2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibition of coronary development is preceded by a decrease in myocyte proliferation and an increase in cardiac apoptosis

Background: 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes cardiovascula r toxicity, culminating in edema, hemorrhage, and mortality in piscine, avi an, and mammalian embryos. To elucidate the mechanism of the cardiovascular teratogenicity of TCDD, we used a chick embryo model to determine whether TCDD alters coronary artery development and whether this alteration was ass ociated with apoptosis and/or changes in myocyte proliferation. Methods: Fertile chicken eggs were injected with corn oil (control), 0.24, or 0.40 pmol TCDD/g in corn oil before incubation, To evaluate effects of T CDD on differentiation of coronary arteries, chick embryo hearts from incub ation days 8 (D8), D10, and D12 were stained with anti-alpha -smooth muscle actin. Myocyte proliferation was measured by BrdU incorporation on D6, 8, 10, and 12 after TCDD treatment. In addition, temporal and spatial patterns of apoptosis were detected by TUNEL on D3, D5, D6, D8, and D10, and immuno histochemistry was used to identify the origin of apoptotic cells on D6. Results: TCDD increased apoptosis in structures where cell death normally o ccurs, including the outflow tract, endocardial cushion of the atrioventric ular canal, and dorsal mesocardium, peaking in intensity on D6. Immunohisto chemistry revealed that cells undergoing TCDD-induced apoptosis in the dors al mesocardium were not neural or epicardial in origin. On D8 and D10 TCDD reduced myocyte proliferation. On D10, TCDD reduced coronary artery size an d on D10 and D12 TCDD induced a dose-dependent decrease in coronary artery number. Conclusions: The reduction of myocyte proliferation by TCDD preceded the re duction in coronary artery number and size, suggesting that changes in coro nary development may be a consequence of reduced myocyte proliferation and a thinner ventricle wall. The peak of TCDD-induced increase in apoptosis oc curred even earlier in embryo development and thus may contribute to change s in myocyte proliferation, coronary development, and cardiac structural ma lformations; however, a cause-and-effect relationship between apoptosis and these other events has yet to be established. (C) 2001 Wiley-Liss, Inc.