Figure 3.

Diversity of cilia distal domain architectures. (a) Schematic representation of representative cilia tip structures. (I) In trachea cilia, the axoneme structure is (9+2) and at the tip the A-tubules and central pair are bound to a central cap that is attached to the ciliary crown, a structure constituted by a cluster of fibrils that project from the membrane to the outside of the cilia tip. (II) In the ciliate protozoa (Tetrahymena) and green alga (Chlamydomonas), the axoneme structure is (9+2). At the tip, the A-tubules are attached to the membrane by distal filaments that, in their proximal ends, form a plug which inserts into A-tubule lumen, while the central microtubule pair is bound to the central cap. (III). In the parasite protozoa Leishmania mexicana amastigote, the axoneme structure is (9+0). Distally, the 9-fold symmetry is broken by microtubule doublets progressively occupying a more central position. (IV) In the flat worm turbellarian the cilia present a typical (9+2) axonemal pattern through the main part of its length. Near the tip, this pattern changes to one where the microtubule central pair and doublets 1, 2, 3, 8 and 9 terminate in a distal cap that is attached to a ciliary crown, whereas doublets 4 to 7 end at a dense material of a proximal secondary cap. Therefore, the tip is asymmetric. (V) In the kidney primary cilia, the axoneme is (9+0) and progressively, along the axoneme, microtubule doublets suffer a displacement towards the center and the axoneme narrows towards the tip. In the distal segment, doublets are converted into singlets through the loss of the B-tubule. (VI). In the parasite protozoa Trypanosome brucei, the tip is (9+2) but the microtubule central pair is slightly shorter. Microtubule doublets and central pair present a dense material at the ends. (VII) In the adult Caenorhabditis elegans sensory amphid channel cilia (yellow) the axoneme is (9+a few singlets), and the microtubules maintain a radial organization and in vertebrate photoreceptors (purple) the axoneme is (9+0). However, both tips present only microtubule singlets. The differences can be observed at the cross section (VIII). In the adult Caenorhabditis elegans’ sensory amphid wing cilia C, the middle region of the axoneme contains both doublets and singlets that “splay apart” laterally and end together ~0.5 μm below the distal membrane. The tip presents a membranous fan-like structure. (b) Macrocilia of the ctenophore Beroe. Macrocilia present hundreds of ciliary axonemes with a (9+2) structure surrounded by a common membrane, with a giant capping structure. At the tip, doublet microtubules end and only singlet microtubules remain, linked together by electron-dense material. Central microtubule pairs are often still recognizable at this level. Schemes were based on: [58,92,103,109,110,111,112].