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Conjoined Twins: Developmental Malformations and Clinical Implications The embryological theory of the author is that all conjoined twins result from fusion of separate germ discs. This is presented with an evangelistic zeal that betrays a lack of objectivity, with few qualifications or expressions of doubt. She assumes that all patterns and degrees of twinning are due to the same mechanism, and that fusion and fission are the only two mechanisms requiring consideration, so that disproof of one proves the other. Concepts that might apply to some cases are applied universally. For example, on page 416, the author expresses without qualification the opinion that teratomas are parasitic conjoined twins. Her discussion of the theoretical embryology of "parapagus" twinning (pages 246-247) provides a particularly disturbing example of fundamental convictions based upon unconvincing evidence. She dismisses fission as the mechanism of rostral duplications in the following words: Bifurcation of a single notochord has been proposed (Machin and Sperber, 1991), but as the notochord probably elongates by proliferation at the caudal aspect (O'Rahilly, 1998), bifurcation during growth should result in caudal, not cranial duplication [italics mine]. The implied support of a leading authority on embryology lends an aura of respectability to this statement. However, the only reference provided is to a personal communication, and the adverb "probably" seems to be given the status of certainty. That statement constitutes one of two chief arguments offered as proof that "parapagus" twinning results from fusion of originally separate germ discs. The other argument is presented in the following words (p. 246): In a review of the entire spectrum of parapagus—it might seem logical to assume that they arise from two separate, nearly parallel notochords on one embryonic disc, very close together caudally but with varying degrees of separation rostrally. However, secondary fusion of two separate embryonic discs is indicated by the indisputable evidence of duplication of some portion of the umbilicus, umbilical cord, or placenta in a few parapagus [italics mine]. The reasons why these few "indisputable" cases so conclusively prove her point are not explained further. No details are provided concerning these cases, and I searched in vain for reference to where they were published. On page 254, it is mentioned that these cases comprised 2% (i.e., 6) of her 305 pairs of "parapagus" twins. On page 246, it is mentioned that three of them were characterized by "an additional umbilical vessel," without noting whether these were arteries or veins, or how their number was documented. Accessory umbilical vessels can result from several developmental mechanisms other than fusion of separate embryonic discs. These six "indisputable" cases, plus the aforementioned reference to a statement by Dr. O'Rahilly, are considered sufficient to justify the following sweeping generalization (p. 246): All things considered, then, it is postulated that all conjoined twins, including the parapagus, arise from two individual notochords on two originally separate embryonic discs [italics mine]. http://link.springer-ny.com/link/service/journals/10024/contents/03/4038/paper/body.html (2 of 5) [9/9/2003 11:23:52 AM]
Conjoined Twins: Developmental Malformations and Clinical Implications Dr. Spencer's speculations on the fusion-fission controversy resurrect a debate that was popular during the 18th and early 19th centuries, when embryology was primarily the domain of philosophers. During the latter part of the 19th century, this question was subjected to scientific investigation, especially in the great embryological laboratories of Germany. It was soon recognized that the original philosophical concepts, designated in German as the "Spaltungstheorie" and "Verwachsungstheorie" ("fission" and "fusion" theory), were too simplistic. Though the evidence was more favorable to the "fission" theory (i.e., incomplete separation of monozygotic twins), it was also apparent that the induction and organization of body axes was more complex than these terms implied. Fusion versus fission was rephrased in terms more relevant to evolving concepts of developmental mechanics. The monographs of Schwalbe [1] and Hiibner [2] provide detailed overviews of experimental studies into the developmental mechanisms of conjoined twinning in the decades preceding the First World War. Hiibner's monograph [2], published in 1911, includes an appropriate epitaph for the simplistic fusion-fission controversy (p. 792): "Verschmelzung 1 oder "Spaltung 1 , dies beiden Worte, die einst so heftige Discussionen verursachten, sind heute blosse Worte ['Fusion' or 'splitting,' these words that once provoked such violent controversy, are now empty words]. In 1923, Newman's analytical review of the embryology of twins [3] added several new arguments refuting fusion as the mechanism for most cases of conjoined twinning, and that concept received little support in the following decades. The experimental embryology of conjoined twinning has now been extended to the molecular level. One model involves the production of axial duplications in xenopus larvae by injection of components of the Wnt/beta-catenin cascade at the 2 to 8-celled stage [4 5]. Comparable results are reported in zebrafish [6]. The formation and development of two-headed embryos and other axial duplication anomalies can be observed continuously in vivo. These and numerous other experimental models developed over more than a century provide substantial evidence inconsistent with Spencer's fusion theory of "parapagus" twinning. The existence of that literature is briefly mentioned in Spencer's chapter 2, but is dismissed on the basis that animal models do not necessarily apply to humans. Perhaps experimental embryology would not have been dismissed so casually if it had provided more support for her speculations. Armchair embryology may suffice for politicians and theologians, but is a poor substitute for descriptive and experimental science. Chapter 9 is devoted to the "rachipagus" pattern, a novel category of conjunction introduced by Dr. Spencer in 1995 [7]. She considers this to be the pattern providing the most convincing proof of her developmental theory. Twins joined back-to-back, with fusion of spinal columns above the sacrum, were not believed to exist until Dr. Spencer discovered a suggestive case report in a French regional medical journal [8]. That report (which I have not seen) was a radiographic study of 7-month fetuses said to be joined back-to-back. No photographs or anatomical details are provided. The radiograph, as reproduced on page 333 and in Spencer's 1995 article, depicts two individuals that appear to have posterior fusion of skulls, two faces directed in opposite directions, and apparent coalescence of vertebrae T6 to L3. The spinal columns diverged in the lumbar and cervical regions. The reproduced image is small and http://link.springer-ny.com/link/service/journals/10024/contents/03/4038/paper/body.html (3 of 5) [9/9/2003 11:23:52 AM]
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