CEM, CEP and OLQ ciliary ultrastructure is abnormal in nphp-4 mutants. TEM of wild type (a–e) and nphp-4 (f–j) mutant animals in transverse thin sections. Black arrows, OLQ; white arrows, CEP; arrowheads, CEM. As depicted in the top drawings, the distal-most region of the CEM axoneme is narrow, curved outward, and exposed to the environment. Microtubule singlets are seen longitudinally near the opening (a). Traveling posteriorly, the distal segment aligns with the body axis and nears the edge of CEP socket cell (b). The CEM middle segment widens and is filled with microtubule outer singlets and dispersed inner singlets (c). The middle segment twists around the CEP away from the OLQ such that the CEM axoneme is furthest away from OLQ (d). CEM TZs are similar to CEP and OLQ TZs with 9 + 0 microtubule doublets (e). In nphp-4 mutants, OLQ distal segments are occasionally missing (f) but middle segments are present (g) with missing microtubules (g and h). CEM and OLQ TZs appear normal (i and j). CEP axonemes are irregular with large unidentified accumulations (g–i). Panel j is from a different section of the same worm illustrating well-formed OLQ and CEM TZs but apparently missing a CEP TZ, replaced by a large mass of TAM surrounded by microtubule singlets. CEM axonemes can lack distal segments (empty channel in f with no microtubules). Drawings at bottom illustrate nphp-4 defects. Bars: (top) 100 nm; (bottom) 0.5 μm.