(A) 3D model of a tomographic reconstruction of a growing, long T. brucei axoneme. The doublets are color coded in a gradient from yellow (dMT1) to red (dMT9). (B) A cut through of the 3D model shows that (1) at the end of the flagellum, the circular arrangement of the MTs is completely lost, (2) <0.5 μm before the flagella tip, the MTs start losing their circular organization, and (3) >0.5 μm from its tip the axoneme is well organized. (C) Individual traces of the nine dMTs in A reveals them bending both toward and away from the CP. dMTs not touching the membrane also showed this random bending (e.g., dMTs 2 and 5) (D) CP projections are arranged as an electron-dense ladder (arrows) extending from the central pair. (E) In the tip of the T. brucei old flagellum the associated structural proteins are visible all the way to the tip. In the longitudinal view, we see the distal end that then curves and the final 200 nm is shown in cross-sectional view (insert). (F) Central pair projections are clearly seen all the way to the end of the central pair in the short growing T. brucei flagellum. (G) In the T. brucei long growing flagellum such associated proteins are not visible (red arrows). (H) The structural proteins are clearly visible along the length of a mature C. reinhardtii flagellum. (I and J) In the growing short C. reinhardtii flagellum, these proteins were also present at the axoneme’s tip, indicating that microtubules and associated structures are simultaneously assembled.