title: Normal and abnormal embryonic development of the anorectum in rats.
abstract: BACKGROUND
  The embryologic and pathologic aspect of anorectal malformation (ARM) remains poorly understood. There is no universally accepted theory to explain anorectal embryology and the abnormal development that produces ARM. The aim of this study was to observe the developmental processes of anorectum in rats and to explore the abnormal embryonic development that leads to ARM.
  
  
  METHODS
  Rat embryos with ARM were obtained by treating pregnant rats with administration of ethylenethiourea (ETU). Normal rat embryos and embryos with ARM from gestational days 12.5 to 20 were sectioned serially and sagittally and stained with H & E. The relevant structure including cloaca and urorectal septum (URS) were examined in a temporospatial sequence.
  
  
  RESULTS
  Characteristics of anorectum development in ARM rat embryos treated by ETU were as follows: (1) URS never fused with cloaca membrane. (2) Dorsal cloacal membrane was maldeveloped. (3) Cloacal configuration was abnormal. (4) Tail groove never appeared. All type of ARM was the rectourethal fistula and common cloaca in ETU-treated rat embryos and was discernible on gestation day 16.
  
  
  CONCLUSIONS
  Absence of the tail groove and maldevelopment of the dorsal cloacal membrane, cloacal configuration, and urorectal septum are likely to be responsible for the formation of ARM. Failure of fusion of the URS with cloacal membrane is the immediate reason for rectourethral fistula or common cloaca in ETU-treated rats.
authors.affiliation: Department of Pediatric Surgery, The Second Affiliated Hospital, China Medical University, Shenyang, People's Republic of China.
authors.forename: YuZuo
authors.initials: Y
authors.lastname: Bai
authors.forename: Hui
authors.initials: H
authors.lastname: Chen
authors.forename: Zheng Wei
authors.initials: ZW
authors.lastname: Yuan
authors.forename: WeiLin
authors.initials: W
authors.lastname: Wang
mesh_terms: D000014:Abnormalities, Drug-Induced / Q000196:embryology; D001003:Anal Canal / Q000002:abnormalities* / Q000196:embryology*; D000818:Animals; D002988:Cloaca / Q000002:abnormalities / Q000196:embryology*; D005031:Ethylenethiourea / Q000633:toxicity; D005260:Female; D005865:Gestational Age; D009024:Morphogenesis; D011247:Pregnancy; D051381:Rats / Q000002:abnormalities* / Q000196:embryology*; D012003:Rectal Fistula / Q000151:congenital / Q000196:embryology; D013623:Tail / Q000002:abnormalities / Q000196:embryology; D013723:Teratogens / Q000633:toxicity; D014521:Urethra / Q000002:abnormalities / Q000196:embryology; D014522:Urethral Diseases / Q000151:congenital / Q000196:embryology; D014548:Urinary Fistula / Q000151:congenital / Q000196:embryology
chemical_list: D013723:Teratogens; D005031:Ethylenethiourea