Distribution and structural diversity of cilia in tadpole larvae of the ascidian Ciona intestinalis

Accumulating evidence demonstrates that cilia play important roles in a variety of processes in embryogenesis. For functional survey of larval cilia at the cellular level, we exploited the simple cell organization of tadpole larvae in the ascidian Ciona intestinalis. Immunofluorescent microscopy showed distribution of cilia not only in previously described tissues but also in a subpopulation of ependymal cells in the sensory vesicle, gut primordium, papillae, apical trunk epidermal neurons, and the endodermal strand. Transmission electron microscopy revealed a variety of axonemal structures, including a 9 + 0 structure similar to vertebrate primary cilia, a 9 + 0 structure with electron-dense materials in the center, a 9 + 2 structure with no dynein arms, and an axoneme with a disorganized structure at the distal end. Extensive description of cilia in the present study gives important insights into the evolution of the ciliary structure and provides a basis for analysis of ciliary functions in establishment of chordate body plan.