origin and conservation of mechanisms underlying embryonic asymmetry.

Developmental mechanism and evolutionary origin of vertebrate left/right asymmetries

The systematically 'handed', or directionally asymmetrical way in which the major viscera are packed within the vertebrate body is known as situs. Other less obvious vertebrate lateralisations concern cognitive neural function, and include the human phenomena of hand-use preference and language-associated cognitive partitioning. An overview, rather than an exhaustive scholarly review, is given of recent advances in molecular understanding of the mechanism that ensures normal development of 'correct' situs. While the asymmetry itself and its left/right direction are clearly vertebrate-conserved characters, data available from various embryo types are compared in order to assess the likelihood that the developmental mechanism is evolutionarily conserved in its entirety. A conserved post-gastrular 'phylotypic' stage, with left- and right-specific cascades of key, orthologous gene expressions, clearly exists. It now seems probable that earlier steps, in which symmetry-breaking information is reliably transduced to trigger these cascades on the correct sides, are also conserved at depth although it remains unclear exactly how these steps operate. Earlier data indicated that the initiation of symmetry-breaking had been transformed, among the different vertebrate classes, as drastically as has the anatomy of pre-gastrular development itself, but it now seems more likely that this apparent diversity is deceptive. Ideas concerning the functional advantages to the vertebrate lifestyle of a systematically asymmetrical visceral packing arrangement, while untestable, are accepted because they form a plausible adaptationist 'just-so' story. Nevertheless, two contrasting beliefs are possible about the evolutionary origins of situs. Major recent advances in analysis of its developmental mechanism are largely due not to zoologists, comparative anatomists or evolutionary systematists, but to molecular geneticists, and these workers have generally assumed that the asymmetry is an evolutionary novelty imposed on a true bilateral symmetry, at or close to the origin of the vertebrate clade. A major purpose of this review is to advocate an alternative view, on the grounds of comparative anatomy and molecular systematics together with the comparative study of expressions of orthologous genes in different forms. This view is that situs represents a co-optation of a pre-existing, evolutionarily ancient non-bilaterality of the adult form in a vertebrate ancestor. Viewed this way, vertebrate or chordate origins are best understood as the novel imposition of an adaptively bilateral locomotory-skeletal-neural system, around a retained non-symmetrical 'visceral' animal. One component of neuro-anatomical asymmetry, the habenular/parapineal one that originates in the diencephalon, has recently been found (in teleosts) to be initiated from the same 'phylotypic' gene cascade that controls situs development. But the function of this particular diencephalic asymmetry is currently unclear. Other left-right partitionings of brain function, including the much more recently evolved, cerebral cortically located one associated with human language and hand-use, may be controlled entirely separately from situs even though their directionality has a particular relation to it in a majority of individuals. Finally, possible relationships are discussed between the vertebrate directional asymmetries and those that occur sporadically among protostome bilaterian forms. These may have very different evolutionary and molecular bases, such that there may have been constraints, in protostome evolution, upon any exploitation of left and right for complex organismic, and particularly cognitive neural function.

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.

Conjoined twins: morphogenesis of the heart and a review

Five cases of conjoined twins have been studied. These included three thoracopagus twins, one monocephalus diprosopus (prosop = face), and one dicephalus dipus dibrachus. The thoracopagus twins were conjoined only from the upper thorax to the umbilicus with a normal foregut. These three cases shared a single complex multiventricular heart, one with a four chambered heart with one atrium and one ventricle belonging to each twin with complex venous and arterial connection; two had a seven chambered heart with four atria and three ventricles. The mono-cephalus diprosopus twins had a single heart with tetralogy of Fallot. The dicephalus twins had two separate axial skeletons to the sacrum, two separate hearts were connected between the right atria with a shared inferior vena cava. Thoracopagus twinning is associated with complex cardiac malformations. The cardiac anlagen in cephalopagus or diprosopus are diverted and divided along with the entire rostral end of the embryonic disc and result in two relatively normal shared hearts. However, in thoracopagus twins the single heart is multiventricular and suggests very early union with fusion of the cardiac anlagen before significant differentiation. Cardiac morphogenesis in conjoined twins therefore appears to depend on the site of the conjoined fusion and the temporal and spatial influence that determines morphogenesis as well as abnormally oriented embryonic axes.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Conjoined twins cephalopagus janiceps monosymmetros: a case report

BACKGROUND

We present an extremely rare case of Janiceps type of female cephalopagus conjoined twins. Various pathological mechanisms that may be responsible for abnormal morphological development (limited to the conjoined part and normal morphological development in free lower abdomen and pelvis) are discussed.

METHODS

Conjoined twins were obtained from a hospital, fixed in formalin, and dissected to observe and report the abnormalities present.

RESULTS

Anatomical features included development of normal face on one side, rudimentary face (two pinna only) on the opposite side, two cerebra each connected to the eye ball of its own side byoptic nerve without any chiasma formation, two brain stems, two spinal cords, normally developed vertebral columns, four upper limbs, four lower limbs, a single shared foregut and small intestine up to Meckel's diverticulum beyond which it duplicated for the two twins, patent large intestines and anal canals. The genitourinary system was developed normally in both twins. The heart of one twin was relatively hypoplastic with the lungs. The liver was herniated on one side and the other side it was hypoplastic without any herniation. Gall bladder was absent on both sides. Conjoined hepatic ducts from both livers and the only pancreatic duct drained into esophagogastric junction.

CONCLUSIONS

We propose that the development of upper abnormally conjoined parts and lower normally developed parts of the body of both the twins may be attributed to the factors associated with variable degrees of the conjoining, dysgenetic (developmental) defects, and deformations because of crowding in utero.

Node and midline defects are associated with left-right development in Delta1 mutant embryos

Axes formation is a fundamental process of early embryonic development. In addition to the anteroposterior and dorsoventral axes, the determination of the left-right axis is crucial for the proper morphogenesis of internal organs and is evolutionarily conserved in vertebrates. Genes known to be required for the normal establishment and/or maintenance of left-right asymmetry in vertebrates include, for example, components of the TGF-beta family of intercellular signalling molecules and genes required for node and midline function. We report that Notch signalling, which previously had not been implicated in this morphogenetic process, is required for normal left-right determination in mice. We show, that the loss-of-function of the delta 1 (Dll1) gene causes a situs ambiguous phenotype, including randomisation of the direction of heart looping and embryonic turning. The most probable cause for this left-right defect in Dll1 mutant embryos is a failure in the development of proper midline structures. These originate from the node, which is disrupted and deformed in Dll1 mutant embryos. Based on expression analysis in wild-type and mutant embryos, we suggest a model, in which Notch signalling is required for the proper differentiation of node cells and node morphology.