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Learning Developmental Biology
Collaborative Resources for Learning Developmental Biology
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Conjoined Twins
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Anna Ferrer Vaquer

Additional Author(s): Anna-Katerina Hadjantonakis

Early Embryogenesis: Blastula
Endoderm-derived: Digestive (Gut) Tract; Respiratory System
Mesoderm-derived: Notochord; Cardiovascular System; Somites; Reproductive & Excretory; Limb Formation; Muscle
Ectoderm-derived: Nervous System; Epidermis
Extraembryonic: Extraembryonic Tissues
Organism: Human
Stage of Development: Larva/Juvenile

Object Description

Conjoined twins are classified according to the major site of attachment followed by the suffix pagus (from the Greek, pagos, fixed) and are grouped into dorsal and ventral attachments. Ventral attachments involve the sharing of the yolk sac or abdomen, while dorsal attachments involve union at the neural tube. Ventral unions characterize cephalopagus, thoracopagus, omphalopagus, parapagus and ischiopagus twins. Dorsal unions distinguish craniopagus, pygopagus and rachipagus twins. Cephalopagus twins have a fused head and often a fused thorax. Thoracopagus twins have a fused thorax and shared cardiac anatomy from a single intercardiac vessel to a shared heart. Omphalopagus twins are fused at the umbilicus displaying similar fusion patterns as their thoracopagus counterparts but without a shared heart. Parapagus twins are joined caudolaterally resulting in a joined abdomen, pelvis and lower limbs having different levels of thoracic and cranial union. Ischiopagus twins share a pelvis and may be oriented face-to-face or end-to-end. Craniopagus twins are united at any portion of the cranial vault but not including the face or the vertebrae. They share the cranium, the meninges, the dural venous sinuses and less frequently, the brain. Pygopagus twins are joined at the sacrum sharing the sacrococcygeal and perineal region, while rachipagus twins are fused dorsally at the spine, sharing part of the spinal cord.


Ferrer-Vaquer, A., Hadjantonakis, A.K. Birth defects associated with perturbations in preimplantation, gastrulation, and axis extension: from conjoined twinning to caudal dysgenesis. WIREs Dev Biol, 2012, Published Online: Nov 26 2012.

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