arl-13(tm2322) mutant cilia possess ultrastructural defects. Transmission electron microscopy serial cross sections of amphid pore from N2 and tm2322 worms. (A–E) Distal pore showing 10 axonemes in N2 worms (A and B) but only 4 axonemes in tm2322 mutants (C–E). (F–J) 2 µm proximal to A–E (through MSs) showing axonemes still missing in tm2322 worms (H). Also, abnormal accumulation of electron-dense material (J, arrowhead) and unzipping of MTs (J, arrow) are observed. (K–O) 3 µm proximal to A–E (through MSs) showing most ciliary axonemes are present in tm2322 mutants (M). Further abnormal axonemal accumulations of amorphous electron-dense material (N, arrowheads) and missing, misplaced, and unzipped doublet MTs (N and O, arrows) are also shown. (P–T) 5 µm proximal to A and B, through MSs, transition zones, and transitional fibers. Some tm2322 cilia are abnormally enlarged (R and S), filled with electron-dense material that is amorphous, or contained within vesicle-type structures (R and S). (S) Arrowheads indicate amorphous and vesicular-like accumulations. Transition zones appear normal (T). (U) Schematics of amphid channel cilia from N2 and tm2322 mutants, showing the major ultrastructural defects observed. tz, transition zone. Bars, 200 µm.