Anatomic findings in dicephalic conjoined twins: implications for morphogenesis

A review of the mechanisms and evidence for typical and atypical twinning

The mechanisms responsible for twinning and disorders of twin gestations have been the subject of considerable interest by physicians and scientists, and cases of atypical twinning have called for a reexamination of the fundamental theories invoked to explain twin gestations. This article presents a review of the literature focusing on twinning and atypical twinning with an emphasis on the phenomena of chimeric twins, phenotypically discordant monozygotic twins, mirror-image twins, polar body twins, complete hydatidiform mole with a coexistent twin, vanishing twins, fetus papyraceus, fetus in fetu, superfetation, and superfecundation. The traditional models attributing monozygotic twinning to a fission event, and more recent models describing monozygotic twinning as a fusion event, are critically reviewed. Ethical restrictions on scientific experimentation with human embryos and the rarity of cases of atypical twinning have limited opportunities to elucidate the exact mechanisms by which these phenomena occur. Refinements in the modeling of early embryonic development in twin pregnancies may have significant clinical implications. The article includes a series of figures to illustrate the phenomena described.

Development of handed body asymmetry in mammals

We have proposed a three step model for the specification of left-right in mammalian embryos. The fundamental assumption is that handedness is imparted by an asymmetrical molecule. Conversion of molecular asymmetry to the cellular level gives a property to one side of the embryo to bias an otherwise random generation of an asymmetrical gradient which can be interpreted by developing organs. Rat embryos, treated at discrete stages, show a window of sensitivity for disruption of handedness, which may reflect the time of conversion/biasing. Heat shock and several chemicals cause left-right inversion in up to 50% of embryos exposed during neural groove formation. Earlier stages are less sensitive; no treatment begun after foregut pocket formation influences asymmetry. Evidence for cellular interactions in left-right specification comes from the apparent rescue of iv/iv mutant embryos in chimeras. We are looking for molecular left-right disparity before morphological asymmetry but detect no differences in two-dimensional protein profiles. Using an indirect measure, we find a right-left gradient of tissue oxygen in embryos at the 20-30 somite stage. This may reflect asymmetrical vasculature, as we have suggested to explain drug-induced asymmetrical limb malformations.

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.

Association of cloacal anomalies, caudal duplication, and twinning

Cloacal anomalies exhibit a wide variety of morphologic types and accompanying clinical severity. The association of malformations of the cloaca with partial, complete, or conjoined twinning has been appreciated for some time, but, with the advent of prenatal ultrasound technology, appears to occur with a greater frequency than once thought. This observation has important implications for pathogenesis. We present 2 representative cases, a 19-week-old female fetus with duplication of several caudal structures and a 21-week-old male fetus with cloacal exstrophy variant and demised co-twin with lower abdominal wall defect, extruded intestinal tract, absent external genitalia, and imperforate anus. These findings and previously published theories suggest that certain models of monozygotic twinning may apply to the pathogenesis of cloacal anomalies. Specifically, the partial or complete duplication of the organizing center within a single embryonic disc may increase the risk of mesodermal insufficiency and thus account for the failure of complete development of the cloacal membrane and consequent exstrophy or other aberration.

Dicephalus conjoined twins: a historical review with emphasis on viability

Dicephalus conjoined twins (2 heads on 1 body) form a distinct subgroup. They are often stillborn because of cardiopulmonary malformations. This article describes 17 historical cases with particular emphasis on viability. There are reliable reports of unseparated dicephali reaching maturity; 1 pair (the Tocci brothers) reached at least 34 years of age. In the absence of concomitant cardiac, pulmonary, and intestinal malformations, dicephalus conjoined twins may well attain adulthood. This appears to be unrelated to whether they have 4 (dicephalus tetrabrachius) or 3 (dicephalus tribrachius) arms at birth.

