title: Diprosopia revisited in light of the recognized role of neural crest cells in facial development.
abstract: OBJECTIVE
  The aim of this study is to compare the theory of embryogenesis of the face with human diprosopia. This peculiar form of conjoined twinning is of great interest because 1) only the facial structures are duplicated and 2) almost all cases have a rather monomorphic pattern. The hypothesis is that an initial duplication of the notochord leads to two neural plates and subsequently duplicated neural crests. In those conditions, derivatives of the neural crests will be partially or totally duplicated; therefore, in diprosopia, the duplicated facial structures would be considered to be neural crest derivatives. If these structures are identical to those that are experimentally demonstrated to be neural crest derivatives in animals, these findings are an argument to apply this theory of facial embryogenesis in man.
  
  
  METHODS
  Serial horizontal sections of the face of two diprosopic fetuses (11 and 21 weeks gestation) were studied macro- and microscopically to determine the external and internal structures that are duplicated. Complete postmortem examination was performed in search for additional malformations.
  
  
  RESULTS
  The face of both fetuses showed a very similar morphologic pattern with duplication of ocular, nasal, and buccal structures. The nasal fossae and the anterior part of the tongue were also duplicated, albeit the posterior part and the pharyngolaryngeal structures were unique. Additional facial clefts were present in both fetuses. Extrafacial anomalies were represented by a craniorachischisis, two fused vertebral columns and, in the older fetus, by a complex cardiac malformation morphologically identical to malformations induced by removal or grafting of additional cardiac neural crest cells in animals.
  
  
  CONCLUSIONS
  These pathological findings could identify the facial structures that are neural crest derivatives in man. They are similar to those experimentally demonstrated to be neural crest derivatives in animals. In this respect, diprosopia could be considered as the end of a spectrum, whereas the other end is agnathia-holoprosencephaly complex. This assumption has to be discussed, but we want to draw attention to the fact that diprosopia must not be considered as a curious form of conjoined twinning, but as a major means of bringing us a better knowledge of the facial embryogenesis in man.
authors.affiliation: Unité de Foetopathologie, Groupe Hospitalier Pellegrin, Bordeaux, France.
authors.forename: D
authors.initials: D
authors.lastname: Carles
authors.forename: W
authors.initials: W
authors.lastname: Weichhold
authors.forename: E M
authors.initials: EM
authors.lastname: Alberti
authors.forename: F
authors.initials: F
authors.lastname: Léger
authors.forename: F
authors.initials: F
authors.lastname: Pigeau
authors.forename: J
authors.initials: J
authors.lastname: Horovitz
mesh_terms: D000015:Abnormalities, Multiple / Q000196:embryology; D005314:Embryonic and Fetal Development; D005145:Face / Q000002:abnormalities* / Q000196:embryology*; D005260:Female; D006330:Heart Defects, Congenital / Q000196:embryology; D006801:Humans; D009432:Neural Crest / Q000002:abnormalities*; D009436:Neural Tube Defects / Q000196:embryology; D014428:Twins, Conjoined