Figure 3. The cilium has hybrid features between primary and motile cilia.

(A) Representative section from FIB-SEM tomogram of human primary nasal multiciliated cells. Arrows indicate basal bodies with multiple basal feet. Scale bar represents 1 μm. (B) High-magnification view of boxed area in (A) at different z positions of the tomogram from z=0 nm to z=1660 nm. Note the hybrid cilium axoneme and central pair (z=0 nm), transition fibers (z=1000–1040 nm), multiple basal feet (z=1160–1160 nm) and the absence of the endocytic pocket (z=1660 nm). Scale bar represents 500 nm. (C) A high-magnification view of boxed area in (B) highlighting the basal body with a central pair and multiple basal feet. Scale bar represents 100 nm. (D) 2D projection micrographs of 3D-SIM volume of human airway multiciliated cells (left), and high-magnification views of boxed areas (right), labeled with anti-CNTRL (green), anti-RSPH4A (red, top) and anti-GAS8 (red, bottom) antibodies. Scale bars represent 2 μm. (E) Cartoon representation of hybrid cilium structure relative to the primary cilium and motile cilium. The axoneme extends from the basal body (grey), each with a basal foot (red triangle). The basal body of the primary cilium presents multiple basal feet, while the one of the motile cilium has one basal foot. In regard to the axoneme, the motile cilium presents outer dynein arms, inner dynein arms, nexin-dynein regulatory complexes, radial spokes and central pair complexes (black) , which are critical for in plane ciliary beating. The hybrid cilium has features of both the primary and motile cilium, namely multiple basal feet and protein complexes critical for ciliary beating.