Mythos and logos in Hesiod's Theogony, circa 700 B.C

Hesiod's Theogony, a completely preserved poem, is dated from about 700 B.C. It depicts partly a development, partly a situation that has arisen in the course of time in the world. It constitutes an attempt to understand the cosmos as the product of a genealogical evolution, which can be seen as a process of successive separation, differentiation, and hierarchization. In this attempted picture of the world myth and reality are inextricably interwoven. Observations of natural phenomena as far as of congenital malformations are accordingly exaggerated and undergone mythical transformation. Entities enumerated in this genealogical poem, no matter whether they stand for parts and concrete phenomena of the physical world or intellectual properties and abstract concepts, behave, think, and act, and are accordingly though of, in anthropomorphic terms. Monogamy, polygamy, endogamy, exogamy, asexual and sexual reproduction, multiple conceptions and births, dominantly and recessively inherited traits, normal and abnormal offspring, and perceptions and notions on cosmogony, isogamy, and teratogenesis could be observed in the writing of the Theogony and interpreted by rational modern concepts.

A model for specification of the left-right axis in vertebrates

The mechanisms that underlie the formation of the left-right embryonic axis in vertebrates are not known. The programmed pattern of cell-type change in fission yeast results from the inheritance of specific chromatids of the parental chromosome. Here, I address how such a model may explain left-right specification of the viscera in mice. The model proposes that DNA replication produces different chromatids, and that these specific chromatids of both homologs are nonrandomly segregated to daughter cells to specify the left-right axis of the embryo. Such a model presents a simple explanation of the interesting phenotype of the newly discovered insertional mutation inv in mice, which causes reversal of the left-right axis, proposing that it is caused by a chromosomal inversion.

Diamniotic placentation associated with omphalopagus conjoined twins: implications for a contemporary model of conjoined twinning

We have studied omphalopagus conjoined twins with a diamniotic monochorionic placenta. Although conjoined twins usually present in a single amniotic sac, one other example of diamniotic placenta has been reported in omphalopagus twins [Weston et al., 1990: Am J Med Genet 37:558-561]. Most theories concerning the pathogenesis of conjoined twinning exclude the possibility of diamniotic placentation. However, Spencer [1992: Teratology 45:591-602] recently elaborated a model for conjoined twinning based on duplication of organizing centers (primitive streaks) during gastrulation. We have considered the fate of embryonic membranes according to this model of omphalopagus twinning and show that diamniotic placentation is a predictable outcome.

Morphology of the cardiovascular system in conjoined twins: spatial and sequential segmental arrangements in 36 cases

Thirty-six pairs of conjoined twins are reviewed from the joint experience of the authors. The external forms of conjunctions were 18 thoracopagus (joined at chest), eight cephalothoracopagus (joined at head and chest), eight dicephalus (single trunk and two heads), one craniopagus (joined at head), and one omphalopagus (joined at abdomen). There was one case with a single conventional heart in association with isomerism of the left lungs and absence of the heart in the left-side twin. Six cases had two conventional hearts. All of them had associated lesions, which were more severe in three cases (50%) with abnormal laterality. All of the cephalothoracopagus (eight cases) had two shared hearts. Cardiac lesions were commonly seen in the posterior heart, but the arrangement of atrial appendages were normal in every heart. Twenty-one cases of either dicephalus or thoracopagus had a compound heart. Three cases (14%) had fusion only at the level of venous sinus; three others (14%) were fused at the atrial level only, but 15 cases (71%) had fusion at both atrial and ventricular levels. Abnormal laterality was an associated lesion in ten cases (48%) with a compound heart. The fusion at the atrial level always occurred between right atriums, but ventricular fusion was more commonly between two left ventricles. Four atrial or ventricular chambers, if fused, were arranged in a cruciate arrangement. Mode of conjunction was as important, in the formation of the cardiovascular system, as was the degree of fusion. Abnormal laterality, which is commonly associated in lateral or mixed lateral/facing conjunction, was the most important factor affecting the general morphology of the cardiovascular system.