Twenty-eight pregnant rats (Wistar-Imamichi) on the 11th gestation day were treated with a single intragastric administration of
ethylenethiourea (ETU) of 100, 125, 150, and 200 mg/kg
body weight, and received
cesarean section on the 20th gestation day. No dam died following the ETU treatment. In these transplacentally treated rat fetuses, various types of externally visible malformations were frequently observed; absent or kinked short tail in 100% and
spina bifida or myeloschisis in 48.6% to 86.8%. The incidences of these anomalies were significantly higher in males than females. The fetuses were fixed in
Bouin's solution and embedded in
paraffin, and the step-serial section were sagittally prepared. Then they were stained with
phosphotungstic acid hematoxylin for the light microscopic examination. In a preparation of just midsagittal section, malformations of rectum and anus were investigated. The results were as follows: (1) the most appropriate dose of ETU might be 125 mg/kg,
body weight of dam, which was suggested by number of the fetuses, prenatal development of the fetuses and incidences of externally visible anomalies in the fetuses; (2) various types of
anorectal anomalies, including rectourethral
fistula, rectocloacal
fistula, covered anus-complete, anoperineal
fistula, and anal membrane
stenosis, were induced; (3) the incidence of
anorectal anomalies was significantly higher in males than females; (4) in every type of
anorectal anomalies, inner circular layer of smooth muscle was hypertrophied at the rectum end, which seemed to be considered as the internal sphincter muscle; and (5) the external sphincter muscle complex was hypoplastic in the high
deformity fetuses associated with sacrococcygeal anomaly.