Urorectal septum malformation sequence: Insights into pathogenesis.

We characterize the urorectal septum malformation sequence (URSMS) in discordant fetal lambs and relate it to the human syndromes with which URSMS is associated. We found abnormal external genitalia, imperforate anus, and fistulous connections between the rectum, bladder, and vagina. Discordance among the dizygous twins eliminated teratogens as a likely etiologic factor. We summarize the relevant literature and propose a molecular model for the pathogenesis of the URSMS, in which alterations in sonic hedgehog and homeobox genes lead to caudal mesodermal deficiency during blastogenesis.