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Fig. 2 TCDD prevents closure of the nascent AV valve during systole. Newly fertilized zebrafish eggs were treated with TCDD or DMSO as a vehicle control as described in the "Materials and Methods." Representative lateral view micrographs are shown for hearts at the indicated time in development. The arrows indicate an open passage between the atrium and ventricle. Open: the image was taken during diastole in which the AV junction is relaxed. Closed: the image was taken during systole, during which the AV junction has contracted to its fullest extent. An open passage between the ventricle and atrium was consistently observed associated with retrograde blood movement at 72 hpf (see supplemental movie: 72hpfAB.mov).