Impaired distribution of retinoic acid receptors in the hindgut-tailgut region of murine embryos with anorectal malformations.

BACKGROUND/PURPOSE: Retinoid-mediated signal transduction plays a crucial role in the organogenesis of various organs. To investigate the pathogenesis of anorectal malformations (ARM), the authors studied the distribution pattern of retinoic acid receptors (RARs) in ARM murine embryos induced by overdose of all-trans retinoic acid (ATRA). METHODS: Pregnant mice were gavage-fed 100 mg/kg of ATRA on the ninth gestational day (E9.0). Embryos were obtained between E11.0 and E14.0 and were fixed immediately in a 4% paraformaldehyde solution. Frozen sections were prepared for immunohistochemistry using antibodies specifically raised against RAR-alpha, RAR-beta, and RAR-gamma. RESULTS: Over 98% of the embryos administered ATRA had ARM; rectoprostatic urethral and rectocloacal fistulas were the most frequent anomalies. The immunoreactivity of RAR-alpha was found equally in the epithelium of hindgut-tailgut in normal embryos on E11.5. However, it was absent in the hindgut in the treated embryos. The immunoreactivities of RAR-beta and RAR-gamma showed no difference in the distal hindgut. CONCLUSIONS: Impaired distribution of RAR-alpha in the hindgut-tailgut on E11.5 resulted in the incomplete partitioning of the cloaca and the rectourethral or rectocloacal fistula on E14.0. These results suggest that overdose of ATRA affects the distal hindgut development by directly disrupting the retinoid-mediated signalling pathway.