Surface rendering of a short retrograde IFT train from a WT flagellum tomogram. The density map was semiautomatically segmented, and colors have been assigned as in previous models (white, flagellar membrane [m]; red, IFT particles [IFT]; yellow, MT doublets [md]). (a) Longitudinal view of the model. The IFT train is located in the space between the flagellar membrane and the MT doublet. The two ends of the IFT train have different appearances, which suggests the presence of a polarity in the retrograde train. (b) The retrograde IFT train as seen from the interior of the axoneme. (c) The IFT train as seen from the exterior of the axoneme. Note in b and c the asymmetry between the head and the tail ends, and the presence of repeating structures along the train. Black arrows in each frame indicate the flagellar tip orientation.