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Conjoined twins: theoretical embryologic basis.

R Spencer1

  • 1Louisiana State University School of Medicine, New Orleans.

Teratology
|June 1, 1992
PubMed
Summary
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Conjoined twins result from embryonic fusion, not fission, with specific union sites explaining all seven types. This study proposes a new term, parapagus, for anterolaterally united twins.

Area of Science:

  • Embryology
  • Teratology
  • Developmental Biology

Background:

  • Conjoined twinning is a rare congenital anomaly.
  • Existing theories on twinning mechanisms lack comprehensive explanations.
  • Clinical observations and literature review are crucial for understanding twinning.

Purpose of the Study:

  • To establish a theoretical basis for the embryology of conjoined twins.
  • To explain the formation of all seven types of conjoined twins.
  • To propose a new classification term for specific twin types.

Main Methods:

  • Clinical experience with ten cases of conjoined twins.
  • Extensive review of embryologic and clinical literature (over 500 cases).
  • Formulation of a theoretical model based on fusion of embryonic structures.

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Main Results:

  • Concluded that conjoined twins form exclusively through fusion, not fission.
  • Identified seven specific sites of embryonic union corresponding to seven types of conjoined twins.
  • Proposed the term 'parapagus' for anterolaterally united twins, formerly classified as dicephalus or diprosopos.

Conclusions:

  • The fusion theory adequately explains the embryogenesis of all conjoined twin types.
  • Specific embryonic fusion sites (notochords, neuropores, membranes, cardiac anlage, umbilicus) dictate twin morphology.
  • Parasitic twins result from the embryonic death of one twin, with the survivor vascularizing remaining structures.