title: Development of the rectus abdominis and its sheath in the human fetus.
abstract: OBJECTIVE
  Although the rectus abdominis and its sheath are well known structures, their development in the human fetus is poorly understood.
  
  
  METHODS
  We examined rectus abdominis and sheath development in semiserial horizontal sections of 18 fetuses at 5-9 weeks of gestation.
  
  
  RESULTS
  Rectus muscle differentiation was found to commence above the umbilicus at 6 weeks and extend inferiorly. Until closure of the anterior chest wall via fusion of the bilateral sternal anlagen (at 7 weeks), the anterior rectal sheath originated from the external oblique and developed towards the medial margin of the rectus abdominis at all levels, including the supracostal part. After formation of the anterior sheath, fascial laminae from the internal oblique and transversus abdominis contributed to formation of the posterior rectus sheath. However, the posterior sheath was absent along the supracostal part of the rectus abdominis, as the transversus muscle fibers reached the sternum or the midline area. Therefore, it appeared that resolution of the physiological umbilical hernia (8-9 weeks) as well as chest wall closure was not required for development of the rectus abdominis and its sheath. Conversely, in the inferior part of the two largest fetal specimens, after resolution of the hernia, the posterior sheath underwent secondary disappearance, possibly due to changes in mechanical stress.
  
  
  CONCLUSIONS
  Upward extension of the rectus abdominis suddenly stopped at the margin of the inferiorly developing pectoralis major without facing the external intercostalis. The rectus thoracis, if present, might correspond to the pectoralis.
authors.affiliation: Department of Surgery, Chonbuk National University Medical School, Jeonju 561-712, Korea.
authors.forename: Jae Do
authors.initials: JD
authors.lastname: Yang
authors.forename: Hong Pil
authors.initials: HP
authors.lastname: Hwang
authors.forename: Ji Hyun
authors.initials: JH
authors.lastname: Kim
authors.forename: Jose Francisco
authors.initials: JF
authors.lastname: Rodríguez-Vázquez
authors.forename: Shin-ichi
authors.initials: S
authors.lastname: Abe
authors.forename: Gen
authors.initials: G
authors.lastname: Murakami
authors.forename: Baik Hwan
authors.initials: BH
authors.lastname: Cho
mesh_terms: D004622:Embryo, Mammalian / Q000196:embryology; D005333:Fetus / Q000196:embryology*; D006554:Hernia, Umbilical / Q000196:embryology; D006555:Hernia, Ventral / Q000196:embryology; D006801:Humans; D017568:Rectus Abdominis / Q000196:embryology*; D013249:Sternum / Q000196:embryology