102. The Embryological Basis of Craniopagus Twinning: Joining the Fusionists and the Fissionists

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Authors: Andrew J. Kobets, MD; Alan Cohen, MD; James Goodrich, MD, PhD (Bronx, NY)


Twins occur once in 87 live births, conjoined twins once in 50,000 births, and craniopagi once in 2.5 million births. Due to the rarity and ethical dilemmas involved in performing embryological experiments investigating the etiology of craniopagus twins, several theories have arisen surrounding conjoining events in these twins, namely ‘fusion’ and ‘fission’ theories. This report aims to describe the literature regarding these theories and presents a coalescence toward a unified theory of conjoined twinning, with a craniopagus focus.


All relevant literature pertaining to the embryological basis of conjoined twinning was reviewed.


The theories of fission and fusion, in their attempts to singularly explain the full spectrum of conjoined twinning, fail to account for key phenomenology supporting one theory over the other in different types of twins. Fusion fails to explain symmetry exhibited by conjoined twins (i.e. situs inversus), and disregards exogenous twinning stimuli (alcohol, mitotic inhibitors) which result in incomplete embryonic separation in animal models. Fissionists disregard reports of rare diamniotic conjoining, insufficient evidence describing why cranial and caudal sites are final locations of failed separation, and ‘joined’ tissues, as with the brains of craniopagi, which abut and compress each other rather than form a singular, contiguous structure.


A dual theory of conjoined twinning is proposed. In cases of omphalopagi, thoracopagi, cephalopagi, and parapagi, large ventral or lateral segments are fused between twins who demonstrate symmetry, may rarely be diamniotic omphalopagi with fused umbilical regions, may share several body parts, and may be the result of incomplete twinning stimuli consistent with a fissionist theory. Rachipagi, ischiopagi, and craniopagi are joined dorsally or along neuropore closure sites, asymmetrically, with any possible rotational component, consistent with a fusion theory. Therefore, to account for all phenomenology, a dual hypothesis may exist for the embryological development of conjoined twins.