Gestational Exposure to 2,3,7,8-Tetrachlorodibenzo-p-dioxin Induces Developmental Defects in the Rat Vagina

At puberty, female rats exposed in utero to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) exhibit a persistent thread of mesenchymal tissue surrounded by keratinized epithelium that partially occludes the vaginal opening. Our objective was to determine the earliest time during fetal development that morphological signs of this vaginal canal malformation could be detected and to obtain greater insight into mechanisms involved in this effect. Pregnant rats were administered a single dose of vehicle (control) or TCDD (1.0 microg/kg, po) on gestation day (GD) 15 and were sacrificed on GD 18, 19, 20, and 21 for histological evaluation of female. Gestational exposure to TCDD affected vaginal morphogenesis as early as GD 19, 4 days after exposure of pregnant dams. In exposed fetuses, the thickness of mesenchymal tissue between the caudal Mullerian ducts was increased, which resulted in a failure of the Mullerian ducts to fuse, a process normally completed prior to parturition. In addition, TCDD exposure appeared to inhibit the regression of Wolffian ducts. Thus, TCDD interferes with vaginal development by impairing regression of the Wolffian ducts, by increasing the size of interductal mesenchyme, and by preventing fusion of the Mullerian ducts. Taken together, these effects appear to cause the persistent vaginal thread defect observed in rats following in utero and lactational TCDD exposure.