An accumulation of electron-dense material precedes flagellum elongation. (A) Cross-sections through the flagellum base of Chlamydomonas cells that undergo regeneration fixed shortly after pH shock-induced deflagellation. IFT particles (arrowheads) are visible in all flagella. In short flagella, numerous particles fill the space distal to the basal body, but by the time microtubules have formed (D), particles have become organized to form linear arrays. IFT particles are linked to the microtubules (small arrowheads) and to the membrane (small arrowheads). Scale bars: 0.1 μm. Reproduced with permission from Dentler . (B) Cross-sections through the flagellar pocket in which the new flagellum is built in procyclic Trypanosoma brucei. The short new flagellum contains a large amount of electron-dense material, while microtubules are not yet assembled. Once microtubules have started to elongate, this material is much more discrete. Scale bars: 500 nm, except where indicated. Reproduced with permission from Pazour et al. . IFT, intraflagellar transport.