Fig. 2. (A–C) Endothelin-1-like immunoreactivity (ET-1 IR) in Xenopus laevis NF stage 50 tadpoles. Cross sections of the head region cranial to the heart and truncus arteriosus with the ventral side at the top of the figure. (A) The immunoreactive endothelial cells of the main head arteries (arrows). Note that the two main head veins lack ET-1 IR (arrowheads). The body wall showed nonspecific labelling (top), which was also present in control sections. Scale bar, 400 µm. (B) The two main head arteries with the endothelium, showing ET-1 IR. The smooth muscle layer, the connective tissue and the melanophores covering parts of the blood vessels cannot be seen in this micrograph. Scale bar, 200 µm. (C) A higher-power view of one of the main head arteries. Scale bar, 100 µm.