Distribution of opsin-expressing cone cells in eel retina. Transverse sections of eel retina from different developmental stages showing localisation of cone opsin expression as detected by in situ hybridisation of the 5' end of rh2 or sws2 opsin mRNA. Cone cell bodies in the eel are arranged in a single layer between the inner nuclear layer (INL) and the rod outer segments (ROS). Glass eel: (A) the layer of cone cells expresses rh2 opsin (horizontal arrowhead), (B) sws2 opsin expression was not detected in the glass eel retina. Yellow eel: (C) the layer of cone cells expressing rh2 opsin (horizontal arrowhead), (D) occasional cone cells expressing sws2 opsin (vertical arrowheads). Silver eel: (E) monolayer of cone cells expressing rh2 opsin (horizontal arrowhead), (F) occasional cone cells express sws2 opsin (vertical arrowheads). Scale bar is 100μm, all panels are to same scale. RPE, retinal pigment epithelium; ROS, rod outer segments; ONL, outer nuclear layer; INL, inner nuclear layer.