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 cardiovascular toxicity, culminating in edema, hemorrhage, and mortality in piscine, avian, 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 associated 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 TCDD on differentiation of coronary arteries, chick embryo hearts from incubation 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 immunohistochemistry was used to identify the origin of apoptotic cells on D6. Results: TCDD increased apoptosis in structures where cell death normally occurs, including the outflow tract, endocardial cushion of the atrioventricular canal, and dorsal mesocardium, peaking in intensity on D6. Immunohistochemistry revealed that cells undergoing TCDD-induced apoptosis in the dorsal mesocardium were not neural or epicardial in origin. On D8 and D10 TCDD reduced myocyte proliferation. On D10, TCDD reduced coronary artery size and on D10 and D12 TCDD induced a dose-dependent decrease in coronary artery number. Conclusions: The reduction of myocyte proliferation by TCDD preceded the reduction in coronary artery number and size, suggesting that changes in coronary development may be a consequence of reduced myocyte proliferation and a thinner ventricle wall. The peak of TCDD-induced increase in apoptosis occurred even earlier in embryo development and thus may contribute to changes in myocyte proliferation, coronary development, and cardiac structural malformations; however, a cause-and-effect relationship between apoptosis and these other events has yet to be established. (C) 2001 Wiley-Liss, Inc.