2,3,7,8-Tetrachlorodibenzo-p-dioxin alters embryonic palatal medial epithelial cell differentiation in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, inducing cleft palate and hydronephrosis. After exposure in vivo, TCDD specifically alters differentiation of embryonic palatal medial epithelial cells. In this study, the palatal epithelial cell response to TCDD is determined in vitro. C57BL/6N palatal shelves were placed in organ culture on gestation day (GD) 12 in Richter's improved modified Eagle's medium:Ham's F12 medium (1:1) with 1% fetal bovine serum for 3 or 4 days. Medium contained 0.1% dimethylsulfoxide and TCDD at 0, 10(-13), 10(-12), 10(-11), 10(-10), and 10(-9) M, with some doses at 5 x 10(-11), 7.5 x 10(-11), and 5 x 10(-12) M. Epithelial cell responses to TCDD occurred over a narrow range of concentrations, with maximal response at 5 x 10(-11) M. Cytotoxicity was detected at 1 x 10(-10) M. At a stage when control medial cells ceased proliferation and EGF receptors were not detected immunohistochemically. TCDD-exposed medial cells incorporated [3H]thymidine and high levels of epidermal growth factor receptors were detected. TCDD prevented programmed cell death of medial peridermal cells, and induced a shift in the differentiation of medial cells toward an oral-like phenotype. The responses to TCDD observed after exposure in vitro were indistinguishable from previously reported effects observed after exposure in vivo. In the present study, the distribution of TCDD in the fetus after exposure in vivo was examined. The levels of exposure to TCDD are similar for in vitro and in vivo exposure routes. The levels of TCDD in 1 x 10(-11) to 1 x 10(-10) M solutions (3 to 32 pg/ml) were comparable to levels observed in fetal tissues after in vivo exposure on GD 11 to 30 microns/kg [3H]TCDD, where the palatal shelf contained 1.4 to 3.5. pg TCDD, representing 0.0003% of the total dose. In vivo, TCDD was detected in the GD 11 embryo 3 hr postexposure and the TCDD was equally distributed between the embryonic head and body. At 72 hr postexposure, 0.035% of the total dose was in fetal tissues, and 1% of the TCDD in the fetus was found in the palatal shelf. The present study shows that the palatal epithelium responds to TCDD in vitro in a manner comparable to that observed after in vivo exposure, and that the response occurs at a concentration comparable to in vivo levels in the fetus. The availability of an in vitro system will facilitate studies of TCDD toxicity that are difficult or impossible to perform in vivo, such as comparisons of TCDD effects between species, including human tissues.