title: Timetable for intestinal rotation in staged human embryos and fetuses.
abstract: BACKGROUND
  The existing data on intestinal rotation during human development are contradictory regarding the timing of major events, and as such an exact timetable for rotation of the intestine in humans is not yet available.
  
  
  METHODS
  We studied the initial formation and rotation of the intestine by microdissection and histological observations in 72 human embryos and fetuses at two to 12 weeks postfertilization. The embryos were classified according to the Carnegie staging system.
  
  
  RESULTS
  The primordium of the primitive gut was first observed as a yolk sac at stage 5. With the formation of the embryonic foldings, three divisions of the primitive gut (the foregut, midgut, and hindgut) were observed at stage 10. At stage 12, the primitive gut was located on the midline. At stage 15, a 90 degrees counterclockwise rotation of the intestine began. At stage 16, herniation of the intestine into the umbilical cord was not evident in observations of the external form or a transversely sectioned embryo, but was evident in a sagittally sectioned embryo. There was another 90 degrees counterclockwise rotation at stage 20. Reduction of the intestine was a rapid process, since it was still in the cord in fetuses of <40 mm crown-rump length (CRL), and was reduced above 40 mm in general during nine weeks of development. When the intestine returned to the abdominal cavity, the cecum was located in the right lower quadrant (the adult position).
  
  
  CONCLUSIONS
  We have developed a standard timetable to describe the rotation of the intestine. The current results will be helpful in studies describing the pathogenesis of some developmental abnormalities in the intestine due to abnormal rotation.
authors.affiliation: Department of Anatomy, College of Medicine, Hanyang University, Seoul, South Korea. hwoopark@yumc.yonsei.ac.kr
authors.forename: Won Kyu
authors.initials: WK
authors.lastname: Kim
authors.forename: Hyun
authors.initials: H
authors.lastname: Kim
authors.forename: Dae Ho
authors.initials: DH
authors.lastname: Ahn
authors.forename: Myoung Hee
authors.initials: MH
authors.lastname: Kim
authors.forename: Hyoung Woo
authors.initials: HW
authors.lastname: Park
mesh_terms: D004622:Embryo, Mammalian / Q000033:anatomy & histology* / Q000473:pathology; D004628:Embryonic Structures; D005314:Embryonic and Fetal Development; D005333:Fetus / Q000033:anatomy & histology* / Q000473:pathology; D006801:Humans; D007422:Intestines / Q000196:embryology*; D013997:Time Factors