Structural diversity and distribution of cilia in the apical sense organ of the ctenophore Bolinopsis mikado.

The apical organ of ctenophores is the center of sensory information that controls locomotion. Previous studies have described several types of cilia in this organ. However, detailed ciliary structures, particularly axonemal structures, have not been extensively investigated. Here, we reported that the apical organ of the ctenophore Bolinopsis mikado contains six types of cilia with different axonemal structures. These include the typical "9 + 2" motile axonemes, with both outer and inner dynein arms, only the inner dynein arm, or no dynein arm; axonemes with electron-dense structures in the A-tubules; "9 + 0" axonemes lacking the central pair of microtubules; and axonemes with compartmenting lamellae. Considering that "9 + 2" axonemal structures with both dynein arms are thought to be ancestral forms of cilia, the apical organ of ctenophores would comprise an elaborate assembly of modified ciliary forms that sense and transmit extracellular stimuli and generate various fluid flows.