Types of cilia found at the left-right-organizer of vertebrates. In mouse, two types of ciliated cells at the node have been demonstrated (A). Pit cells possess motile cilia visualized by a GFP-fusion protein of DNAH11 (LRD-GFP; arrowhead). Most crown cells possess immotile cilia containing polycystin-2 but lacking LRD-GFP (arrow). It is suggested that immotile cilia of crown cells act as flow sensors. Primary cilia containing polycystin-2 are found on endodermal cells adjacent to the node (asterix). [11,101,118]. Classical motile cilia (B) possess nine outer microtubule doublets forming a ring around a central pair (CP) of single microtubules, called 9 + 2 axoneme. Outer microtubule doublets possess dynein arms that drive rhythmic movement of the axonemes. Radial spoke complexes project from each outer doublet towards the CP and are thought to be important in regulating motion of the axoneme (B). Motile monocilia (B) are found in cells of the LR-organizer in various vertebrates, such as ventral node in mouse, notochordal plate in rabbit or Kupffer’s vesicle in zebrafish and medaka (C). In mouse and medaka, cilia of the LR-organizer usually display the 9 + 0 configuration lacking the CP. In other vertebrates, such as zebrafish, they display 9 + 2 or in rabbit 9 + 0, 9 + 2 and 9 + 4 configuration. Irrespective of the structure, these cilia move in a rotational manner, establishing a leftward-directed fluid flow within the cavity of the LR-organizer. Immotile cilia (B) lack motility components such as dynein arms, CP and radial spokes and act as specialized sensors of the cell transducing signals from extracellular stimuli to a cellular response [125,131]. A: Immunofluorescent staining detecting polycystin-2 (red) in node monocilia of a 2 somite stage LRD-GFP (green) mouse embryo. A: anterior; l: left; p: posterior; r: right. Scale bar: 20 μm [132-142].