Clarification of the processes that lead to anorectal malformations in the ETU-induced rat model of imperforate anus.

Background: Ethylenethiourea (ETU) administered to timed-pregnant rats can induce anorectal malformations (ARMs) in about 80% of rat fetuses, thus providing an ideal animal model to study the embryogenesis of ARMs. The current study was undertaken to investigate the embryogenetic events that may be responsible for the development of ARMs in rats.

Methods: Time-mated pregnant rats were divided randomly into control and experimental groups. The experimental rats received 1% ETU (125 mg/kg) by gavage on gestational day 10, and control rats received only the vehicle. Their embryos were harvested by cesarean section on gestational days 13, 14, 15, and 16. They were fixed and embedded in paraffin and serially sectioned in either the sagittal or transverse plane. The sections were stained with H&E, examined, and photographed. The comparative morphogenesis of the hindgut, cloaca, and tailgut of age-matched embryos was studied.

Results: The key abnormalities in the experimental embryos were: (1) maldevelopment of cloaca and urorectal septum with no sign of the fusion between the urorectal septum and the cloacal membrane, (2) delay of tailgut regression, (3) abnormal and massive apoptotic cell death involving the posterior cloacal wall, and (4) underdevelopment of the dorsal aspect of the cloaca and its membrane. The type of ARM that was developing was discernible by gestational day 15 and 16.

Conclusions: ARMs induced by ETU in rat embryos seem to be caused by the cumulative effect of aberrations in the development of several components of the hindgut and cloaca. Variation in the extent of maldevelopment of these structures may result in a spectrum of ARMs.