Fig. 2. Photomicrographs showing whole-mount preparations of eel dorsal aorta (A,C) and intestinal vein (B,D) following processing for NADPH diaphorase histochemistry (A,B) and neural nitric oxide synthase (nNOS) immunohistochemistry (C,D). Using both techniques a plexus of nNOS-positive perivascular nerve bundles (white arrows) was observed in the outer layers of the wall of each vessel. In addition, some larger nNOS-positive nerve bundles (B, arrowheads) were observed. Within the nerve bundles, nNOS-positive cell bodies were observed (B, black arrow). Inset, high power magnification of a nerve cell body. Scale bars, 100 µm.