Two is a Crowd

Abstract

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.

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

“…Although the proposed genesis of conjoined twins has been extensively described in another review (Boer et al, 2019), we will outline it briefly here to introduce and summarize the theories and descriptions of shared anomalies proposed in this review.…”

“…The degree of aplasia depends not only on the approximation of the two primordia, but also on their mutual angle (Boer et al, 2019). The closer the approximation and the more acute the angle, the more prominent the interaction aplasia.…”

“…The red line indicates the midline of the twins; indicating that somewhat more than 50% of each twin is still present external variability depends solely on the mutual position (in both orientation and approximation) of the duplicated embryonic primordia and their subsequent outgrowth. For more background information, the reader is referred to a previous paper on this subject (Boer et al, 2019).…”

“…Symmetrical cephalothoracoileopagus twins are considered to be formed by duplicated embryonic primordia located exactly opposite each other, and their phenotypic appearance is profoundly influenced by neo-axial orientation (Boer et al, 2019). In addition to thoracic and abdominal conjunction, this results in two complete F I G U R E 4 (a) Ilioischiopagus tripus twin with a single shared lower limb on the side affected by regional interaction aplasia.…”

Dutch teratological collections and their artistic portrayals

“…In Leiden, 86 cases of conjoined twins or twin-related anomalies were found in a recent inventory of its teratological collection (Boer et al, 2018). These twins remain a source of contemporary research as etiopathogenetic questions remain hidden behind a multitude of uncertainties (Boer, Schepens-Franke, & Oostra, 2019). It becomes even more intriguing when one is confronted with conjoined twins in which discordant phenotypes are observable (Boer et al, 2021).…”